Abiogenesis, or informally the origin of life, is the natural process by which life arises from non-living matter, such as simple organic compounds. The transition from non-living to living entities was not a single event, but a gradual process of increasing complexity that involved molecular self-replication, self-assembly, autocatalysis and cell membranes.
Researchers study abiogenesis through a combination of molecular biology, paleontology, astrobiology, oceanography, biophysics, geochemistry and biochemistry, and aim to determine how pre-life chemical reactions gave rise to life. The study of abiogenesis can be geophysical, chemical, or biological, with more recent approaches attempting a synthesis of all three, as life arose under conditions that are strikingly different from those on Earth today. Life functions through the specialized chemistry of carbon and water and builds largely upon four key families of chemicals: lipids (fatty cell walls), carbohydrates (sugars, cellulose), amino acids (protein metabolism), and nucleic acids (self-replicating DNA and RNA). Any successful theory of abiogenesis must explain the origins and interactions of these classes of molecules. Many approaches to abiogenesis investigate how self-replicating molecules, or their components, came into existence. Researchers generally think that current life on Earth descends from an RNA world, although RNA-based life may not have been the first life to have existed.
The classic 1952 Miller–Urey experiment and similar research demonstrated that most amino acids, the chemical constituents of the proteins used in all living organisms, can be synthesized from inorganic compounds under conditions intended to replicate those of the early Earth. Scientists have proposed various external sources of energy that may have triggered these reactions, including lightning and radiation. Other approaches ("metabolism-first" hypotheses) focus on understanding how catalysis in chemical systems on the early Earth might have provided the precursor molecules necessary for self-replication. Complex organic molecules occur in the Solar System and in interstellar space, and these molecules may have provided starting material for the development of life on Earth.
The biochemistry of life may have begun shortly after the Big Bang, 13.8 billion years ago, during a habitable epoch when the age of the universe was only 10 to 17 million years. The panspermia hypothesis suggests that microscopic life was distributed to the early Earth by space dust, meteoroids, asteroids and other small Solar System bodies and that life may exist throughout the universe. The panspermia hypothesis proposes that life originated outside the Earth, but does not definitively explain its origin.
Nonetheless, Earth remains the only place in the universe known to harbour life, and fossil evidence from the Earth informs most studies of abiogenesis. The age of the Earth is about 4.54 billion years; the earliest undisputed evidence of life on Earth dates from at least 3.5 billion years ago, and possibly as early as the Eoarchean Era (between 3.6 and 4.0 billion years ago), after geological crust started to solidify following the molten Hadean Eon. In May 2017 scientists found possible evidence of early life on land in 3.48-billion-year-old geyserite and other related mineral deposits (often found around hot springs and geysers) uncovered in the Pilbara Craton of Western Australia. However, a number of discoveries suggest that life may have appeared on Earth even earlier. As of 2017, microfossils, or fossilised microorganisms, within hydrothermal-vent precipitates dated from 3.77 to 4.28 billion years old found in Quebec, Canada may be the oldest record of life on Earth, suggesting life started soon after ocean formation 4.4 billion years ago. According to biologist Stephen Blair Hedges, "If life arose relatively quickly on Earth … then it could be common in the universe."
Early geophysical conditions on Earth
The Hadean Earth is thought to have had a secondary atmosphere, formed through degassing of the rocks that accumulated from planetesimal impactors. At first, it was thought that the Earth's atmosphere consisted of hydrogen compounds—methane, ammonia and water vapour—and that life began under such reducing conditions, which are conducive to the formation of organic molecules. According to later models, suggested by study of ancient minerals, the atmosphere in the late Hadean period consisted largely of water vapour, nitrogen and carbon dioxide, with smaller amounts of carbon monoxide, hydrogen, and sulfur compounds. During its formation, the Earth lost a significant part of its initial mass, with a nucleus of the heavier rocky elements of the protoplanetary disk remaining. As a consequence, Earth lacked the gravity to hold any molecular hydrogen in its atmosphere, and rapidly lost it during the Hadean period, along with the bulk of the original inert gases. The solution of carbon dioxide in water is thought to have made the seas slightly acidic, giving it a pH of about 5.5. The atmosphere at the time has been characterized as a "gigantic, productive outdoor chemical laboratory." It may have been similar to the mixture of gases released today by volcanoes, which still support some abiotic chemistry.
Oceans may have appeared first in the Hadean Eon, as soon as two hundred million years (200 Ma) after the Earth was formed, in a hot 100 °C (212 °F) reducing environment, and the pH of about 5.8 rose rapidly towards neutral. This has been supported by the dating of 4.404 Ga-old zircon crystals from metamorphosed quartzite of Mount Narryer in the Western Australia Jack Hills of the Pilbara, which are evidence that oceans and continental crust existed within 150 Ma of Earth's formation. Despite the likely increased volcanism and existence of many smaller tectonic "platelets," it has been suggested that between 4.4 and 4.3 Ga (billion year), the Earth was a water world, with little if any continental crust, an extremely turbulent atmosphere and a hydrosphere subject to intense ultraviolet (UV) light, from a T Tauri stage Sun, cosmic radiation and continued bolide impacts.
The Hadean environment would have been highly hazardous to modern life. Frequent collisions with large objects, up to 500 kilometres (310 mi) in diameter, would have been sufficient to sterilize the planet and vaporize the ocean within a few months of impact, with hot steam mixed with rock vapour becoming high altitude clouds that would completely cover the planet. After a few months, the height of these clouds would have begun to decrease but the cloud base would still have been elevated for about the next thousand years. After that, it would have begun to rain at low altitude. For another two thousand years, rains would slowly have drawn down the height of the clouds, returning the oceans to their original depth only 3,000 years after the impact event.
Earliest biological evidence for life
The most commonly accepted location of the root of the tree of life is between a monophyletic domain Bacteria and a clade formed by Archaea and Eukaryota of what is referred to as the "traditional tree of life" based on several molecular studies starting with C. Woese. A very small minority of studies have concluded differently, namely that the root is in the Domain Bacteria, either in the phylum Firmicutes or that the phylum Chloroflexi is basal to a clade with Archaea+Eukaryotes and the rest of Bacteria as proposed by Thomas Cavalier-Smith. More recently Peter Ward has established an alternative view which is rooted in abiotic RNA synthesis which becomes enclosed within a capsule and then creates RNA ribozyme replicates. It is proposed that this then bifurcates between Dominion Ribosa (RNA life), and after the loss of ribozymes RNA viruses as Domain Viorea, and Dominion Terroa, which after creating a large cell within a lipid wall, creating DNA the 20 based amino acids and the triplet code, is established as the last universal common ancestor or LUCA, of earlier phylogenic trees.
The earliest life on Earth existed more than 3.5 billion years ago, during the Eoarchean Era when sufficient crust had solidified following the molten Hadean Eon. The earliest physical evidence so far found consists of microfossils in the Nuvvuagittuq Greenstone Belt of Northern Quebec, in "banded iron formation" rocks at least 3.77 billion and possibly 4.28 billion years old. This finding suggested that there was almost instant development of life after oceans were formed. The structure of the microbes was noted to be similar to bacteria found near hydrothermal vents in the modern era, and provided support for the hypothesis that abiogenesis began near hydrothermal vents.
Also noteworthy is biogenic graphite in 3.7 billion-year-old metasedimentary rocks from southwestern Greenland and microbial mat fossils found in 3.48 billion-year-old sandstone from Western Australia. Evidence of early life in rocks from Akilia Island, near the Isua supracrustal belt in southwestern Greenland, dating to 3.7 billion years ago have shown biogenic carbon isotopes. In other parts of the Isua supracrustal belt, graphite inclusions trapped within garnet crystals are connected to the other elements of life: oxygen, nitrogen, and possibly phosphorus in the form of phosphate, providing further evidence for life 3.7 billion years ago. At Strelley Pool, in the Pilbara region of Western Australia, compelling evidence of early life was found in pyrite-bearing sandstone in a fossilized beach, that showed rounded tubular cells that oxidized sulfur by photosynthesis in the absence of oxygen. Further research on zircons from Western Australia in 2015 suggested that life likely existed on Earth at least 4.1 billion years ago.
Traditionally it was thought that during the period between 4.28 and 3.8 Ga, changes in the orbits of the giant planets may have caused a heavy bombardment by asteroids and comets that pockmarked the Moon and the other inner planets (Mercury, Mars, and presumably Earth and Venus). This would likely have repeatedly sterilized the planet, had life appeared before that time. Geologically, the Hadean Earth would have been far more active than at any other time in its history. Studies of meteorites suggests that radioactive isotopes such as aluminium-26 with a half-life of 7.17×105 (717 thousand) years, and potassium-40 with a half-life of 1.250×109 (1.25 billion) years, isotopes mainly produced in supernovae, were much more common. Internal heating as a result of gravitational sorting between the core and the mantle would have caused a great deal of mantle convection, with the probable result of many more smaller and more active tectonic plates than now exist.
The time periods between such devastating environmental events give time windows for the possible origin of life in the early environments. If the deep marine hydrothermal setting was the site for the origin of life, then abiogenesis could have happened as early as 4.0 to 4.2 Ga. If the site was at the surface of the Earth, abiogenesis could only have occurred between 3.7 and 4.0 Ga.
In 2016, a set of 355 genes likely present in the Last Universal Common Ancestor (LUCA) of all organisms living on Earth was identified. A total of 6.1 million prokaryotic protein coding genes from various phylogenic trees were sequenced, identifying 355 protein clusters from amongst 286,514 protein clusters that were probably common to LUCA. The results "depict LUCA as anaerobic, CO2-fixing, H2-dependent with a Wood–Ljungdahl pathway, N2-fixing and thermophilic. LUCA’s biochemistry was replete with FeS clusters and radical reaction mechanisms. Its cofactors reveal dependence upon transition metals, flavins, S-adenosyl methionine, coenzyme A, ferredoxin, molybdopterin, corrins and selenium. Its genetic code required nucleoside modifications and S-adenosylmethionine-dependent methylations." The results depict methanogenic clostridia as a basal clade in the 355 phylogenies examined, and suggest that LUCA inhabited an anaerobic hydrothermal vent setting in a geochemically active environment rich in H2, CO2 and iron. M.D. Brazier has shown that early micro-fossils came from a hot world of gases such as methane, ammonia, carbon dioxide and hydrogen sulphide, which are toxic to much current life. Another analysis of the conventional threefold tree of life shows thermophilic and hyperthermophilic bacteria and archaea are closest to the root, suggesting that life may have evolved in a hot environment.
Belief in spontaneous generation of certain forms of life from non-living matter goes back to Aristotle and ancient Greek philosophy and continued to have support in Western scholarship until the 19th century. This belief was paired with a belief in heterogenesis, i.e., that one form of life derived from a different form (e.g., bees from flowers). Classical notions of spontaneous generation held that certain complex, living organisms are generated by decaying organic substances. According to Aristotle, it was a readily observable truth that aphids arise from the dew that falls on plants, flies from putrid matter, mice from dirty hay, crocodiles from rotting logs at the bottom of bodies of water, and so on. In the 17th century, people began to question such assumptions. In 1646, Sir Thomas Browne published his Pseudodoxia Epidemica (subtitled Enquiries into Very many Received Tenets, and commonly Presumed Truths), which was an attack on false beliefs and "vulgar errors." His contemporary, Alexander Ross, erroneously refuted him, stating: "To question this [spontaneous generation], is to question Reason, Sense, and Experience: If he doubts of this, let him go to Ægypt, and there he will finde the fields swarming with mice begot of the mud of Nylus, to the great calamity of the Inhabitants."
In 1665, Robert Hooke published the first drawings of a microorganism. Hooke was followed in 1676 by Antonie van Leeuwenhoek, who drew and described microorganisms that are now thought to have been protozoa and bacteria. Many felt the existence of microorganisms was evidence in support of spontaneous generation, since microorganisms seemed too simplistic for sexual reproduction, and asexual reproduction through cell division had not yet been observed. Van Leeuwenhoek took issue with the ideas common at the time that fleas and lice could spontaneously result from putrefaction, and that frogs could likewise arise from slime. Using a broad range of experiments ranging from sealed and open meat incubation and the close study of insect reproduction he became, by the 1680s, convinced that spontaneous generation was incorrect.
The first experimental evidence against spontaneous generation came in 1668 when Francesco Redi showed that no maggots appeared in meat when flies were prevented from laying eggs. It was gradually shown that, at least in the case of all the higher and readily visible organisms, the previous sentiment regarding spontaneous generation was false. The alternative seemed to be biogenesis: that every living thing came from a pre-existing living thing (omne vivum ex ovo, Latin for "every living thing from an egg").
In 1768, Lazzaro Spallanzani demonstrated that microbes were present in the air, and could be killed by boiling. In 1861, Louis Pasteur performed a series of experiments that demonstrated that organisms such as bacteria and fungi do not spontaneously appear in sterile, nutrient-rich media, but could only appear by invasion from without.
The belief that self-ordering by spontaneous generation was impossible begged for an alternative. By the middle of the 19th century, the theory of biogenesis had accumulated so much evidential support, due to the work of Pasteur and others, that the alternative theory of spontaneous generation had been effectively disproven. John Desmond Bernal, a pioneer in X-ray crystallography, suggested that earlier theories such as spontaneous generation were based upon an explanation that life was continuously created as a result of chance events.
The term biogenesis is usually credited to either Henry Charlton Bastian or to Thomas Henry Huxley. Bastian used the term around 1869 in an unpublished exchange with John Tyndall to mean "life-origination or commencement". In 1870, Huxley, as new president of the British Association for the Advancement of Science, delivered an address entitled Biogenesis and Abiogenesis. In it he introduced the term biogenesis (with an opposite meaning to Bastian's) as well as abiogenesis:
- And thus the hypothesis that living matter always arises by the agency of pre-existing living matter, took definite shape; and had, henceforward, a right to be considered and a claim to be refuted, in each particular case, before the production of living matter in any other way could be admitted by careful reasoners. It will be necessary for me to refer to this hypothesis so frequently, that, to save circumlocution, I shall call it the hypothesis of Biogenesis; and I shall term the contrary doctrine–that living matter may be produced by not living matter–the hypothesis of Abiogenesis.
- A word of explanation seems necessary with regard to the introduction of the new term Archebiosis. I had originally, in unpublished writings, adopted the word Biogenesis to express the same meaning—viz., life-origination or commencement. But in the mean time the word Biogenesis has been made use of, quite independently, by a distinguished biologist [Huxley], who wished to make it bear a totally different meaning. He also introduced the word Abiogenesis. I have been informed, however, on the best authority, that neither of these words can—with any regard to the language from which they are derived—be supposed to bear the meanings which have of late been publicly assigned to them. Wishing to avoid all needless confusion, I therefore renounced the use of the word Biogenesis, and being, for the reason just given, unable to adopt the other term, I was compelled to introduce a new word, in order to designate the process by which living matter is supposed to come into being, independently of pre-existing living matter.
Louis Pasteur and Charles Darwin
Louis Pasteur remarked, about a finding of his in 1864 which he considered definitive, "Never will the doctrine of spontaneous generation recover from the mortal blow struck by this simple experiment." One alternative was that life's origins on Earth had come from somewhere else in the universe. Periodically resurrected (see Panspermia, above) Bernal said that this approach "is equivalent in the last resort to asserting the operation of metaphysical, spiritual entities... it turns on the argument of creation by design by a creator or demiurge." Such a theory, Bernal said, was unscientific. A theory popular around the same time was that life was the result of an inner "life force", which in the late 19th century was championed by Henri Bergson.
The idea of evolution by natural selection proposed by Charles Darwin put an end to these metaphysical theologies. In a letter to Joseph Dalton Hooker on 1 February 1871, Darwin discussed the suggestion that the original spark of life may have begun in a "warm little pond, with all sorts of ammonia and phosphoric salts, light, heat, electricity, &c., present, that a proteine compound was chemically formed ready to undergo still more complex changes." He went on to explain that "at the present day such matter would be instantly devoured or absorbed, which would not have been the case before living creatures were formed." He had written to Hooker in 1863 stating that, "It is mere rubbish, thinking at present of the origin of life; one might as well think of the origin of matter." In On the Origin of Species, he had referred to life having been "created", by which he "really meant 'appeared' by some wholly unknown process", but had soon regretted using the Old Testament term "creation".
"Primordial soup" hypothesis
No new notable research or hypothesis on the subject appeared until 1924, when Alexander Oparin reasoned that atmospheric oxygen prevents the synthesis of certain organic compounds that are necessary building blocks for the evolution of life. In his book The Origin of Life, Oparin proposed that the "spontaneous generation of life" that had been attacked by Louis Pasteur did in fact occur once, but was now impossible because the conditions found on the early Earth had changed, and preexisting organisms would immediately consume any spontaneously generated organism. Oparin argued that a "primeval soup" of organic molecules could be created in an oxygenless atmosphere through the action of sunlight. These would combine in ever more complex ways until they formed coacervate droplets. These droplets would "grow" by fusion with other droplets, and "reproduce" through fission into daughter droplets, and so have a primitive metabolism in which factors that promote "cell integrity" survive, and those that do not become extinct. Many modern theories of the origin of life still take Oparin's ideas as a starting point.
Robert Shapiro has summarized the "primordial soup" theory of Oparin and J. B. S. Haldane in its "mature form" as follows:
- The early Earth had a chemically reducing atmosphere.
- This atmosphere, exposed to energy in various forms, produced simple organic compounds ("monomers").
- These compounds accumulated in a "soup" that may have concentrated at various locations (shorelines, oceanic vents etc.).
- By further transformation, more complex organic polymers – and ultimately life – developed in the soup.
About this time, Haldane suggested that the Earth's prebiotic oceans (quite different from their modern counterparts) would have formed a "hot dilute soup" in which organic compounds could have formed. Bernal called this idea biopoiesis or biopoesis, the process of living matter evolving from self-replicating but non-living molecules, and proposed that biopoiesis passes through a number of intermediate stages.
One of the most important pieces of experimental support for the "soup" theory came in 1952. Stanley L. Miller and Harold C. Urey performed an experiment that demonstrated how organic molecules could have spontaneously formed from inorganic precursors under conditions like those posited by the Oparin-Haldane hypothesis. The now-famous Miller–Urey experiment used a highly reducing mixture of gases – methane, ammonia, and hydrogen, as well as water vapour – to form simple organic monomers such as amino acids. The mixture of gases was cycled through an apparatus that delivered electrical sparks to the mixture. After one week, it was found that about 10% to 15% of the carbon in the system was then in the form of a racemic mixture of organic compounds, including amino acids, which are the building blocks of proteins. This provided direct experimental support for the second point of the "soup" theory, and it is around the remaining two points of the theory that much of the debate now centres.
Bernal showed that based upon this and subsequent work there is no difficulty in principle in forming most of the molecules we recognize as the necessary molecules for life from their inorganic precursors. The underlying hypothesis held by Oparin, Haldane, Bernal, Miller and Urey, for instance, was that multiple conditions on the primeval Earth favoured chemical reactions that synthesized the same set of complex organic compounds from such simple precursors. A 2011 reanalysis of the saved vials containing the original extracts that resulted from the Miller and Urey experiments, using current and more advanced analytical equipment and technology, has uncovered more biochemicals than originally discovered in the 1950s. One of the more important findings was 23 amino acids, far more than the five originally found. However, Bernal said that "it is not enough to explain the formation of such molecules, what is necessary, is a physical-chemical explanation of the origins of these molecules that suggests the presence of suitable sources and sinks for free energy."
In trying to uncover the intermediate stages of abiogenesis mentioned by Bernal, Sidney W. Fox in the 1950s and 1960s studied the spontaneous formation of peptide structures (small chains of amino acids) under conditions that might plausibly have existed early in Earth's history. In one of his experiments, he allowed amino acids to dry out as if puddled in a warm, dry spot in prebiotic conditions. He found that, as they dried, the amino acids formed long, often cross-linked, thread-like, submicroscopic polypeptide molecules now named "proteinoid microspheres".
In another experiment to set suitable conditions for life to form, Fox collected volcanic material from a cinder cone in Hawaii. He discovered that the temperature was over 100 °C (212 °F) just 4 inches (100 mm) beneath the surface of the cinder cone, and suggested that this might have been the environment in which life was created—molecules could have formed and then been washed through the loose volcanic ash into the sea. He placed lumps of lava over amino acids derived from methane, ammonia and water, sterilized all materials, and baked the lava over the amino acids for a few hours in a glass oven. A brown, sticky substance formed over the surface, and when the lava was drenched in sterilized water, a thick, brown liquid leached out. The amino acids had combined to form proteinoids, and the proteinoids had combined to form small globules that Fox called "microspheres". His proteinoids were not cells, although they formed clumps and chains reminiscent of cyanobacteria, but they contained no functional nucleic acids or any encoded information. Based upon such experiments, Colin S. Pittendrigh stated in December 1967 that "laboratories will be creating a living cell within ten years," a remark that reflected the typical contemporary naivety about the complexity of cell structures.
There is no single, generally accepted model for the origin of life. Scientists have proposed several plausible hypotheses, which share some common elements. While differing in the details, these hypotheses are based on the framework laid out by Alexander Oparin (in 1924) and by J. B. S. Haldane (in 1925), who postulated the molecular or chemical evolution theory of life. According to them, the first molecules constituting the earliest cells "were synthesized under natural conditions by a slow process of molecular evolution, and these molecules then organized into the first molecular system with properties with biological order". Oparin and Haldane suggested that the atmosphere of the early Earth may have been chemically reducing in nature, composed primarily of methane (CH4), ammonia (NH3), water (H2O), hydrogen sulfide (H2S), carbon dioxide (CO2) or carbon monoxide (CO), and phosphate (PO43−), with molecular oxygen (O2) and ozone (O3) either rare or absent. According to later models, the atmosphere in the late Hadean period consisted largely of nitrogen (N2) and carbon dioxide, with smaller amounts of carbon monoxide, hydrogen (H2), and sulfur compounds; while it did lack molecular oxygen and ozone, it was not as chemically reducing as Oparin and Haldane supposed. In the atmosphere proposed by Oparin and Haldane, electrical activity can produce certain small molecules (monomers) of life, such as amino acids. The Miller–Urey experiment reported in 1953 demonstrated this.
- the origin of biological monomers
- the origin of biological polymers
- the evolution from molecules to cells
Bernal suggested that evolution commenced between stages 1 and 2. Bernal regarded the third stage – discovering methods by which biological reactions were incorporated behind a cell's boundary – as the most difficult. Modern work on the way that cell membranes self-assemble, and the work on micropores in various substrates may be a halfway house towards the development of independent free-living cells.
The chemical processes that took place on the early Earth are called chemical evolution. Since the end of the nineteenth century, 'evolutive abiogenesis' means increasing complexity and evolution of matter from inert to living state. Both Manfred Eigen and Sol Spiegelman demonstrated that evolution, including replication, variation, and natural selection, can occur in populations of molecules as well as in organisms. Spiegelman took advantage of natural selection to synthesize the Spiegelman Monster, which had a genome with just 218 nucleotide bases, having deconstructively evolved from a 4500-base bacterial RNA. Eigen built on Spiegelman's work and produced a similar system further degraded to just 48 or 54 nucleotides – the minimum required for the binding of the replication enzyme.
Following on from chemical evolution came the initiation of biological evolution, which led to the first cells. No one has yet synthesized a "protocell" using simple components with the necessary properties of life (the so-called "bottom-up-approach"). Without such a proof-of-principle, explanations have tended to focus on chemosynthesis. However, some researchers work in this field, notably Steen Rasmussen and Jack W. Szostak. Others have argued that a "top-down approach" is more feasible. One such approach, successfully attempted by Craig Venter and others at J. Craig Venter Institute, involves engineering existing prokaryotic cells with progressively fewer genes, attempting to discern at which point the most minimal requirements for life are reached.
The NASA strategy on abiogenesis states that it is necessary to identify interactions, intermediary structures and functions, energy sources, and environmental factors that contributed to the diversity, selection, and replication of evolvable macromolecular systems. Emphasis must continue to map the chemical landscape of potential primordial informational polymers. The advent of polymers that could replicate, store genetic information, and exhibit properties subject to selection likely was a critical step in the emergence of prebiotic chemical evolution.
Chemical origin of organic molecules
The elements, except for hydrogen and helium, ultimately derive from stellar nucleosynthesis. On 12 October 2016, astronomers reported that the very basic chemical ingredients of life — the carbon-hydrogen molecule (CH, or methylidyne radical), the carbon-hydrogen positive ion (CH+) and the carbon ion (C+) — are largely the result of ultraviolet light from stars, rather than other forms of radiation from supernovae and young stars, as thought earlier. Complex molecules, including organic molecules, form naturally both in space and on planets. There are two possible sources of organic molecules on the early Earth:
- Terrestrial origins – organic molecule synthesis driven by impact shocks or by other energy sources (such as UV light, redox coupling, or electrical discharges; e.g., Miller's experiments)
- Extraterrestrial origins – formation of organic molecules in interstellar dust clouds, which rain down on planets. (See pseudo-panspermia)
Based on recent computer model studies, the complex organic molecules necessary for life may have formed in the protoplanetary disk of dust grains surrounding the Sun before the formation of the Earth. According to the computer studies, this same process may also occur around other stars that acquire planets. (Also see Extraterrestrial organic molecules).
Estimates of the production of organics from these sources suggest that the Late Heavy Bombardment before 3.5 Ga within the early atmosphere made available quantities of organics comparable to those produced by terrestrial sources.
It has been estimated that the Late Heavy Bombardment may also have effectively sterilized the Earth's surface to a depth of tens of metres. If life evolved deeper than this, it would have also been shielded from the early high levels of ultraviolet radiation from the T Tauri stage of the Sun's evolution. Simulations of geothermically heated oceanic crust yield far more organics than those found in the Miller-Urey experiments (see below). In the deep hydrothermal vents, Everett Shock has found "there is an enormous thermodynamic drive to form organic compounds, as seawater and hydrothermal fluids, which are far from equilibrium, mix and move towards a more stable state." Shock has found that the available energy is maximized at around 100 – 150 degrees Celsius, precisely the temperatures at which the hyperthermophilic bacteria and thermoacidophilic archaea have been found, at the base of the phylogenetic tree of life closest to the Last Universal Common Ancestor (LUCA).
The accumulation and concentration of organic molecules on a planetary surface is also considered an essential early step for the origin of life. Identifying and understanding the mechanisms that led to the production of prebiotic molecules in various environments is critical for establishing the inventory of ingredients from which life originated on Earth, assuming that the abiotic production of molecules ultimately influenced the selection of molecules from which life emerged.
While features of self-organization and self-replication are often considered the hallmark of living systems, there are many instances of abiotic molecules exhibiting such characteristics under proper conditions. Stan Palasek suggested based on a theoretical model that self-assembly of ribonucleic acid (RNA) molecules can occur spontaneously due to physical factors in hydrothermal vents. Virus self-assembly within host cells has implications for the study of the origin of life, as it lends further credence to the hypothesis that life could have started as self-assembling organic molecules.
Multiple sources of energy were available for chemical reactions on the early Earth. For example, heat (such as from geothermal processes) is a standard energy source for chemistry. Other examples include sunlight and electrical discharges (lightning), among others. Computer simulations also suggest that cavitation in primordial water reservoirs such as breaking sea waves, streams and oceans can potentially lead to the synthesis of biogenic compounds. Unfavourable reactions can also be driven by highly favourable ones, as in the case of iron-sulfur chemistry. For example, this was probably important for carbon fixation (the conversion of carbon from its inorganic form to an organic one). Carbon fixation via iron-sulfur chemistry is highly favourable, and occurs at neutral pH and 100 °C (212 °F). Iron-sulfur surfaces, which are abundant near hydrothermal vents, are also capable of producing small amounts of amino acids and other biological metabolites.
As early as the 1860s, experiments have demonstrated that biologically relevant molecules can be produced from interaction of simple carbon sources with abundant inorganic catalysts. In particular, experiments by Butlerov (the formose reaction) showed that tetroses, pentoses, and hexoses are produced when formaldehyde is heated under basic conditions with divalent metal ions like calcium. The reaction was scrutinized and subsequently proposed to be autocatalytic by Breslow in 1959. Similar experiments (see below) demonstrate that nucleobases like guanine and adenine could be synthesized from simple carbon and nitrogen sources like hydrogen cyanide and ammonia.
Formamide produces all four ribonucleotides and other biological molecules when warmed in the presence of various terrestrial minerals. Formamide is ubiquitous in the Universe, produced by the reaction of water and hydrogen cyanide (HCN). It has several advantages as a biotic precursor, including the ability to easily become concentrated through the evaporation of water. Although HCN is poisonous, it only affects aerobic organisms (eukaryotes and aerobic bacteria), which did not yet exist. It can play roles in other chemical processes as well, such as the synthesis of the amino acid glycine.
In 1961, it was shown that the nucleic acid purine base adenine can be formed by heating aqueous ammonium cyanide solutions. Other pathways for synthesizing bases from inorganic materials were also reported. Leslie E. Orgel and colleagues have shown that freezing temperatures are advantageous for the synthesis of purines, due to the concentrating effect for key precursors such as hydrogen cyanide. Research by Stanley L. Miller and colleagues suggested that while adenine and guanine require freezing conditions for synthesis, cytosine and uracil may require boiling temperatures. Research by the Miller group notes the formation of seven different amino acids and 11 types of nucleobases in ice when ammonia and cyanide were left in a freezer from 1972 to 1997. Other work demonstrated the formation of s-triazines (alternative nucleobases), pyrimidines (including cytosine and uracil), and adenine from urea solutions subjected to freeze-thaw cycles under a reductive atmosphere (with spark discharges as an energy source). The explanation given for the unusual speed of these reactions at such a low temperature is eutectic freezing. As an ice crystal forms, it stays pure: only molecules of water join the growing crystal, while impurities like salt or cyanide are excluded. These impurities become crowded in microscopic pockets of liquid within the ice, and this crowding causes the molecules to collide more often. Mechanistic exploration using quantum chemical methods provide a more detailed understanding of some of the chemical processes involved in chemical evolution, and a partial answer to the fundamental question of molecular biogenesis.
At the time of the Miller–Urey experiment, scientific consensus was that the early Earth had a reducing atmosphere with compounds relatively rich in hydrogen and poor in oxygen (e.g., CH4 and NH3 as opposed to CO2 and nitrogen dioxide (NO2)). However, current scientific consensus describes the primitive atmosphere as either weakly reducing or neutral (see also Oxygen Catastrophe). Such an atmosphere would diminish both the amount and variety of amino acids that could be produced, although studies that include iron and carbonate minerals (thought present in early oceans) in the experimental conditions have again produced a diverse array of amino acids. Other scientific research has focused on two other potential reducing environments: outer space and deep-sea thermal vents.
The spontaneous formation of complex polymers from abiotically generated monomers under the conditions posited by the "soup" theory is not at all a straightforward process. Besides the necessary basic organic monomers, compounds that would have prohibited the formation of polymers were also formed in high concentration during the Miller–Urey and Joan Oró experiments. The Miller–Urey experiment, for example, produces many substances that would react with the amino acids or terminate their coupling into peptide chains.
A research project completed in March 2015 by John D. Sutherland and others found that a network of reactions beginning with hydrogen cyanide and hydrogen sulfide, in streams of water irradiated by UV light, could produce the chemical components of proteins and lipids, as well as those of RNA, while not producing a wide range of other compounds. The researchers used the term "cyanosulfidic" to describe this network of reactions.
Autocatalysts are substances that catalyze the production of themselves and therefore are "molecular replicators." The simplest self-replicating chemical systems are autocatalytic, and typically contain three components: a product molecule and two precursor molecules. The product molecule joins together the precursor molecules, which in turn produce more product molecules from more precursor molecules. The product molecule catalyzes the reaction by providing a complementary template that binds to the precursors, thus bringing them together. Such systems have been demonstrated both in biological macromolecules and in small organic molecules. Systems that do not proceed by template mechanisms, such as the self-reproduction of micelles and vesicles, have also been observed.
It has been proposed that life initially arose as autocatalytic chemical networks. British ethologist Richard Dawkins wrote about autocatalysis as a potential explanation for the origin of life in his 2004 book The Ancestor's Tale. In his book, Dawkins cites experiments performed by Julius Rebek Jr. and his colleagues in which they combined amino adenosine and pentafluorophenyl esters with the autocatalyst amino adenosine triacid ester (AATE). One product was a variant of AATE, which catalyzed the synthesis of themselves. This experiment demonstrated the possibility that autocatalysts could exhibit competition within a population of entities with heredity, which could be interpreted as a rudimentary form of natural selection.
In the early 1970s, Manfred Eigen and Peter Schuster examined the transient stages between the molecular chaos and a self-replicating hypercycle in a prebiotic soup. In a hypercycle, the information storing system (possibly RNA) produces an enzyme, which catalyzes the formation of another information system, in sequence until the product of the last aids in the formation of the first information system. Mathematically treated, hypercycles could create quasispecies, which through natural selection entered into a form of Darwinian evolution. A boost to hypercycle theory was the discovery of ribozymes capable of catalyzing their own chemical reactions. The hypercycle theory requires the existence of complex biochemicals, such as nucleotides, which do not form under the conditions proposed by the Miller–Urey experiment.
Geoffrey W. Hoffmann has shown that an early error-prone translation machinery can be stable against an error catastrophe of the type that had been envisaged as problematical for the origin of life, and was known as "Orgel's paradox".
Hoffmann has furthermore argued that a complex nucleation event as the origin of life involving both polypeptides and nucleic acid is compatible with the time and space available in the primitive oceans of Earth Hoffmann suggests that volcanic ash may provide the many random shapes needed in the postulated complex nucleation event. This aspect of the theory can be tested experimentally.
Homochirality refers to a geometric uniformity of some materials composed of chiral units. Chiral refers to nonsuperimposable 3D forms that are mirror images of one another, as are left and right hands. Living organisms use molecules that have the same chirality ("handedness"): with almost no exceptions, amino acids are left-handed while nucleotides and sugars are right-handed. Chiral molecules can be synthesized, but in the absence of a chiral source or a chiral catalyst, they are formed in a 50/50 mixture of both enantiomers (called a racemic mixture). Known mechanisms for the production of non-racemic mixtures from racemic starting materials include: asymmetric physical laws, such as the electroweak interaction; asymmetric environments, such as those caused by circularly polarized light, quartz crystals, or the Earth's rotation, statistical fluctuations during racemic synthesis, and spontaneous symmetry breaking.
Once established, chirality would be selected for. A small bias (enantiomeric excess) in the population can be amplified into a large one by asymmetric autocatalysis, such as in the Soai reaction. In asymmetric autocatalysis, the catalyst is a chiral molecule, which means that a chiral molecule is catalyzing its own production. An initial enantiomeric excess, such as can be produced by polarized light, then allows the more abundant enantiomer to outcompete the other.
Clark has suggested that homochirality may have started in outer space, as the studies of the amino acids on the Murchison meteorite showed that L-alanine is more than twice as frequent as its D form, and L-glutamic acid was more than three times prevalent than its D counterpart. Various chiral crystal surfaces can also act as sites for possible concentration and assembly of chiral monomer units into macromolecules. Compounds found on meteorites suggest that the chirality of life derives from abiogenic synthesis, since amino acids from meteorites show a left-handed bias, whereas sugars show a predominantly right-handed bias, the same as found in living organisms.
Self-enclosement, reproduction, duplication and the RNA world
A protocell is a self-organized, self-ordered, spherical collection of lipids proposed as a stepping-stone to the origin of life. A central question in evolution is how simple protocells first arose and differed in reproductive contribution to the following generation driving the evolution of life. Although a functional protocell has not yet been achieved in a laboratory setting, there are scientists who think the goal is well within reach.
Self-assembled vesicles are essential components of primitive cells. The second law of thermodynamics requires that the universe move in a direction in which entropy increases, yet life is distinguished by its great degree of organization. Therefore, a boundary is needed to separate life processes from non-living matter. Researchers Irene A. Chen and Jack W. Szostak amongst others, suggest that simple physicochemical properties of elementary protocells can give rise to essential cellular behaviours, including primitive forms of differential reproduction competition and energy storage. Such cooperative interactions between the membrane and its encapsulated contents could greatly simplify the transition from simple replicating molecules to true cells. Furthermore, competition for membrane molecules would favour stabilized membranes, suggesting a selective advantage for the evolution of cross-linked fatty acids and even the phospholipids of today. Such micro-encapsulation would allow for metabolism within the membrane, the exchange of small molecules but the prevention of passage of large substances across it. The main advantages of encapsulation include the increased solubility of the contained cargo within the capsule and the storage of energy in the form of a electrochemical gradient.
A 2012 study led by Armen Y. Mulkidjanian of Germany's University of Osnabrück, suggests that inland pools of condensed and cooled geothermal vapour have the ideal characteristics for the origin of life. Scientists confirmed in 2002 that by adding a montmorillonite clay to a solution of fatty acid micelles (lipid spheres), the clay sped up the rate of vesicles formation 100-fold.
Another protocell model is the Jeewanu. First synthesized in 1963 from simple minerals and basic organics while exposed to sunlight, it is still reported to have some metabolic capabilities, the presence of semipermeable membrane, amino acids, phospholipids, carbohydrates and RNA-like molecules. However, the nature and properties of the Jeewanu remains to be clarified.
The RNA world hypothesis describes an early Earth with self-replicating and catalytic RNA but no DNA or proteins. It is generally accepted that current life on Earth descends from an RNA world, although RNA-based life may not have been the first life to exist. This conclusion is drawn from many independent lines of evidence, such as the observations that RNA is central to the translation process and that small RNAs can catalyze all of the chemical groups and information transfers required for life. The structure of the ribosome has been called the "smoking gun," as it showed that the ribosome is a ribozyme, with a central core of RNA and no amino acid side chains within 18 angstroms of the active site where peptide bond formation is catalyzed. The concept of the RNA world was first proposed in 1962 by Alexander Rich, and the term was coined by Walter Gilbert in 1986.
Possible precursors for the evolution of protein synthesis include a mechanism to synthesize short peptide cofactors or form a mechanism for the duplication of RNA. It is likely that the ancestral ribosome was composed entirely of RNA, although some roles have since been taken over by proteins. Major remaining questions on this topic include identifying the selective force for the evolution of the ribosome and determining how the genetic code arose.
Eugene Koonin said, "Despite considerable experimental and theoretical effort, no compelling scenarios currently exist for the origin of replication and translation, the key processes that together comprise the core of biological systems and the apparent pre-requisite of biological evolution. The RNA World concept might offer the best chance for the resolution of this conundrum but so far cannot adequately account for the emergence of an efficient RNA replicase or the translation system. The MWO ["many worlds in one"] version of the cosmological model of eternal inflation could suggest a way out of this conundrum because, in an infinite multiverse with a finite number of distinct macroscopic histories (each repeated an infinite number of times), emergence of even highly complex systems by chance is not just possible but inevitable."
Recent evidence for a "virus first" hypothesis, which may support theories of the RNA world, has been suggested. One of the difficulties for the study of the origins of viruses is their high rate of mutation; this is particularly the case in RNA retroviruses like HIV. A 2015 study compared protein fold structures across different branches of the tree of life, where researchers can reconstruct the evolutionary histories of the folds and of the organisms whose genomes code for those folds. They argue that protein folds are better markers of ancient events as their three-dimensional structures can be maintained even as the sequences that code for those begin to change. Thus, the viral protein repertoire retain traces of ancient evolutionary history that can be recovered using advanced bioinformatics approaches. Those researchers think that "the prolonged pressure of genome and particle size reduction eventually reduced virocells into modern viruses (identified by the complete loss of cellular makeup), meanwhile other coexisting cellular lineages diversified into modern cells. The data suggest that viruses originated from ancient cells that co-existed with the ancestors of modern cells. These ancient cells likely contained segmented RNA genomes.
RNA synthesis and replication
A number of hypotheses of formation of RNA have been put forward. As of 1994, there were difficulties in the explanation of the abiotic synthesis of the nucleotides cytosine and uracil. Subsequent research has shown possible routes of synthesis; for example, formamide produces all four ribonucleotides and other biological molecules when warmed in the presence of various terrestrial minerals. Early cell membranes could have formed spontaneously from proteinoids, which are protein-like molecules produced when amino acid solutions are heated while in the correct concentration of aqueous solution. These are seen to form micro-spheres which are observed to behave similarly to membrane-enclosed compartments. Other possible means of producing more complicated organic molecules include chemical reactions that take place on clay substrates or on the surface of the mineral pyrite.
Factors supporting an important role for RNA in early life include its ability to act both to store information and to catalyze chemical reactions (as a ribozyme); its many important roles as an intermediate in the expression of and maintenance of the genetic information (in the form of DNA) in modern organisms; and the ease of chemical synthesis of at least the components of the RNA molecule under the conditions that approximated the early Earth. Relatively short RNA molecules have been synthesized, capable of replication. Such replicase RNA, which functions as both code and catalyst provides its own template upon which copying can occur. Jack W. Szostak has shown that certain catalytic RNAs can join smaller RNA sequences together, creating the potential for self-replication. If these conditions were present, Darwinian natural selection would favour the proliferation of such autocatalytic sets, to which further functionalities could be added. Such autocatalytic systems of RNA capable of self-sustained replication have been identified. The RNA replication systems, which include two ribozymes that catalyze each other's synthesis, showed a doubling time of the product of about one hour, and were subject to natural selection under the conditions that existed in the experiment. In evolutionary competition experiments, this led to the emergence of new systems which replicated more efficiently. This was the first demonstration of evolutionary adaptation occurring in a molecular genetic system.
Depending on the definition, life started when RNA chains began to self-replicate, initiating the three mechanisms of Darwinian selection: heritability, variation of type, and differential reproductive output. The fitness of an RNA replicator (its per capita rate of increase) would likely be a function of its intrinsic adaptive capacities, determined by its nucleotide sequence, and the availability of resources. The three primary adaptive capacities may have been: (1) replication with moderate fidelity, giving rise to both heritability while allowing variation of type, (2) resistance to decay, and (3) acquisition of process resources. These capacities would have functioned by means of the folded configurations of the RNA replicators resulting from their nucleotide sequences.
Carl Zimmer has speculated that the chemical conditions, including the presence of boron, molybdenum and oxygen needed for the initial production of RNA, may have been better on early Mars than on early Earth. If so, life-suitable molecules originating on Mars may have later migrated to Earth via meteor ejections.
It is possible that a different type of nucleic acid, such as PNA, TNA or GNA, was the first to emerge as a self-reproducing molecule, only later replaced by RNA. Larralde et al., say that "the generally accepted prebiotic synthesis of ribose, the formose reaction, yields numerous sugars without any selectivity." and they conclude that their "results suggest that the backbone of the first genetic material could not have contained ribose or other sugars because of their instability." The ester linkage of ribose and phosphoric acid in RNA is known to be prone to hydrolysis.
Pyrimidine ribonucleosides and their respective nucleotides have been prebiotically synthesized by a sequence of reactions which by-pass the free sugars, and are assembled in a stepwise fashion by using nitrogenous or oxygenous chemistries. Sutherland has demonstrated high yielding routes to cytidine and uridine ribonucleotides built from small 2 and 3 carbon fragments such as glycolaldehyde, glyceraldehyde or glyceraldehyde-3-phosphate, cyanamide and cyanoacetylene. One of the steps in this sequence allows the isolation of enantiopure ribose aminooxazoline if the enantiomeric excess of glyceraldehyde is 60% or greater. This can be viewed as a prebiotic purification step, where the said compound spontaneously crystallized out from a mixture of the other pentose aminooxazolines. Ribose aminooxazoline can then react with cyanoacetylene in a mild and highly efficient manner to give the alpha cytidine ribonucleotide. Photoanomerization with UV light allows for inversion about the 1' anomeric centre to give the correct beta stereochemistry. In 2009 they showed that the same simple building blocks allow access, via phosphate controlled nucleobase elaboration, to 2',3'-cyclic pyrimidine nucleotides directly, which are known to be able to polymerize into RNA. This paper also highlights the possibility for the photo-sanitization of the pyrimidine-2',3'-cyclic phosphates.
Origin of biological metabolism
Metabolism-like reactions could have occurred naturally in early oceans, before the first organisms evolved. Metabolism may predate the origin of life, which may have evolved from the chemical conditions in the earliest oceans. Reconstructions in laboratories show that some of these reactions can produce RNA, and some others resemble two essential reaction cascades of metabolism: glycolysis and the pentose phosphate pathway, that provide essential precursors for nucleic acids, amino acids and lipids. A study at the University of Düsseldorf created phylogenic trees based upon 6 million genes from bacteria and archaea, and identified 355 protein families that were probably present in the LUCA. They were based upon an anaerobic metabolism fixing carbon dioxide and nitrogen. It suggests that the LUCA evolved in an environment rich in hydrogen, carbon dioxide and iron. Following are some observed discoveries and related hypotheses.
In the 1980s, Günter Wächtershäuser, encouraged and supported by Karl R. Popper, postulated his iron–sulfur world, a theory of the evolution of pre-biotic chemical pathways as the starting point in the evolution of life. It systematically traces today's biochemistry to primordial reactions which provide alternative pathways to the synthesis of organic building blocks from simple gaseous compounds.
In contrast to the classical Miller experiments, which depend on external sources of energy (simulated lightning, ultraviolet irradiation), "Wächtershäuser systems" come with a built-in source of energy: sulfides of iron (iron pyrite) and other minerals. The energy released from redox reactions of these metal sulfides is available for the synthesis of organic molecules, and such systems may have evolved into autocatalytic sets constituting self-replicating, metabolically active entities predating the life forms known today. Experiments with such sulfides in an aqueous environment at 100 °C produced a relatively small yield of dipeptides (0.4% to 12.4%) and a smaller yield of tripeptides (0.10%) although under the same conditions, dipeptides were quickly broken down.
Several models reject the self-replication of a "naked-gene", postulating instead the emergence of a primitive metabolism providing a safe environment for the later emergence of RNA replication. The centrality of the Krebs cycle (citric acid cycle) to energy production in aerobic organisms, and in drawing in carbon dioxide and hydrogen ions in biosynthesis of complex organic chemicals, suggests that it was one of the first parts of the metabolism to evolve. Concordantly, geochemist Michael Russell has proposed that "the purpose of life is to hydrogenate carbon dioxide" (as part of a "metabolism-first," rather than a "genetics-first," scenario). Physicist Jeremy England of MIT has proposed that life was inevitable from general thermodynamic considerations: "... when a group of atoms is driven by an external source of energy (like the sun or chemical fuel) and surrounded by a heat bath (like the ocean or atmosphere), it will often gradually restructure itself in order to dissipate increasingly more energy. This could mean that under certain conditions, matter inexorably acquires the key physical attribute associated with life."
One of the earliest incarnations of this idea was put forward in 1924 with Oparin's notion of primitive self-replicating vesicles which predated the discovery of the structure of DNA. Variants in the 1980s and 1990s include Wächtershäuser's iron–sulfur world theory and models introduced by Christian de Duve based on the chemistry of thioesters. More abstract and theoretical arguments for the plausibility of the emergence of metabolism without the presence of genes include a mathematical model introduced by Freeman Dyson in the early 1980s and Stuart Kauffman's notion of collectively autocatalytic sets, discussed later that decade.
Orgel summarized his analysis by stating, "There is at present no reason to expect that multistep cycles such as the reductive citric acid cycle will self-organize on the surface of FeS/FeS2 or some other mineral." It is possible that another type of metabolic pathway was used at the beginning of life. For example, instead of the reductive citric acid cycle, the "open" acetyl-CoA pathway (another one of the five recognized ways of carbon dioxide fixation in nature today) would be compatible with the idea of self-organization on a metal sulfide surface. The key enzyme of this pathway, carbon monoxide dehydrogenase/acetyl-CoA synthase, harbours mixed nickel-iron-sulfur clusters in its reaction centres and catalyzes the formation of acetyl-CoA (similar to acetyl-thiol) in a single step. There are increasing concerns, however, that prebiotic thiolated and thioester compounds are thermodynamically and kinetically unfavourable to accumulate in presumed prebiotic conditions (i.e. hydrothermal vents). It has also been proposed that cysteine and homocysteine may have reacted with nitriles resulting from the Stecker reaction, readily forming catalytic thiol-reach poplypeptides.
The Zn-world (zinc world) theory of Armen Y. Mulkidjanian is an extension of Wächtershäuser's pyrite hypothesis. Wächtershäuser based his theory of the initial chemical processes leading to informational molecules (RNA, peptides) on a regular mesh of electric charges at the surface of pyrite that may have facilitated the primeval polymerization by attracting reactants and arranging them appropriately relative to each other. The Zn-world theory specifies and differentiates further. Hydrothermal fluids rich in H2S interacting with cold primordial ocean (or Darwin's "warm little pond") water leads to the precipitation of metal sulfide particles. Oceanic vent systems and other hydrothermal systems have a zonal structure reflected in ancient volcanogenic massive sulfide deposits (VMS) of hydrothermal origin. They reach many kilometres in diameter and date back to the Archean Eon. Most abundant are pyrite (FeS2), chalcopyrite (CuFeS2), and sphalerite (ZnS), with additions of galena (PbS) and alabandite (MnS). ZnS and MnS have a unique ability to store radiation energy, e.g. from UV light. During the relevant time window of the origins of replicating molecules, the primordial atmospheric pressure was high enough (>100 bar, about 100 atmospheres) to precipitate near the Earth's surface, and UV irradiation was 10 to 100 times more intense than now; hence the unique photosynthetic properties mediated by ZnS provided just the right energy conditions to energize the synthesis of informational and metabolic molecules and the selection of photostable nucleobases.
The Zn-world theory has been further filled out with experimental and theoretical evidence for the ionic constitution of the interior of the first proto-cells before archaea, bacteria and proto-eukaryotes evolved. Archibald Macallum noted the resemblance of body fluids such as blood and lymph to seawater; however, the inorganic composition of all cells differ from that of modern seawater, which led Mulkidjanian and colleagues to reconstruct the "hatcheries" of the first cells combining geochemical analysis with phylogenomic scrutiny of the inorganic ion requirements of universal components of modern cells. The authors conclude that ubiquitous, and by inference primordial, proteins and functional systems show affinity to and functional requirement for K+, Zn2+, Mn2+, and phosphate. Geochemical reconstruction shows that the ionic composition conducive to the origin of cells could not have existed in what we today call marine settings but is compatible with emissions of vapour-dominated zones of what we today call inland geothermal systems. Under the oxygen depleted, CO2-dominated primordial atmosphere, the chemistry of water condensates and exhalations near geothermal fields would resemble the internal milieu of modern cells. Therefore, the precellular stages of evolution may have taken place in shallow "Darwin ponds" lined with porous silicate minerals mixed with metal sulfides and enriched in K+, Zn2+, and phosphorus compounds.
Deep sea vent hypothesis
The deep sea vent, or alkaline hydrothermal vent, theory posits that life may have begun at submarine hydrothermal vents, William Martin and Michael Russell have suggested "that life evolved in structured iron monosulphide precipitates in a seepage site hydrothermal mound at a redox, pH, and temperature gradient between sulphide-rich hydrothermal fluid and iron(II)-containing waters of the Hadean ocean floor. The naturally arising, three-dimensional compartmentation observed within fossilized seepage-site metal sulphide precipitates indicates that these inorganic compartments were the precursors of cell walls and membranes found in free-living prokaryotes. The known capability of FeS and NiS to catalyze the synthesis of the acetyl-methylsulphide from carbon monoxide and methylsulphide, constituents of hydrothermal fluid, indicates that pre-biotic syntheses occurred at the inner surfaces of these metal-sulphide-walled compartments,..." These form where hydrogen-rich fluids emerge from below the sea floor, as a result of serpentinization of ultra-mafic olivine with seawater and a pH interface with carbon dioxide-rich ocean water. The vents form a sustained chemical energy source derived from redox reactions, in which electron donors (molecular hydrogen) react with electron acceptors (carbon dioxide); see Iron–sulfur world theory. These are highly exothermic reactions.
Michael Russell demonstrated that alkaline vents created an abiogenic proton motive force (PMF) chemiosmotic gradient, in which conditions are ideal for an abiogenic hatchery for life. Their microscopic compartments "provide a natural means of concentrating organic molecules," composed of iron-sulfur minerals such as mackinawite, endowed these mineral cells with the catalytic properties envisaged by Wächtershäuser. This movement of ions across the membrane depends on a combination of two factors:
- Diffusion force caused by concentration gradient—all particles including ions tend to diffuse from higher concentration to lower.
- Electrostatic force caused by electrical potential gradient—cations like protons H+ tend to diffuse down the electrical potential, anions in the opposite direction.
These two gradients taken together can be expressed as an electrochemical gradient, providing energy for abiogenic synthesis. The proton motive force can be described as the measure of the potential energy stored as a combination of proton and voltage gradients across a membrane (differences in proton concentration and electrical potential).
Jack W. Szostak suggested that geothermal activity provides greater opportunities for the origination of life in open lakes where there is a buildup of minerals. In 2010, based on spectral analysis of sea and hot mineral water, Ignat Ignatov and Oleg Mosin demonstrated that life may have predominantly originated in hot mineral water. The hot mineral water that contains bicarbonate and calcium ions has the most optimal range. This case is similar to the origin of life in hydrothermal vents, but with bicarbonate and calcium ions in hot water. This water has a pH of 9–11 and is possible to have the reactions in seawater. According to Melvin Calvin, certain reactions of condensation-dehydration of amino acids and nucleotides in individual blocks of peptides and nucleic acids can take place in the primary hydrosphere with pH 9-11 at a later evolutionary stage. Some of these compounds like hydrocyanic acid (HCN) have been proven in the experiments of Miller. This is the environment in which the stromatolites have been created. David Ward of Montana State University described the formation of stromatolites in hot mineral water at the Yellowstone National Park. Stromatolites survive in hot mineral water and in proximity to areas with volcanic activity. Processes have evolved in the sea near geysers of hot mineral water. In 2011, Tadashi Sugawara from the University of Tokyo created a protocell in hot water.
Experimental research and computer modelling suggest that the surfaces of mineral particles inside hydrothermal vents have catalytic properties similar to those of enzymes and are able to create simple organic molecules, such as methanol (CH3OH) and formic, acetic and pyruvic acid out of the dissolved CO2 in the water.
The research reported above by William F. Martin in July 2016 supports the thesis that life arose at hydrothermal vents, that spontaneous chemistry in the Earth’s crust driven by rock–water interactions at disequilibrium thermodynamically underpinned life’s origin and that the founding lineages of the archaea and bacteria were H2-dependent autotrophs that used CO2 as their terminal acceptor in energy metabolism. Martin suggests, based upon this evidence that LUCA "may have depended heavily on the geothermal energy of the vent to survive".
Today's bioenergetic process of fermentation is carried out by either the aforementioned citric acid cycle or the Acetyl-CoA pathway, both of which have been connected to the primordial Iron–sulfur world. In a different approach, the thermosynthesis hypothesis considers the bioenergetic process of chemiosmosis, which plays an essential role in cellular respiration and photosynthesis, more basal than fermentation: the ATP synthase enzyme, which sustains chemiosmosis, is proposed as the currently extant enzyme most closely related to the first metabolic process.
First, life needed an energy source to bring about the condensation reaction that yielded the peptide bonds of proteins and the phosphodiester bonds of RNA. In a generalization and thermal variation of the binding change mechanism of today's ATP synthase, the "first protein" would have bound substrates (peptides, phosphate, nucleosides, RNA 'monomers') and condensed them to a reaction product that remained bound until after a temperature change it was released by thermal unfolding.
The energy source under the thermosynthesis hypothesis was thermal cycling, the result of suspension of protocells in a convection current, as is plausible in a volcanic hot spring; the convection accounts for the self-organization and dissipative structure required in any origin of life model. The still ubiquitous role of thermal cycling in germination and cell division is considered a relic of primordial thermosynthesis.
By phosphorylating cell membrane lipids, this "first protein" gave a selective advantage to the lipid protocell that contained the protein. This protein also synthesized a library of many proteins, of which only a minute fraction had thermosynthesis capabilities. As proposed by Dyson, it propagated functionally: it made daughters with similar capabilities, but it did not copy itself. Functioning daughters consisted of different amino acid sequences.
Whereas the Iron–sulfur world identifies a circular pathway as the most simple, the thermosynthesis hypothesis does not even invoke a pathway: ATP synthase's binding change mechanism resembles a physical adsorption process that yields free energy, rather than a regular enzyme's mechanism, which decreases the free energy. It has been claimed that the emergence of cyclic systems of protein catalysts is implausible.
Montmorillonite, an abundant clay, is a catalyst for the polymerization of RNA and for the formation of membranes from lipids. A model for the origin of life using clay was forwarded by Alexander Graham Cairns-Smith in 1985 and explored as a plausible mechanism by several scientists. The clay hypothesis postulates that complex organic molecules arose gradually on pre-existing, non-organic replication surfaces of silicate crystals in solution.
At the Rensselaer Polytechnic Institute, James P. Ferris' studies have also confirmed that clay minerals of montmorillonite catalyze the formation of RNA in aqueous solution, by joining nucleotides to form longer chains.
In 2007, Bart Kahr from the University of Washington and colleagues reported their experiments that tested the idea that crystals can act as a source of transferable information, using crystals of potassium hydrogen phthalate. "Mother" crystals with imperfections were cleaved and used as seeds to grow "daughter" crystals from solution. They then examined the distribution of imperfections in the new crystals and found that the imperfections in the mother crystals were reproduced in the daughters, but the daughter crystals also had many additional imperfections. For gene-like behaviour to be observed, the quantity of inheritance of these imperfections should have exceeded that of the mutations in the successive generations, but it did not. Thus Kahr concluded that the crystals "were not faithful enough to store and transfer information from one generation to the next."
Gold's "deep-hot biosphere" model
In the 1970s, Thomas Gold proposed the theory that life first developed not on the surface of the Earth, but several kilometres below the surface. It is claimed that discovery of microbial life below the surface of another body in our Solar System would lend significant credence to this theory. Thomas Gold also asserted that a trickle of food from a deep, unreachable, source is needed for survival because life arising in a puddle of organic material is likely to consume all of its food and become extinct. Gold's theory is that the flow of such food is due to out-gassing of primordial methane from the Earth's mantle; more conventional explanations of the food supply of deep microbes (away from sedimentary carbon compounds) is that the organisms subsist on hydrogen released by an interaction between water and (reduced) iron compounds in rocks.
Panspermia is the hypothesis that life exists throughout the universe, distributed by meteoroids, asteroids, comets, planetoids, and, also, by spacecraft in the form of unintended contamination by microorganisms.
The panspermia hypothesis does not attempt to explain how life first originated, but merely shifts it to another planet or a comet. The advantage of an extraterrestrial origin of primitive life is that life is not required to have formed on each planet it occurs on, but rather in a single location, and then spread about the galaxy to other star systems via cometary and/or meteorite impact. Evidence to support the hypothesis is scant, but it finds support in studies of Martian meteorites found in Antarctica and in studies of extremophile microbes' survival in outer space tests. (See also: List of microorganisms tested in outer space.)
Extraterrestrial organic molecules
An organic compound is any member of a large class of gaseous, liquid, or solid chemicals whose molecules contain carbon. Carbon is the fourth most abundant element in the Universe by mass after hydrogen, helium, and oxygen. Carbon is abundant in the Sun, stars, comets, and in the atmospheres of most planets. Organic compounds are relatively common in space, formed by "factories of complex molecular synthesis" which occur in molecular clouds and circumstellar envelopes, and chemically evolve after reactions are initiated mostly by ionizing radiation. Based on computer model studies, the complex organic molecules necessary for life may have formed on dust grains in the protoplanetary disk surrounding the Sun before the formation of the Earth. According to the computer studies, this same process may also occur around other stars that acquire planets.
Observations suggest that the majority of organic compounds introduced on Earth by interstellar dust particles are considered principal agents in the formation of complex molecules, thanks to their peculiar surface-catalytic activities. Studies reported in 2008, based on 12C/13C isotopic ratios of organic compounds found in the Murchison meteorite, suggested that the RNA component uracil and related molecules, including xanthine, were formed extraterrestrially. On 8 August 2011, a report based on NASA studies of meteorites found on Earth was published suggesting DNA components (adenine, guanine and related organic molecules) were made in outer space. Scientists also found that the cosmic dust permeating the universe contains complex organics ("amorphous organic solids with a mixed aromatic–aliphatic structure") that could be created naturally, and rapidly, by stars. Sun Kwok of The University of Hong Kong suggested that these compounds may have been related to the development of life on Earth said that "If this is the case, life on Earth may have had an easier time getting started as these organics can serve as basic ingredients for life."
Glycolaldehyde, the first example of an interstellar sugar molecule, was detected in the star-forming region near the centre of our galaxy. It was discovered in 2000 by Jes Jørgensen and Jan M. Hollis. In 2012, Jørgensen's team reported the detection of glycolaldehyde in a distant star system. The molecule was found around the protostellar binary IRAS 16293-2422 400 light years from Earth. Glycolaldehyde is needed to form RNA, which is similar in function to DNA. These findings suggest that complex organic molecules may form in stellar systems prior to the formation of planets, eventually arriving on young planets early in their formation. Because sugars are associated with both metabolism and the genetic code, two of the most basic aspects of life, it is thought the discovery of extraterrestrial sugar increases the likelihood that life may exist elsewhere in our galaxy.
NASA announced in 2009 that scientists had identified another fundamental chemical building block of life in a comet for the first time, glycine, an amino acid, which was detected in material ejected from comet Wild 2 in 2004 and grabbed by NASA's Stardust probe. Glycine has been detected in meteorites before. Carl Pilcher, who leads the NASA Astrobiology Institute commented that "The discovery of glycine in a comet supports the idea that the fundamental building blocks of life are prevalent in space, and strengthens the argument that life in the universe may be common rather than rare." Comets are encrusted with outer layers of dark material, thought to be a tar-like substance composed of complex organic material formed from simple carbon compounds after reactions initiated mostly by ionizing radiation. It is possible that a rain of material from comets could have brought significant quantities of such complex organic molecules to Earth. Amino acids which were formed extraterrestrially may also have arrived on Earth via comets. It is estimated that during the Late Heavy Bombardment, meteorites may have delivered up to five million tons of organic prebiotic elements to Earth per year.
Polycyclic aromatic hydrocarbons (PAH) are the most common and abundant of the known polyatomic molecules in the observable universe, and are considered a likely constituent of the primordial sea. In 2010, PAHs, along with fullerenes (or "buckyballs"), have been detected in nebulae. In March 2015, NASA scientists reported that, for the first time, complex DNA and RNA organic compounds of life, including uracil, cytosine and thymine, have been formed in the laboratory under outer space conditions, using starting chemicals, such as pyrimidine, found in meteorites. Pyrimidine, like PAHs, the most carbon-rich chemical found in the Universe, may have been formed in red giant stars or in interstellar dust and gas clouds. A group of Czech scientists reported that all four RNA-bases may be synthesized from formamide in the course of high-energy density events like extraterrestrial impacts.
The lipid world theory postulates that the first self-replicating object was lipid-like. It is known that phospholipids form lipid bilayers in water while under agitation—the same structure as in cell membranes. These molecules were not present on early Earth, but other amphiphilic long-chain molecules also form membranes. Furthermore, these bodies may expand (by insertion of additional lipids), and under excessive expansion may undergo spontaneous splitting which preserves the same size and composition of lipids in the two progenies. The main idea in this theory is that the molecular composition of the lipid bodies is the preliminary way for information storage, and evolution led to the appearance of polymer entities such as RNA or DNA that may store information favourably. Studies on vesicles from potentially prebiotic amphiphiles have so far been limited to systems containing one or two types of amphiphiles. This in contrast to the output of simulated prebiotic chemical reactions, which typically produce very heterogeneous mixtures of compounds. Within the hypothesis of a lipid bilayer membrane composed of a mixture of various distinct amphiphilic compounds there is the opportunity of a huge number of theoretically possible combinations in the arrangements of these amphiphiles in the membrane. Among all these potential combinations, a specific local arrangement of the membrane would have favoured the constitution of a hypercycle, actually a positive feedback composed of two mutual catalysts represented by a membrane site and a specific compound trapped in the vesicle. Such site/compound pairs are transmissible to the daughter vesicles leading to the emergence of distinct lineages of vesicles which would have allowed Darwinian natural selection.
A problem in most scenarios of abiogenesis is that the thermodynamic equilibrium of amino acid versus peptides is in the direction of separate amino acids. What has been missing is some force that drives polymerization. The resolution of this problem may well be in the properties of polyphosphates. Polyphosphates are formed by polymerization of ordinary monophosphate ions PO4−3. Several mechanisms of organic molecule synthesis have been investigated. Polyphosphates cause polymerization of amino acids into peptides. They are also logical precursors in the synthesis of such key biochemical compounds as adenosine triphosphate (ATP). A key issue seems to be that calcium reacts with soluble phosphate to form insoluble calcium phosphate (apatite), so some plausible mechanism must be found to keep calcium ions from causing precipitation of phosphate. There has been much work on this topic over the years, but an interesting new idea is that meteorites may have introduced reactive phosphorus species on the early Earth.
PAH world hypothesis
Polycyclic aromatic hydrocarbons (PAH) are known to be abundant in the universe, including in the interstellar medium, in comets, and in meteorites, and are some of the most complex molecules so far found in space.
Other sources of complex molecules have been postulated, including extraterrestrial stellar or interstellar origin. For example, from spectral analyses, organic molecules are known to be present in comets and meteorites. In 2004, a team detected traces of PAHs in a nebula. In 2010, another team also detected PAHs, along with fullerenes, in nebulae. The use of PAHs has also been proposed as a precursor to the RNA world in the PAH world hypothesis. The Spitzer Space Telescope has detected a star, HH 46-IR, which is forming by a process similar to that by which the Sun formed. In the disk of material surrounding the star, there is a very large range of molecules, including cyanide compounds, hydrocarbons, and carbon monoxide. In September 2012, NASA scientists reported that PAHs, subjected to interstellar medium conditions, are transformed, through hydrogenation, oxygenation and hydroxylation, to more complex organics—"a step along the path toward amino acids and nucleotides, the raw materials of proteins and DNA, respectively." Further, as a result of these transformations, the PAHs lose their spectroscopic signature which could be one of the reasons "for the lack of PAH detection in interstellar ice grains, particularly the outer regions of cold, dense clouds or the upper molecular layers of protoplanetary disks."
NASA maintains a database for tracking PAHs in the universe. More than 20% of the carbon in the universe may be associated with PAHs, possible starting materials for the formation of life. PAHs seem to have been formed shortly after the Big Bang, are widespread throughout the universe, and are associated with new stars and exoplanets.
Radioactive beach hypothesis
Zachary Adam claims that tidal processes that occurred during a time when the Moon was much closer may have concentrated grains of uranium and other radioactive elements at the high-water mark on primordial beaches, where they may have been responsible for generating life's building blocks. According to computer models, a deposit of such radioactive materials could show the same self-sustaining nuclear reaction as that found in the Oklo uranium ore seam in Gabon. Such radioactive beach sand might have provided sufficient energy to generate organic molecules, such as amino acids and sugars from acetonitrile in water. Radioactive monazite material also has released soluble phosphate into the regions between sand-grains, making it biologically "accessible." Thus amino acids, sugars, and soluble phosphates might have been produced simultaneously, according to Adam. Radioactive actinides, left behind in some concentration by the reaction, might have formed part of organometallic complexes. These complexes could have been important early catalysts to living processes.
John Parnell has suggested that such a process could provide part of the "crucible of life" in the early stages of any early wet rocky planet, so long as the planet is large enough to have generated a system of plate tectonics which brings radioactive minerals to the surface. As the early Earth is thought to have had many smaller plates, it might have provided a suitable environment for such processes.
The 19th-century Austrian physicist Ludwig Boltzmann first recognized that the struggle for existence of living organisms was neither over raw material nor energy, but instead had to do with entropy production derived from the conversion of the solar spectrum into heat by these systems. Boltzmann thus realized that living systems, like all irreversible processes, were dependent on the dissipation of a generalized chemical potential for their existence. In his book "What is Life", the 20th-century Austrian physicist Erwin Schrödinger emphasized the importance of Boltzmann’s deep insight into the irreversible thermodynamic nature of living systems, suggesting that this was the physics and chemistry behind the origin and evolution of life. However, irreversible processes, and much less living systems, could not be conveniently analyzed under this perspective until Lars Onsager, and later Ilya Prigogine, developed an elegant mathematical formalism for treating the "self-organization" of material under a generalized chemical potential. This formalism became known as Classical Irreversible Thermodynamics and Prigogine was awarded the Nobel Prize in Chemistry in 1977 "for his contributions to non-equilibrium thermodynamics, particularly the theory of dissipative structures". The analysis of Prigogine showed that if a system were left to evolve under an imposed external potential, material could spontaneously organize (lower its entropy) forming what he called "dissipative structures" which would increase the dissipation of the externally imposed potential (augment the global entropy production). Non-equilibrium thermodynamics has since been successfully applied to the analysis of living systems, from the biochemical production of ATP to optimizing bacterial metabolic pathways to complete ecosystems.
In his "Thermodynamic Dissipation Theory of the Origin and Evolution of Life", Karo Michaelian has taken the insight of Boltzmann and the work of Prigogine to its ultimate consequences regarding the origin of life. This theory postulates that the hallmark of the origin and evolution of life is the microscopic dissipative structuring of organic pigments and their proliferation over the entire Earth surface. Present day life augments the entropy production of Earth in its solar environment by dissipating ultraviolet and visible photons into heat through organic pigments in water. This heat then catalyzes a host of secondary dissipative processes such as the water cycle, ocean and wind currents, hurricanes, etc. Michaelian argues that if the thermodynamic function of life today is to produce entropy through photon dissipation in organic pigments, then this probably was its function at its very beginnings. It turns out that both RNA and DNA when in water solution are very strong absorbers and extremely rapid dissipaters of ultraviolet light within the 230–290 nm wavelength (UV-C) region, which is a part of the Sun's spectrum that could have penetrated the prebiotic atmosphere. In fact, not only RNA and DNA, but many fundamental molecules of life (those common to all three domains of life) are also pigments that absorb in the UV-C, and many of these also have a chemical affinity to RNA and DNA. Nucleic acids may thus have acted as acceptor molecules to the UV-C photon excited antenna pigment donor molecules by providing an ultrafast channel for dissipation. Michaelian has shown using the formalism of non-linear irreversible thermodynamics that there would have existed during the Archean a thermodynamic imperative to the abiogenic UV-C photochemical synthesis and proliferation of these pigments over the entire Earth surface if they acted as catalysts to augment the dissipation of the solar photons. By the end of the Archean, with life-induced ozone dissipating UV-C light in the Earth’s upper atmosphere, it would have become ever more improbable for a completely new life to emerge that didn’t rely on the complex metabolic pathways already existing since now the free energy in the photons arriving at Earth’s surface would have been insufficient for direct breaking and remaking of covalent bonds. It has been suggested, however, that such changes in the surface flux of ultraviolet radiation due to geophysical events affecting the atmosphere could have been what promoted the development of complexity in life based on existing metabolic pathways, for example during the Cambrian explosion
Many salient characteristics of the fundamental molecules of life (those found in all three domains) all point directly to the involvement of UV-C light in the dissipative structuring of incipient life. Some of the most difficult problems concerning the origin of life, such as enzyme-less replication of RNA and DNA, homochirality of the fundamental molecules, and the origin of information encoding in RNA and DNA, also find an explanation within the same dissipative thermodynamic framework by considering the probable existence of a relation between primordial replication and UV-C photon dissipation. Michaelian suggests that it is erroneous to expect to describe the emergence, proliferation, or even evolution, of life without overwhelming reference to entropy production through the dissipation of a generalized chemical potential, in particular, the prevailing solar photon flux.
Different forms of life with variable origin processes may have appeared quasi-simultaneously in the early history of Earth. The other forms may be extinct (having left distinctive fossils through their different biochemistry—e.g., hypothetical types of biochemistry). It has been proposed that:
The first organisms were self-replicating iron-rich clays which fixed carbon dioxide into oxalic and other dicarboxylic acids. This system of replicating clays and their metabolic phenotype then evolved into the sulfide rich region of the hotspring acquiring the ability to fix nitrogen. Finally phosphate was incorporated into the evolving system which allowed the synthesis of nucleotides and phospholipids. If biosynthesis recapitulates biopoiesis, then the synthesis of amino acids preceded the synthesis of the purine and pyrimidine bases. Furthermore the polymerization of the amino acid thioesters into polypeptides preceded the directed polymerization of amino acid esters by polynucleotides.
Fluctuating hydrothermal pools on volcanic islands or proto-continents
Armid Mulkidjanian and co-authors think that the marine environments did not provide the ionic balance and composition universally found in cells, as well as of ions required by essential proteins and ribozymes found in virtually all living organisms, especially with respect to K+/Na+ ratio, Mn2+, Zn2+ and phosphate concentrations. The only known environments that mimic the needed conditions on Earth are found in terrestrial hydrothermal pools fed by steam vents. Additionally, mineral deposits in these environments under an anoxic atmosphere would have suitable pH (as opposed to current pools in an oxygenated atmosphere), contain precipitates of sulfide minerals that block harmful UV radiation, have wetting/drying cycles that concentrate substrate solutions to concentrations amenable to spontaneous formation of polymers of nucleic acids, polyesters and depsipeptides, both by chemical reactions in the hydrothermal environment, as well as by exposure to UV light during transport from vents to adjacent pools. Their hypothesized pre-biotic environments are similar to the deep-oceanic vent environments most commonly hypothesized, but add additional components that help explain peculiarities found in reconstructions of the Last Universal Common Ancestor (LUCA) of all living organisms.
Bruce Damer and David Deamer have come to the conclusion that cell membranes cannot be formed in salty seawater, and must therefore have originated in freshwater. Before the continents formed, the only dry land on Earth would be volcanic islands, where rainwater would form ponds where lipids could form the first stages towards cell membranes. These predecessors of true cells are assumed to have behaved more like a superorganism rather than individual structures, where the porous membranes would house molecules which would leak out and enter other protocells. Only when true cells had evolved would they gradually adapt to saltier environments and enter the ocean.
Colín-García et al. (2016) discuss the advantages and disadvantages of hydrothermal vents as primitive environments. They mention the exergonic reactions in such systems could have been a source of free energy that promoted chemical reactions, additional to their high mineralogical diversity which implies the induction of important chemical gradients, thus favoring the interaction between electron donors and acceptors. Colín-García et al. (2016) also summarize a set of experiments proposed to test the role of hydrothermal vents in prebiotic synthesis.
A theory that speaks to the origin of life on Earth and other rocky planets posits life as an information system in which information content grows because of selection. Life must start with minimum possible information, or minimum possible departure from thermodynamic equilibrium, and it requires thermodynamically free energy accessible by means of its information content. The most benign circumstances, minimum entropy variations with abundant free energy, suggest the pore space in the first few kilometres of the surface. Free energy is derived from the condensed products of the chemical reactions taking place in the cooling nebula.
- Anthropic principle
- Artificial cell
- Artificial life
- Biological immortality
- Carbon Mineral Challenge
- Common descent
- Entropy and life
- Formamide-based prebiotic chemistry
- GADV protein world
- Mediocrity principle
- Mycoplasma laboratorium
- Nexus for Exoplanet System Science
- Planetary habitability
- Rare Earth hypothesis
- Shadow biosphere
- Also occasionally called biopoiesis.
- The reactions are:
- FeS + H2S → FeS2 + 2H+ + 2e−
- FeS + H2S + CO2 → FeS2 + HCOOH
- The reactions are:
Reaction 1: Fayalite + water → magnetite + aqueous silica + hydrogen
- 3Fe2SiO4 + 2H2O → 2Fe3O4 + 3SiO2 + 2H2
- 3Mg2SiO4 + SiO2 + 4H2O → 2Mg3Si2O5(OH)4
- 2Mg2SiO4 + 3H2O → Mg3Si2O5(OH)4 + Mg(OH)2
- 2 Ca2SiO4 + 4 H2O → 3 CaO · 2 SiO2 · 3 H2O + Ca(OH)2
- Dodd, Matthew S.; Papineau, Dominic; Grenne, Tor; Slack, John F.; Rittner, Martin; Pirajno, Franco; O'Neil, Jonathan; Little, Crispin T. S. (1 March 2017). "Evidence for early life in Earth's oldest hydrothermal vent precipitates". Nature. 543 (7643): 60–64. Bibcode:2017Natur.543...60D. doi:10.1038/nature21377. Archived from the original on 8 September 2017. Retrieved 2 March 2017.
- Zimmer, Carl (1 March 2017). "Scientists Say Canadian Bacteria Fossils May Be Earth's Oldest". The New York Times. Archived from the original on 2 March 2017. Retrieved 2 March 2017.
- Oparin, Aleksandr Ivanovich (2003) . The Origin of Life. Phoenix Edition Series. Translated by Morgulis, Sergius (2 ed.). Mineola, New York: Courier Corporation. ISBN 9780486495224. Retrieved 2018-06-16.
- Peretó, Juli (2005). "Controversies on the origin of life" (PDF). International Microbiology. Barcelona: Spanish Society for Microbiology. 8 (1): 23–31. ISSN 1139-6709. PMID 15906258. Archived from the original (PDF) on 24 August 2015. Retrieved 2015-06-01.
Ever since the historical contributions by Aleksandr I. Oparin, in the 1920s, the intellectual challenge of the origin of life enigma has unfolded based on the assumption that life originated on Earth through physicochemical processes that can be supposed, comprehended, and simulated; that is, there were neither miracles nor spontaneous generations.
- Compare: Scharf, Caleb; et al. (18 December 2015). "A Strategy for Origins of Life Research". Astrobiology. 15 (12): 1031–1042. Bibcode:2015AsBio..15.1031S. doi:10.1089/ast.2015.1113. PMC 4683543. PMID 26684503. Retrieved 28 November 2016.
What do we mean by the origins of life (OoL)? [...] Since the early 20th century the phrase OoL has been used to refer to the events that occurred during the transition from non-living to living systems on Earth, i.e., the origin of terrestrial biology (Oparin, 1924; Haldane, 1929). The term has largely replaced earlier concepts such as abiogenesis (Kamminga, 1980; Fry, 2000).
- Oparin 1953, p. vi
- Warmflash, David; Warmflash, Benjamin (November 2005). "Did Life Come from Another World?". Scientific American. Stuttgart: Georg von Holtzbrinck Publishing Group. 293 (5): 64–71. Bibcode:2005SciAm.293e..64W. doi:10.1038/scientificamerican1105-64. ISSN 0036-8733.
According to the conventional hypothesis, the earliest living cells emerged as a result of chemical evolution on our planet billions of years ago in a process called abiogenesis.
- Yarus 2010, p. 47
- Howell, Elizabeth (8 December 2014). "How Did Life Become Complex, And Could It Happen Beyond Earth?". Astrobiology Magazine. Retrieved 14 February 2018.
- Davis, John (29 October 2013). "Paleontologist presents origin of life theory". Texas Tech University. Retrieved 14 February 2018.
- Tirard, Stephane (20 April 2015). "Abiogenesis - Definition". Encyclopedia of Astrobiology. Springer. doi:10.1007/978-3-642-27833-4_2-4. Retrieved 14 February 2018.
Thomas Huxley (1825–1895) used the term abiogenesis in an important text published in 1870. He strictly made the difference between spontaneous generation, which he did not accept, and the possibility of the evolution of matter from inert to living, without any influence of life. [...] Since the end of the nineteenth century, evolutive abiogenesis means increasing complexity and evolution of matter from inert to living state in the abiotic context of evolution of primitive Earth.
- Voet & Voet 2004, p. 29
- Dyson 1999
- Davies, Paul (1998). The Fifth Miracle, Search for the origin and meaning of life. Penguin.
- Ward, Peter; Kirschvink, Joe (2015). A New History of Life: the radical discoveries about the origins and evolution of life on earth. Bloomsbury Press. pp. 39–40.
- Copley, Shelley D.; Smith, Eric; Morowitz, Harold J. (December 2007). "The origin of the RNA world: Co-evolution of genes and metabolism" (PDF). Bioorganic Chemistry. Amsterdam, the Netherlands: Elsevier. 35 (6): 430–443. doi:10.1016/j.bioorg.2007.08.001. ISSN 0045-2068. PMID 17897696. Archived (PDF) from the original on 5 September 2013. Retrieved 8 June 2015.
The proposal that life on Earth arose from an RNA world is widely accepted.
- Orgel, Leslie E. (April 2003). "Some consequences of the RNA world hypothesis". Origins of Life and Evolution of the Biosphere. Kluwer Academic Publishers. 33 (2): 211–218. doi:10.1023/A:1024616317965. ISSN 0169-6149. PMID 12967268.
It now seems very likely that our familiar DNA/RNA/protein world was preceded by an RNA world...
- Robertson & Joyce 2012: "There is now strong evidence indicating that an RNA World did indeed exist before DNA- and protein-based life."
- Neveu, Kim & Benner 2013: "[The RNA world's existence] has broad support within the community today."
- Copley, Shelley D.; Smith, Eric; Morowitz, Harold J. (December 2007). "The origin of the RNA world: Co-evolution of genes and metabolism" (PDF). Bioorganic Chemistry. Amsterdam, the Netherlands: Elsevier. 35 (6): 430–443. doi:10.1016/j.bioorg.2007.08.001. ISSN 0045-2068. PMID 17897696. Archived (PDF) from the original on 5 September 2013. Retrieved 8 June 2015.
- Robertson, Michael P.; Joyce, Gerald F. (May 2012). "The origins of the RNA world". Cold Spring Harbor Perspectives in Biology. Cold Spring Harbor, NY: Cold Spring Harbor Laboratory Press. 4 (5): a003608. doi:10.1101/cshperspect.a003608. ISSN 1943-0264. PMC 3331698. PMID 20739415.
- Cech, Thomas R. (July 2012). "The RNA Worlds in Context". Cold Spring Harbor Perspectives in Biology. Cold Spring Harbor, NY: Cold Spring Harbor Laboratory Press. 4 (7): a006742. doi:10.1101/cshperspect.a006742. ISSN 1943-0264. PMC 3385955. PMID 21441585.
- Keller, Markus A.; Turchyn, Alexandra V.; Ralser, Markus (25 March 2014). "Non‐enzymatic glycolysis and pentose phosphate pathway‐like reactions in a plausible Archean ocean". Molecular Systems Biology. Heidelberg, Germany: EMBO Press on behalf of the European Molecular Biology Organization. 10 (725): 725. doi:10.1002/msb.20145228. ISSN 1744-4292. PMC 4023395. PMID 24771084.
- Ehrenfreund, Pascale; Cami, Jan (December 2010). "Cosmic carbon chemistry: from the interstellar medium to the early Earth". Cold Spring Harbor Perspectives in Biology. Cold Spring Harbor, NY: Cold Spring Harbor Laboratory Press. 2 (12): a002097. doi:10.1101/cshperspect.a002097. ISSN 1943-0264. PMC 2982172. PMID 20554702.
- Perkins, Sid (8 April 2015). "Organic molecules found circling nearby star". Science (News). Washington, D.C.: American Association for the Advancement of Science. ISSN 1095-9203. Retrieved 2015-06-02.
- King, Anthony (14 April 2015). "Chemicals formed on meteorites may have started life on Earth". Chemistry World (News). London: Royal Society of Chemistry. ISSN 1473-7604. Archived from the original on 17 April 2015. Retrieved 17 April 2015.
- Saladino, Raffaele; Carota, Eleonora; Botta, Giorgia; et al. (13 April 2015). "Meteorite-catalyzed syntheses of nucleosides and of other prebiotic compounds from formamide under proton irradiation". Proc. Natl. Acad. Sci. U.S.A. Washington, D.C.: National Academy of Sciences. 112 (21): E2746–E2755. Bibcode:2015PNAS..112E2746S. doi:10.1073/pnas.1422225112. ISSN 1091-6490. PMC 4450408. PMID 25870268.
- Loeb, Abraham (October 2014). "The habitable epoch of the early universe". International Journal of Astrobiology. Cambridge, UK: Cambridge University Press. 13 (4): 337–339. arXiv:1312.0613. Bibcode:2014IJAsB..13..337L. doi:10.1017/S1473550414000196. ISSN 1473-5504.
- Dreifus, Claudia (2 December 2014). "Much-Discussed Views That Go Way Back". The New York Times. New York. p. D2. ISSN 0362-4331. Archived from the original on 3 December 2014. Retrieved 3 December 2014.
- Berera, Arjun (6 November 2017). "Space dust collisions as a planetary escape mechanism". Astrobiology. 17 (12): 1274–1282. arXiv:1711.01895. Bibcode:2017AsBio..17.1274B. doi:10.1089/ast.2017.1662. PMID 29148823.
- Chan, Queenie H. S. (10 January 2018). "Organic matter in extraterrestrial water-bearing salt crystals". Science Advances. 4 (1, eaao3521): eaao3521. Bibcode:2018SciA....4O3521C. doi:10.1126/sciadv.aao3521. Retrieved 11 January 2018.
- Rampelotto, Pabulo Henrique (26 April 2010). Panspermia: A Promising Field Of Research (PDF). Astrobiology Science Conference 2010. Houston, TX: Lunar and Planetary Institute. p. 5224. Bibcode:2010LPICo1538.5224R. Archived (PDF) from the original on 27 March 2016. Retrieved 3 December 2014. Conference held at League City, TX
- Graham, Robert W. (February 1990). "Extraterrestrial Life in the Universe" (PDF) (NASA Technical Memorandum 102363). Lewis Research Center, Cleveland, Ohio: NASA. Archived (PDF) from the original on 3 September 2014. Retrieved 2 June 2015.
- Altermann 2009, p. xvii
- "Age of the Earth". United States Geological Survey. 9 July 2007. Archived from the original on 23 December 2005. Retrieved 10 January 2006.
- Dalrymple 2001, pp. 205–221
- Manhesa, Gérard; Allègre, Claude J.; Dupréa, Bernard; Hamelin, Bruno (May 1980). "Lead isotope study of basic-ultrabasic layered complexes: Speculations about the age of the earth and primitive mantle characteristics". Earth and Planetary Science Letters. Amsterdam, the Netherlands: Elsevier. 47 (3): 370–382. Bibcode:1980E&PSL..47..370M. doi:10.1016/0012-821X(80)90024-2. ISSN 0012-821X.
- Schopf, J. William; Kudryavtsev, Anatoliy B.; Czaja, Andrew D.; Tripathi, Abhishek B. (5 October 2007). "Evidence of Archean life: Stromatolites and microfossils". Precambrian Research. Amsterdam, the Netherlands: Elsevier. 158 (3–4): 141–155. Bibcode:2007PreR..158..141S. doi:10.1016/j.precamres.2007.04.009. ISSN 0301-9268.
- Schopf, J. William (29 June 2006). "Fossil evidence of Archaean life". Philosophical Transactions of the Royal Society B. London: Royal Society. 361 (1470): 869–885. doi:10.1098/rstb.2006.1834. ISSN 0962-8436. PMC 1578735. PMID 16754604.
- Raven & Johnson 2002, p. 68
- Staff (9 May 2017). "Oldest evidence of life on land found in 3.48-billion-year-old Australian rocks". Phys.org. Archived from the original on 10 May 2017. Retrieved 13 May 2017.
- Djokic, Tara; Van Kranendonk, Martin J.; Campbell, Kathleen A.; Walter, Malcolm R.; Ward, Colin R. (9 May 2017). "Earliest signs of life on land preserved in ca. 3.5 Ga hot spring deposits". Nature Communications. 8: 15263. Bibcode:2017NatCo...815263D. doi:10.1038/ncomms15263. Archived from the original on 18 May 2017. Retrieved 13 May 2017.
- Schopf, J. William; Kitajima, Kouki; Spicuzza, Michael J.; Kudryavtsev, Anatolly B.; Valley, John W. (2017). "SIMS analyses of the oldest known assemblage of microfossils document their taxon-correlated carbon isotope compositions". PNAS. 115: 53–58. Bibcode:2018PNAS..115...53S. doi:10.1073/pnas.1718063115. Retrieved 19 December 2017.
- Tyrell, Kelly April (18 December 2017). "Oldest fossils ever found show life on Earth began before 3.5 billion years ago". University of Wisconsin-Madison. Retrieved 18 December 2017.
- Ghosh, Pallab (1 March 2017). "Earliest evidence of life on Earth found". BBC News. Archived from the original on 2 March 2017. Retrieved 2 March 2017.
- Dunham, Will (1 March 2017). "Canadian bacteria-like fossils called oldest evidence of life". Reuters. Archived from the original on 2 March 2017. Retrieved 1 March 2017.
- "Researchers uncover 'direct evidence' of life on Earth 4 billion years ago". Deutsche Welle. Retrieved 5 March 2017.
- Borenstein, Seth (19 October 2015). "Hints of life on what was thought to be desolate early Earth". AP News. Associated Press. Retrieved 9 October 2018.
- Schouten, Lucy (20 October 2015). "When did life first emerge on Earth? Maybe a lot earlier than we thought". The Christian Science Monitor. Boston, Massachusetts: Christian Science Publishing Society. ISSN 0882-7729. Archived from the original on 22 March 2016. Retrieved 9 October 2018.
- Johnston, Ian (2 October 2017). "Life first emerged in 'warm little ponds' almost as old as the Earth itself - Charles Darwin's famous idea backed by new scientific study". The Independent. Archived from the original on 3 October 2017. Retrieved 2 October 2017.
- Kasting, James F. (12 February 1993). "Earth's Early Atmosphere" (PDF). Science. Washington, D.C.: American Association for the Advancement of Science. 259 (5097): 920–6. doi:10.1126/science.11536547. ISSN 0036-8075. PMID 11536547. Archived (PDF) from the original on 10 October 2015. Retrieved 28 July 2015.
- Fesenkov 1959, p. 9
- Follmann, Hartmut; Brownson, Carol (November 2009). "Darwin's warm little pond revisited: from molecules to the origin of life". Naturwissenschaften. 96 (11): 1265–1292. Bibcode:2009NW.....96.1265F. doi:10.1007/s00114-009-0602-1. ISSN 0028-1042. PMID 19760276.
- Morse, John W.; MacKenzie, Fred T. (1998). "Hadean Ocean Carbonate Geochemistry". Aquatic Geochemistry. Kluwer Academic Publishers. 4 (3–4): 301–319. doi:10.1023/A:1009632230875. ISSN 1380-6165.
- Wilde, Simon A.; Valley, John W.; Peck, William H.; Graham, Colin M. (11 January 2001). "Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago" (PDF). Nature. London: Nature Publishing Group. 409 (6817): 175–178. doi:10.1038/35051550. ISSN 0028-0836. PMID 11196637. Archived (PDF) from the original on 5 June 2015. Retrieved 3 June 2015.
- Rosing, Minik T.; Bird, Dennis K.; Sleep, Norman H.; et al. (22 March 2006). "The rise of continents—An essay on the geologic consequences of photosynthesis" (PDF). Palaeogeography, Palaeoclimatology, Palaeoecology. Amsterdam, the Netherlands: Elsevier. 232 (2–4): 99–113. doi:10.1016/j.palaeo.2006.01.007. ISSN 0031-0182. Archived (PDF) from the original on 14 July 2015. Retrieved 8 June 2015.
- Sleep, Norman H.; Zahnle, Kevin J.; Kasting, James F.; et al. (9 November 1989). "Annihilation of ecosystems by large asteroid impacts on early Earth" (PDF). Nature. London: Nature Publishing Group. 342 (6246): 139–142. Bibcode:1989Natur.342..139S. doi:10.1038/342139a0. ISSN 0028-0836. PMID 11536616.
- Boone, David R.; Castenholz, Richard W.; Garrity, George M. (eds.). The Archaea and the Deeply Branching and Phototrophic Bacteria. Bergey's Manual of Systematic Bacteriology. ISBN 978-0-387-21609-6. Archived from the original on 25 December 2014.
- Valas RE, Bourne PE (2011). "The origin of a derived superkingdom: how a gram-positive bacterium crossed the desert to become an archaeon". Biology Direct. 6: 16. doi:10.1186/1745-6150-6-16. PMC 3056875. PMID 21356104.
- Cavalier-Smith T (2006). "Rooting the tree of life by transition analyses". Biology Direct. 1: 19. doi:10.1186/1745-6150-1-19. PMC 1586193. PMID 16834776.
- Ward, Peter (2005), "Life as we do not know it" (Viking Books)
- Mortillaro, Nicole (1 March 2017). "Oldest traces of life on Earth found in Quebec, dating back roughly 3.8 billion years". CBC News. Archived from the original on 1 March 2017. Retrieved 2 March 2017.
- Ohtomo, Yoko; Kakegawa, Takeshi; Ishida, Akizumi; et al. (January 2014). "Evidence for biogenic graphite in early Archaean Isua metasedimentary rocks". Nature Geoscience. London: Nature Publishing Group. 7 (1): 25–28. Bibcode:2014NatGe...7...25O. doi:10.1038/ngeo2025. ISSN 1752-0894.
- Borenstein, Seth (13 November 2013). "Oldest fossil found: Meet your microbial mom". Excite. Yonkers, NY: Mindspark Interactive Network. Associated Press. Archived from the original on 29 June 2015. Retrieved 2 June 2015.
- Noffke, Nora; Christian, Daniel; Wacey, David; Hazen, Robert M. (16 November 2013). "Microbially Induced Sedimentary Structures Recording an Ancient Ecosystem in the ca. 3.48 Billion-Year-Old Dresser Formation, Pilbara, Western Australia". Astrobiology. New Rochelle, NY: Mary Ann Liebert, Inc. 13 (12): 1103–1124. Bibcode:2013AsBio..13.1103N. doi:10.1089/ast.2013.1030. ISSN 1531-1074. PMC 3870916. PMID 24205812.
- Wade, Nicholas (31 August 2016). "World's Oldest Fossils Found in Greenland". The New York Times. Archived from the original on 31 August 2016. Retrieved 31 August 2016.
- Davies 1999
- Hassenkam, T.; Andersson, M. P.; Dalby, K. N.; Mackenzie, D. M. A.; Rosing, M. T. (2017). "Elements of Eoarchean life trapped in mineral inclusions". Nature. 548 (7665): 78–81. Bibcode:2017Natur.548...78H. doi:10.1038/nature23261. PMID 28738409.
- Pearlman, Jonathan (13 November 2013). "Oldest signs of life on Earth found". The Daily Telegraph. London. Archived from the original on 16 December 2014. Retrieved 15 December 2014.
- O'Donoghue, James (21 August 2011). "Oldest reliable fossils show early life was a beach". New Scientist. Archived from the original on 30 June 2015.
- Wacey, David; Kilburn, Matt R.; Saunders, Martin; et al. (October 2011). "Microfossils of sulphur-metabolizing cells in 3.4-billion-year-old rocks of Western Australia". Nature Geoscience. 4 (10): 698–702. Bibcode:2011NatGe...4..698W. doi:10.1038/ngeo1238.
- Bell, Elizabeth A.; Boehnike, Patrick; Harrison, T. Mark; et al. (19 October 2015). "Potentially biogenic carbon preserved in a 4.1 billion-year-old zircon" (PDF). Proc. Natl. Acad. Sci. U.S.A. Washington, D.C.: National Academy of Sciences. 112 (47): 14518–21. Bibcode:2015PNAS..11214518B. doi:10.1073/pnas.1517557112. PMC 4664351. PMID 26483481. Archived (PDF) from the original on 6 November 2015. Retrieved 20 October 2015. Early edition, published online before print.
- Wolpert, Stuart (19 October 2015). "Life on Earth likely started at least 4.1 billion years ago — much earlier than scientists had thought". ULCA. Archived from the original on 20 October 2015. Retrieved 20 October 2015.
- Gomes, Rodney; Levison, Hal F.; Tsiganis, Kleomenis; Morbidelli, Alessandro (26 May 2005). "Origin of the cataclysmic Late Heavy Bombardment period of the terrestrial planets". Nature. 435 (7041): 466–469. Bibcode:2005Natur.435..466G. doi:10.1038/nature03676. PMID 15917802.
- Davies 2007, pp. 61–73
- Maher, Kevin A.; Stevenson, David J. (18 February 1988). "Impact frustration of the origin of life". Nature. 331 (6157): 612–614. Bibcode:1988Natur.331..612M. doi:10.1038/331612a0. PMID 11536595.
- Wade, Nicholas (25 July 2016). "Meet Luca, the Ancestor of All Living Things". The New York Times. Archived from the original on 28 July 2016.
- Weiss, M.C.; Sousa, F.L.; Mrnjavac, N.; Neukirchen, S.; Roettger, M.; Nelson-Sathi, S.; Martin, W.F. (2016). "The physiology and habitat of the last universal common ancestor". Nature Microbiology. 1 (9): 16116. doi:10.1038/NMICROBIOL.2016.116. PMID 27562259.
- M.D> Brasier (2012), "Secret Chambers: The Inside Story of Cells and Complex Life" (Oxford Uni Press), p.298
- Ward, Peter & Kirschvink, Joe, op cit, p.42
- Sheldon 2005
- Vartanian 1973, pp. 307–312
- Lennox 2001, pp. 229–258
- Balme, D. M. (1962). "Development of Biology in Aristotle and Theophrastus: Theory of Spontaneous Generation". Phronesis. Leiden, the Netherlands: Brill Publishers. 7 (1–2): 91–104. doi:10.1163/156852862X00052. ISSN 0031-8868.
- Ross 1652
- Dobell 1960
- Bondeson 1999
- Bernal 1967
- "Biogenesis". Hmolpedia. Ancaster, Ontario, Canada: WikiFoundry, Inc. Archived from the original on 20 May 2014. Retrieved 19 May 2014.
- Huxley 1968
- Bastian 1871
- Bastian 1871, p. xi–xii
- Oparin 1953, p. 196
- Tyndall 1905, IV, XII (1876), XIII (1878)
- Bernal 1967, p. 139
- Priscu, John C. "Origin and Evolution of Life on a Frozen Earth". Arlington County, VA: National Science Foundation. Archived from the original on 18 December 2013. Retrieved 1 March 2014.
- Darwin 1887, p. 18: "It is often said that all the conditions for the first production of a living organism are now present, which could ever have been present. But if (and oh! what a big if!) we could conceive in some warm little pond, with all sorts of ammonia and phosphoric salts, light, heat, electricity, &c., present, that a proteine compound was chemically formed ready to undergo still more complex changes, at the present day such matter would be instantly devoured or absorbed, which would not have been the case before living creatures were formed." — Charles Darwin, 1 February 1871
- Bernal 1967, The Origin of Life (A. I. Oparin, 1924), pp. 199–234
- Oparin 1953
- Shapiro 1987, p. 110
- Bryson 2004, pp. 300–302
- Miller, Stanley L. (15 May 1953). "A Production of Amino Acids Under Possible Primitive Earth Conditions". Science. Washington, D.C.: American Association for the Advancement of Science. 117 (3046): 528–529. Bibcode:1953Sci...117..528M. doi:10.1126/science.117.3046.528. ISSN 0036-8075. PMID 13056598.
- Parker, Eric T.; Cleaves, Henderson J.; Dworkin, Jason P.; et al. (5 April 2011). "Primordial synthesis of amines and amino acids in a 1958 Miller H2S-rich spark discharge experiment" (PDF). Proc. Natl. Acad. Sci. U.S.A. Washington, D.C.: National Academy of Sciences. 108 (14): 5526–5531. Bibcode:2011PNAS..108.5526P. doi:10.1073/pnas.1019191108. ISSN 0027-8424. PMC 3078417. PMID 21422282. Archived (PDF) from the original on 24 September 2015. Retrieved 8 June 2015.
- Bernal 1967, p. 143
- Walsh, J. Bruce (1995). "Part 4: Experimental studies of the origins of life". Origins of life (Lecture notes). Tucson, AZ: University Of Arizona. Archived from the original on 2008-01-13. Retrieved 2015-06-08.
- Woodward 1969, p. 287
- Bahadur, Krishna (1973). "Photochemical Formation of Self–sustaining Coacervates" (PDF). Proceedings of the Indian National Science Academy. New Delhi: Indian National Science Academy. 39B (4): 455–467. ISSN 0370-0046. Archived from the original (PDF) on 19 October 2013.
- Kasting 1993, p. 922
- Kasting 1993, p. 920
- Bernal 1951
- Martin, William F. (January 2003). "On the origins of cells: a hypothesis for the evolutionary transitions from abiotic geochemistry to chemoautotrophic prokaryotes, and from prokaryotes to nucleated cells". Phil. Trans. R. Soc. Lond. A. 358 (1429): 59–83. doi:10.1098/rstb.2002.1183. PMC 1693102. PMID 12594918.
- Bernal, John Desmond (September 1949). "The Physical Basis of Life". Proceedings of the Physical Society. Section A. Bristol, UK. 62 (9): 537–558. Bibcode:1949PPSA...62..537B. doi:10.1088/0370-1298/62/9/301. ISSN 0370-1298.
- Kauffman 1995
- Abiogenesis - Definition. 20 April 2015. Encyclopedia of Astrobiology. doi:10.1007/978-3-642-27833-4_2-4
- Oehlenschläger, Frank; Eigen, Manfred (December 1997). "30 Years Later – a New Approach to Sol Spiegelman's and Leslie Orgel's in vitro EVOLUTIONARY STUDIES Dedicated to Leslie Orgel on the occasion of his 70th birthday". Origins of Life and Evolution of Biospheres. 27 (5–6): 437–457. doi:10.1023/A:1006501326129. ISSN 0169-6149. PMID 9394469.
- McCollom, Thomas; Mayhew, Lisa; Scott, Jim (7 October 2014). "NASA awards CU-Boulder-led team $7 million to study origins, evolution of life in universe" (Press release). Boulder, CO: University of Colorado Boulder. Archived from the original on 31 July 2015. Retrieved 8 June 2015.
- Gibson, Daniel G.; Glass, John I.; Lartigue, Carole; et al. (2 July 2010). "Creation of a Bacterial Cell Controlled by a Chemically Synthesized Genome". Science. Washington, D.C.: American Association for the Advancement of Science. 329 (5987): 52–56. Bibcode:2010Sci...329...52G. doi:10.1126/science.1190719. ISSN 0036-8075. PMID 20488990.
- Swaby, Rachel (20 May 2010). "Scientists Create First Self-Replicating Synthetic Life". Wired. New York. Archived from the original on 17 June 2015. Retrieved 8 June 2015.
- Coughlan, Andy (2016) "Smallest ever genome comes to life: Humans built it but we don't know what a third of its genes actually do" (New Scientist 2nd April 2016 No 3067)p.6
- "NASA Astrobiology Strategy" (PDF). NASA. 2015. Archived (PDF) from the original on 22 December 2016.
- Landau, Elizabeth (12 October 2016). "Building Blocks of Life's Building Blocks Come From Starlight". NASA. Archived from the original on 13 October 2016. Retrieved 13 October 2016.
- Gawlowicz, Susan (6 November 2011). "Carbon-based organic 'carriers' in interstellar dust clouds? Newly discovered diffuse interstellar bands". Science Daily. Rockville, MD: ScienceDaily, LLC. Archived from the original on 11 July 2015. Retrieved 8 June 2015.
Post is reprinted from materials provided by the Rochester Institute of Technology.
- Geballe, Thomas R.; Najarro, Francisco; Figer, Donald F.; et al. (10 November 2011). "Infrared diffuse interstellar bands in the Galactic Centre region". Nature. London: Nature Publishing Group. 479 (7372): 200–202. arXiv:1111.0613. Bibcode:2011Natur.479..200G. doi:10.1038/nature10527. ISSN 0028-0836. PMID 22048316.
- Klyce 2001
- Moskowitz, Clara (29 March 2012). "Life's Building Blocks May Have Formed in Dust Around Young Sun". Space.com. Salt Lake City, UT: Purch. Archived from the original on 8 August 2012. Retrieved 30 March 2012.
- Ciesla, F. J.; Sandford, S. A. (29 March 2012). "Organic Synthesis via Irradiation and Warming of Ice Grains in the Solar Nebula". Science. 336 (6080): 452–454. Bibcode:2012Sci...336..452C. doi:10.1126/science.1217291.
- Chyba, Christopher; Sagan, Carl (9 January 1992). "Endogenous production, exogenous delivery and impact-shock synthesis of organic molecules: an inventory for the origins of life". Nature. London: Nature Publishing Group. 355 (6356): 125–132. Bibcode:1992Natur.355..125C. doi:10.1038/355125a0. ISSN 0028-0836. PMID 11538392.
- Furukawa, Yoshihiro; Sekine, Toshimori; Oba, Masahiro; et al. (January 2009). "Biomolecule formation by oceanic impacts on early Earth". Nature Geoscience. London: Nature Publishing Group. 2 (1): 62–66. Bibcode:2009NatGe...2...62F. doi:10.1038/NGEO383. ISSN 1752-0894.
- Davies 1999, p. 155
- Bock & Goode 1996
- Palasek, Stan (23 May 2013). "Primordial RNA Replication and Applications in PCR Technology". arXiv:1305.5581v1 [q-bio.BM].
- Koonin, Eugene V.; Senkevich, Tatiana G.; Dolja, Valerian V. (19 September 2006). "The ancient Virus World and evolution of cells". Biology Direct. London: BioMed Central. 1: 29. doi:10.1186/1745-6150-1-29. ISSN 1745-6150. PMC 1594570. PMID 16984643.
- Vlassov, Alexander V.; Kazakov, Sergei A.; Johnston, Brian H.; et al. (August 2005). "The RNA World on Ice: A New Scenario for the Emergence of RNA Information". Journal of Molecular Evolution. Berlin: Springer-Verlag. 61 (2): 264–273. Bibcode:2005JMolE..61..264V. doi:10.1007/s00239-004-0362-7. ISSN 0022-2844. PMID 16044244.
- Nussinov, Mark D.; Otroshchenko, Vladimir A.; Santoli, Salvatore (1997). "The emergence of the non-cellular phase of life on the fine-grained clayish particles of the early Earth's regolith". BioSystems. Amsterdam, the Netherlands: Elsevier. 42 (2–3): 111–118. doi:10.1016/S0303-2647(96)01699-1. ISSN 0303-2647. PMID 9184757.
- Cavitation-Induced Synthesis of Biogenic Molecules on Primordial Earth. Natan-Haim Kalson, David Furman, and Yehuda Zeiri. ACS Publications. ACS Cent. Sci., September 11, 2017, 3 (9), pp 1041–1049. doi:10.1021/acscentsci.7b00325.
- Saladino, Raffaele; Crestini, Claudia; Pino, Samanta; et al. (March 2012). "Formamide and the origin of life". Physics of Life Reviews. Amsterdam, the Netherlands: Elsevier. 9 (1): 84–104. Bibcode:2012PhLRv...9...84S. doi:10.1016/j.plrev.2011.12.002. ISSN 1571-0645. PMID 22196896.
- Saladino, Raffaele; Botta, Giorgia; Pino, Samanta; et al. (July 2012). "From the one-carbon amide formamide to RNA all the steps are prebiotically possible". Biochimie. Amsterdam, the Netherlands: Elsevier. 94 (7): 1451–1456. doi:10.1016/j.biochi.2012.02.018. ISSN 0300-9084. PMID 22738728.
- Oró, Joan (16 September 1961). "Mechanism of Synthesis of Adenine from Hydrogen Cyanide under Possible Primitive Earth Conditions". Nature. London: Nature Publishing Group. 191 (4794): 1193–1194. Bibcode:1961Natur.191.1193O. doi:10.1038/1911193a0. ISSN 0028-0836. PMID 13731264.
- Basile, Brenda; Lazcano, Antonio; Oró, Joan (1984). "Prebiotic syntheses of purines and pyrimidines". Advances in Space Research. Amsterdam, the Netherlands: Elsevier. 4 (12): 125–131. Bibcode:1984AdSpR...4..125B. doi:10.1016/0273-1177(84)90554-4. ISSN 0273-1177. PMID 11537766.
- Orgel, Leslie E. (August 2004). "Prebiotic Adenine Revisited: Eutectics and Photochemistry". Origins of Life and Evolution of Biospheres. Kluwer Academic Publishers. 34 (4): 361–369. Bibcode:2004OLEB...34..361O. doi:10.1023/B:ORIG.0000029882.52156.c2. ISSN 0169-6149. PMID 15279171.
- Robertson, Michael P.; Miller, Stanley L. (29 June 1995). "An efficient prebiotic synthesis of cytosine and uracil". Nature. London: Nature Publishing Group. 375 (6534): 772–774. Bibcode:1995Natur.375..772R. doi:10.1038/375772a0. ISSN 0028-0836. PMID 7596408.
- Fox, Douglas (February 2008). "Did Life Evolve in Ice?". Discover. Waukesha, WI: Kalmbach Publishing. ISSN 0274-7529. Archived from the original on 30 June 2008. Retrieved 3 July 2008.
- Levy, Matthew; Miller, Stanley L.; Brinton, Karen; Bada, Jeffrey L. (June 2000). "Prebiotic Synthesis of Adenine and Amino Acids Under Europa-like Conditions". Icarus. Amsterdam, the Netherlands: Elsevier. 145 (2): 609–613. Bibcode:2000Icar..145..609L. doi:10.1006/icar.2000.6365. ISSN 0019-1035. PMID 11543508.
- Menor-Salván, César; Ruiz-Bermejo, Marta; Guzmán, Marcelo I.; Osuna-Esteban, Susana; Veintemillas-Verdaguer, Sabino (20 April 2009). "Synthesis of Pyrimidines and Triazines in Ice: Implications for the Prebiotic Chemistry of Nucleobases". Chemistry: A European Journal. Weinheim, Germany: Wiley-VCH on behalf of ChemPubSoc Europe. 15 (17): 4411–4418. doi:10.1002/chem.200802656. ISSN 0947-6539. PMID 19288488.
- Roy, Debjani; Najafian, Katayoun; von Ragué Schleyer, Paul (30 October 2007). "Chemical evolution: The mechanism of the formation of adenine under prebiotic conditions". Proc. Natl. Acad. Sci. U.S.A. Washington, D.C.: National Academy of Sciences. 104 (44): 17272–17277. Bibcode:2007PNAS..10417272R. doi:10.1073/pnas.0708434104. ISSN 0027-8424. PMC 2077245. PMID 17951429.
- Cleaves, H. James; Chalmers, John H.; Lazcano, Antonio; et al. (April 2008). "A Reassessment of Prebiotic Organic Synthesis in Neutral Planetary Atmospheres". Origins of Life and Evolution of Biospheres. Dordrecht, the Netherlands: Springer. 38 (2): 105–115. Bibcode:2008OLEB...38..105C. doi:10.1007/s11084-007-9120-3. ISSN 0169-6149. PMID 18204914.
- Chyba, Christopher F. (13 May 2005). "Rethinking Earth's Early Atmosphere". Science. Washington, D.C.: American Association for the Advancement of Science. 308 (5724): 962–963. doi:10.1126/science.1113157. ISSN 0036-8075. PMID 15890865.
- Barton et al. 2007, pp. 93–95
- Bada & Lazcano 2009, pp. 56–57
- Bada, Jeffrey L.; Lazcano, Antonio (2 May 2003). "Prebiotic Soup—Revisiting the Miller Experiment" (PDF). Science. Washington, D.C.: American Association for the Advancement of Science. 300 (5620): 745–746. doi:10.1126/science.1085145. ISSN 0036-8075. PMID 12730584. Archived (PDF) from the original on 4 March 2016. Retrieved 13 June 2015.
- Oró, Joan; Kimball, Aubrey P. (February 1962). "Synthesis of purines under possible primitive earth conditions: II. Purine intermediates from hydrogen cyanide". Archives of Biochemistry and Biophysics. Amsterdam, the Netherlands: Elsevier. 96 (2): 293–313. doi:10.1016/0003-9861(62)90412-5. ISSN 0003-9861. PMID 14482339.
- Ahuja, Mukesh, ed. (2006). "Origin of Life". Life Science. 1. Delhi: Isha Books. p. 11. ISBN 81-8205-386-2. OCLC 297208106. Archived from the original on 28 September 2015.
- Service, Robert F. (16 March 2015). "Researchers may have solved origin-of-life conundrum". Science (News). Washington, D.C.: American Association for the Advancement of Science. ISSN 1095-9203. Archived from the original on 12 August 2015. Retrieved 26 July 2015.
- Patel, Bhavesh H.; Percivalle, Claudia; Ritson, Dougal J.; Duffy, Colm D.; Sutherland, John D. (April 2015). "Common origins of RNA, protein and lipid precursors in a cyanosulfidic protometabolism". Nature Chemistry. London: Nature Publishing Group. 7 (4): 301–307. Bibcode:2015NatCh...7..301P. doi:10.1038/nchem.2202. ISSN 1755-4330. PMC 4568310. PMID 25803468. Retrieved 2015-07-22.
- Patel et al. 2015, p. 302
- Paul, Natasha; Joyce, Gerald F. (December 2004). "Minimal self-replicating systems". Current Opinion in Chemical Biology. Amsterdam, the Netherlands: Elsevier. 8 (6): 634–639. doi:10.1016/j.cbpa.2004.09.005. ISSN 1367-5931. PMID 15556408.
- Bissette, Andrew J.; Fletcher, Stephen P. (2 December 2013). "Mechanisms of Autocatalysis". Angewandte Chemie International Edition. Weinheim, Germany: Wiley-VCH on behalf of the German Chemical Society. 52 (49): 12800–12826. doi:10.1002/anie.201303822. ISSN 1433-7851. PMID 24127341.
- Kauffman 1993, chpt. 7
- Dawkins 2004
- Tjivikua, T.; Ballester, Pablo; Rebek, Julius Jr. (January 1990). "Self-replicating system". Journal of the American Chemical Society. Washington, D.C.: American Chemical Society. 112 (3): 1249–1250. doi:10.1021/ja00159a057. ISSN 0002-7863.
- Browne, Malcolm W. (30 October 1990). "Chemists Make Molecule With Hint of Life". The New York Times. New York. ISSN 0362-4331. Archived from the original on 21 July 2015. Retrieved 14 July 2015.
- Eigen & Schuster 1979
- Hoffmann, Geoffrey W. (25 June 1974). "On the origin of the genetic code and the stability of the translation apparatus". Journal of Molecular Biology. Amsterdam, the Netherlands: Elsevier. 86 (2): 349–362. doi:10.1016/0022-2836(74)90024-2. ISSN 0022-2836. PMID 4414916.
- Orgel, Leslie E. (April 1963). "The Maintenance of the Accuracy of Protein Synthesis and its Relevance to Ageing". Proc. Natl. Acad. Sci. U.S.A. Washington, D.C.: National Academy of Sciences. 49 (4): 517–521. Bibcode:1963PNAS...49..517O. doi:10.1073/pnas.49.4.517. ISSN 0027-8424. PMC 299893. PMID 13940312.
- Hoffmann, Geoffrey W. (October 1975). "The Stochastic Theory of the Origin of the Genetic Code". Annual Review of Physical Chemistry. Palo Alto, CA: Annal Reviews. 26: 123–144. Bibcode:1975ARPC...26..123H. doi:10.1146/annurev.pc.26.100175.001011. ISSN 0066-426X.
- Hoffmann, Geoffrey William (24 December 2016). "A network theory of the origin of life". bioRxiv 096701.
- Chaichian, Rojas & Tureanu 2014, pp. 353–364
- Plasson, Raphaël; Kondepudi, Dilip K.; Bersini, Hugues; et al. (August 2007). "Emergence of homochirality in far-from-equilibrium systems: Mechanisms and role in prebiotic chemistry". Chirality. Hoboken, NJ: John Wiley & Sons. 19 (8): 589–600. doi:10.1002/chir.20440. ISSN 0899-0042. PMID 17559107. "Special Issue: Proceedings from the Eighteenth International Symposium on Chirality (ISCD-18), Busan, Korea, 2006"
- Jafarpour, Farshid; Biancalani, Tommaso; Goldenfeld, Nigel (2017). "Noise-induced symmetry breaking far from equilibrium and the emergence of biological homochirality". Physical Review E. APS. 95 (3): 032407. Bibcode:2017PhRvE..95c2407J. doi:10.1103/PhysRevE.95.032407.
- Jafarpour, Farshid; Biancalani, Tommaso; Goldenfeld, Nigel (2015). "Noise-induced mechanism for biological homochirality of early life self-replicators". Physical Review Letters. APS. 115 (15): 158101. arXiv:1507.00044. Bibcode:2015PhRvL.115o8101J. doi:10.1103/PhysRevLett.115.158101. PMID 26550754.
- Frank, F.C. (1953). "On spontaneous asymmetric synthesis". Biochimica et Biophysica Acta. Elsevier. 11: 459–463. doi:10.1016/0006-3002(53)90082-1.
- Clark, Stuart (July–August 1999). "Polarized Starlight and the Handedness of Life". American Scientist. Research Triangle Park, NC: Sigma Xi. 87 (4): 336. Bibcode:1999AmSci..87..336C. doi:10.1511/1999.4.336. ISSN 0003-0996.
- Shibata, Takanori; Morioka, Hiroshi; Hayase, Tadakatsu; et al. (17 January 1996). "Highly Enantioselective Catalytic Asymmetric Automultiplication of Chiral Pyrimidyl Alcohol". Journal of the American Chemical Society. Washington, D.C.: American Chemical Society. 118 (2): 471–472. doi:10.1021/ja953066g. ISSN 0002-7863.
- Soai, Kenso; Sato, Itaru; Shibata, Takanori (2001). "Asymmetric autocatalysis and the origin of chiral homogeneity in organic compounds". The Chemical Record. Hoboken, NJ: John Wiley & Sons on behalf of The Japan Chemical Journal Forum. 1 (4): 321–332. doi:10.1002/tcr.1017. ISSN 1528-0691. PMID 11893072.
- Hazen 2005, p. 184
- Meierhenrich, Uwe (2008). Amino acids and the asymmetry of life caught in the act of formation. Berlin: Springer. pp. 76&ndash, 79. ISBN 9783540768869.
- Mullen, Leslie (5 September 2005). "Building Life from Star-Stuff". Astrobiology Magazine. New York: NASA. Archived from the original on 14 July 2015. Retrieved 15 June 2015.
- Chen, Irene A.; Walde, Peter (July 2010). "From Self-Assembled Vesicles to Protocells" (PDF). Cold Spring Harbor Perspectives in Biology. Cold Spring Harbor, NY: Cold Spring Harbor Laboratory Press. 2 (7): a002170. doi:10.1101/cshperspect.a002170. ISSN 1943-0264. PMC 2890201. PMID 20519344. Archived (PDF) from the original on 17 November 2017. Retrieved 15 June 2015.
- "Exploring Life's Origins: Protocells". Exploring Life's Origins: A Virtual Exhibit. Arlington County, VA: National Science Foundation. Archived from the original on 28 February 2014. Retrieved 18 March 2014.
- Chen, Irene A. (8 December 2006). "The Emergence of Cells During the Origin of Life". Science. Washington, D.C.: American Association for the Advancement of Science. 314 (5805): 1558–1559. doi:10.1126/science.1137541. ISSN 0036-8075. PMID 17158315. Archived from the original on 7 September 2015. Retrieved 15 June 2015.
- Zimmer, Carl (26 June 2004). "What Came Before DNA?". Discover. Waukesha, WI: Kalmbach Publishing. ISSN 0274-7529. Archived from the original on 19 March 2014.
- Shapiro, Robert (June 2007). "A Simpler Origin for Life". Scientific American. Stuttgart: Georg von Holtzbrinck Publishing Group. 296 (6): 46–53. Bibcode:2007SciAm.296f..46S. doi:10.1038/scientificamerican0607-46. ISSN 0036-8733. PMID 17663224. Archived from the original on 14 June 2015. Retrieved 15 June 2015.
- Chang 2007
- Switek, Brian (13 February 2012). "Debate bubbles over the origin of life". Nature. London: Nature Publishing Group. doi:10.1038/nature.2012.10024. ISSN 0028-0836.
- Grote, Mathias (September 2011). "Jeewanu, or the 'particles of life'" (PDF). Journal of Biosciences. Bangalore, India: Indian Academy of Sciences; Springer. 36 (4): 563–570. doi:10.1007/s12038-011-9087-0. ISSN 0250-5991. PMID 21857103. Archived (PDF) from the original on 16 July 2015. Retrieved 15 June 2015.
- Gupta, V. K.; Rai, R. K. (August 2013). "Histochemical localisation of RNA-like material in photochemically formed self-sustaining, abiogenic supramolecular assemblies 'Jeewanu'". International Research Journal of Science & Engineering. Amravati, India. 1 (1): 1–4. ISSN 2322-0015. Archived from the original on 28 June 2017. Retrieved 15 June 2015.
- Welter, Kira (10 August 2015). "Peptide glue may have held first protocell components together". Chemistry World (News). London: Royal Society of Chemistry. ISSN 1473-7604. Archived from the original on 5 September 2015. Retrieved 29 August 2015.
- Kamat, Neha P.; Tobé, Sylvia; Hill, Ian T.; Szostak, Jack W. (29 July 2015). "Electrostatic Localization of RNA to Protocell Membranes by Cationic Hydrophobic Peptides". Angewandte Chemie International Edition. Weinheim, Germany: Wiley-VCH on behalf of the German Chemical Society. 54 (40): 11735–9. doi:10.1002/anie.201505742. ISSN 1433-7851. PMC 4600236. PMID 26223820. Archived from the original on 19 August 2015. "Early View (Online Version of Record published before inclusion in an issue)"
- Wimberly, Brian T.; Brodersen, Ditlev E.; Clemons, William M. Jr.; et al. (21 September 2000). "Structure of the 30S ribosomal subunit". Nature. London: Nature Publishing Group. 407 (6802): 327–339. Bibcode:2000Natur.407..327W. doi:10.1038/35030006. ISSN 0028-0836. PMID 11014182.
- Zimmer, Carl (25 September 2014). "A Tiny Emissary From the Ancient Past". The New York Times. New York. ISSN 0362-4331. Archived from the original on 27 September 2014. Retrieved 26 September 2014.
- Wade, Nicholas (4 May 2015). "Making Sense of the Chemistry That Led to Life on Earth". The New York Times. New York. ISSN 0362-4331. Archived from the original on 9 July 2017. Retrieved 10 May 2015.
- Yarus, Michael (April 2011). "Getting Past the RNA World: The Initial Darwinian Ancestor". Cold Spring Harbor Perspectives in Biology. Cold Spring Harbor, NY: Cold Spring Harbor Laboratory Press. 3 (4): a003590. doi:10.1101/cshperspect.a003590. ISSN 1943-0264. PMC 3062219. PMID 20719875.
- Neveu, Marc; Kim, Hyo-Joong; Benner, Steven A. (22 April 2013). "The 'Strong' RNA World Hypothesis: Fifty Years Old". Astrobiology. New Rochelle, NY: Mary Ann Liebert, Inc. 13 (4): 391–403. Bibcode:2013AsBio..13..391N. doi:10.1089/ast.2012.0868. ISSN 1531-1074. PMID 23551238.
- Gilbert, Walter (20 February 1986). "Origin of life: The RNA world". Nature. London: Nature Publishing Group. 319 (6055): 618. Bibcode:1986Natur.319..618G. doi:10.1038/319618a0. ISSN 0028-0836.
- Noller, Harry F. (April 2012). "Evolution of protein synthesis from an RNA world". Cold Spring Harbor Perspectives in Biology. Cold Spring Harbor, NY: Cold Spring Harbor Laboratory Press. 4 (4): a003681. doi:10.1101/cshperspect.a003681. ISSN 1943-0264. PMC 3312679. PMID 20610545.
- Koonin, Eugene V. (31 May 2007). "The cosmological model of eternal inflation and the transition from chance to biological evolution in the history of life". Biology Direct. London: BioMed Central. 2: 15. doi:10.1186/1745-6150-2-15. ISSN 1745-6150. PMC 1892545. PMID 17540027.
- Yates, Diana (25 September 2015). "Study adds to evidence that viruses are alive" (Press release). Champaign, IL: University of Illinois at Urbana–Champaign. Archived from the original on 19 November 2015. Retrieved 20 October 2015.
- Katzourakis, A (2013). "Paleovirology: Inferring viral evolution from host genome sequence data". Philosophical Transactions of the Royal Society B: Biological Sciences. 368 (1626): 20120493. doi:10.1098/rstb.2012.0493. PMC 3758182. PMID 23938747.
- Arshan, Nasir; Caetano-Anollés, Gustavo (25 September 2015). "A phylogenomic data-driven exploration of viral origins and evolution". Science Advances. Washington, D.C.: American Association for the Advancement of Science. 1 (8): e1500527. Bibcode:2015SciA....1E0527N. doi:10.1126/sciadv.1500527. ISSN 2375-2548.
- Nasir, Arshan; Naeem, Aisha; Jawad Khan, Muhammad; et al. (December 2011). "Annotation of Protein Domains Reveals Remarkable Conservation in the Functional Make up of Proteomes Across Superkingdoms". Genes. Basel, Switzerland: MDPI. 2 (4): 869–911. doi:10.3390/genes2040869. ISSN 2073-4425. PMC 3927607. PMID 24710297.
- Orgel, Leslie E. (October 1994). "The origin of life on Earth". Scientific American. Stuttgart: Georg von Holtzbrinck Publishing Group. 271 (4): 76–83. Bibcode:1994SciAm.271d..76O. doi:10.1038/scientificamerican1094-76. ISSN 0036-8733. PMID 7524147.
- Johnston, Wendy K.; Unrau, Peter J.; Lawrence, Michael S.; et al. (18 May 2001). "RNA-Catalyzed RNA Polymerization: Accurate and General RNA-Templated Primer Extension". Science. Washington, D.C.: American Association for the Advancement of Science. 292 (5520): 1319–1325. Bibcode:2001Sci...292.1319J. doi:10.1126/science.1060786. ISSN 0036-8075. PMID 11358999.
- Szostak, Jack W. (5 February 2015). "The Origins of Function in Biological Nucleic Acids, Proteins, and Membranes". Chevy Chase (CDP), MD: Howard Hughes Medical Institute. Archived from the original on 14 July 2015. Retrieved 16 June 2015.
- Lincoln, Tracey A.; Joyce, Gerald F. (27 February 2009). "Self-Sustained Replication of an RNA Enzyme". Science. Washington, D.C.: American Association for the Advancement of Science. 323 (5918): 1229–1232. Bibcode:2009Sci...323.1229L. doi:10.1126/science.1167856. ISSN 0036-8075. PMC 2652413. PMID 19131595.
- Joyce, Gerald F. (2009). "Evolution in an RNA world" (PDF). Cold Spring Harbor Perspectives in Biology. Cold Spring Harbor, NY: Cold Spring Harbor Laboratory Press. 74 (Evolution: The Molecular Landscape): 17–23. doi:10.1101/sqb.2009.74.004. ISSN 1943-0264. PMC 2891321. PMID 19667013. Archived from the original on 16 June 2017. Retrieved 16 June 2015.
- Bernstein, Harris; Byerly, Henry C.; Hopf, Frederick A.; et al. (June 1983). "The Darwinian Dynamic". The Quarterly Review of Biology. Chicago, IL: University of Chicago Press. 58 (2): 185–207. doi:10.1086/413216. ISSN 0033-5770. JSTOR 2828805.
- Michod 1999
- Zimmer, Carl (12 September 2013). "A Far-Flung Possibility for the Origin of Life". The New York Times. New York. ISSN 0362-4331. Archived from the original on 8 July 2015. Retrieved 15 June 2015.
- Webb, Richard (29 August 2013). "Primordial broth of life was a dry Martian cup-a-soup". New Scientist. London. ISSN 0262-4079. Archived from the original on 24 April 2015. Retrieved 16 June 2015.
- Wentao Ma; Chunwu Yu; Wentao Zhang; et al. (November 2007). "Nucleotide synthetase ribozymes may have emerged first in the RNA world". RNA. Cold Spring Harbor, NY: Cold Spring Harbor Laboratory Press on behalf of the RNA Society. 13 (11): 2012–2019. doi:10.1261/rna.658507. ISSN 1355-8382. PMC 2040096. PMID 17878321.
- Orgel, Leslie E. (17 November 2000). "A Simpler Nucleic Acid". Science. Washington, D.C.: American Association for the Advancement of Science. 290 (5495): 1306–1307. doi:10.1126/science.290.5495.1306. ISSN 0036-8075. PMID 11185405.
- Nelson, Kevin E.; Levy, Matthew; Miller, Stanley L. (11 April 2000). "Peptide nucleic acids rather than RNA may have been the first genetic molecule". Proc. Natl. Acad. Sci. U.S.A. Washington, D.C.: National Academy of Sciences. 97 (8): 3868–3871. Bibcode:2000PNAS...97.3868N. doi:10.1073/pnas.97.8.3868. ISSN 0027-8424. PMC 18108. PMID 10760258.
- Larralde, Rosa; Robertson, Michael P.; Miller, Stanley L. (29 August 1995). "Rates of Decomposition of Ribose and Other Sugars: Implications for Chemical Evolution" (PDF). Proc. Natl. Acad. Sci. U.S.A. Washington, D.C.: National Academy of Sciences. 92 (18): 8158–8160. Bibcode:1995PNAS...92.8158L. doi:10.1073/pnas.92.18.8158. ISSN 0027-8424. PMC 41115. PMID 7667262. Archived (PDF) from the original on 24 September 2015.
- Lindahl, Tomas (22 April 1993). "Instability and decay of the primary structure of DNA". Nature. London: Nature Publishing Group. 362 (6422): 709–715. Bibcode:1993Natur.362..709L. doi:10.1038/362709a0. ISSN 0028-0836. PMID 8469282.
- Anastasi, Carole; Crowe, Michael A.; Powner, Matthew W.; Sutherland, John D. (18 September 2006). "Direct Assembly of Nucleoside Precursors from Two- and Three-Carbon Units". Angewandte Chemie International Edition. Weinheim, Germany: Wiley-VCH on behalf of the German Chemical Society. 45 (37): 6176–6179. doi:10.1002/anie.200601267. ISSN 1433-7851. PMID 16917794.
- Powner, Matthew W.; Sutherland, John D. (13 October 2008). "Potentially Prebiotic Synthesis of Pyrimidine β-D-Ribonucleotides by Photoanomerization/Hydrolysis of α-D-Cytidine-2′-Phosphate". ChemBioChem. Weinheim, Germany: Wiley-VCH. 9 (15): 2386–2387. doi:10.1002/cbic.200800391. ISSN 1439-4227. PMID 18798212.
- Powner, Matthew W.; Gerland, Béatrice; Sutherland, John D. (14 May 2009). "Synthesis of activated pyrimidine ribonucleotides in prebiotically plausible conditions". Nature. London: Nature Publishing Group. 459 (7244): 239–242. Bibcode:2009Natur.459..239P. doi:10.1038/nature08013. ISSN 0028-0836. PMID 19444213.
- Senthilingam, Meera (25 April 2014). "Metabolism May Have Started in Early Oceans Before the Origin of Life" (Press release). Wellcome Trust. EurekAlert!. Archived from the original on 17 June 2015. Retrieved 16 June 2015.
- Nature Vol 535, 28 July 2016,"Early Life Liked it Hot", p.468
- Yue-Ching Ho, Eugene (July–September 1990). "Evolutionary Epistemology and Sir Karl Popper's Latest Intellectual Interest: A First-Hand Report". Intellectus. Hong Kong: Hong Kong Institute of Economic Science. 15: 1–3. OCLC 26878740. Archived from the original on 11 March 2012. Retrieved 13 August 2012.
- Wade, Nicholas (22 April 1997). "Amateur Shakes Up Ideas on Recipe for Life". The New York Times. New York. ISSN 0362-4331. Archived from the original on 17 June 2015. Retrieved 16 June 2015.
- Popper, Karl R. (29 March 1990). "Pyrite and the origin of life". Nature. London: Nature Publishing Group. 344 (6265): 387. Bibcode:1990Natur.344..387P. doi:10.1038/344387a0. ISSN 0028-0836.
- Huber, Claudia; Wächtershäuser, Günter (31 July 1998). "Peptides by Activation of Amino Acids with CO on (Ni,Fe)S Surfaces: Implications for the Origin of Life". Science. Washington, D.C.: American Association for the Advancement of Science. 281 (5377): 670–672. Bibcode:1998Sci...281..670H. doi:10.1126/science.281.5377.670. ISSN 0036-8075. PMID 9685253.
- Lane 2009
- Musser, George (23 September 2011). "How Life Arose on Earth, and How a Singularity Might Bring It Down". Observations (Blog). ISSN 0036-8733. Archived from the original on 17 June 2015. Retrieved 17 June 2015.
- Carroll, Sean (10 March 2010). "Free Energy and the Meaning of Life". Cosmic Variance (Blog). Discover. ISSN 0274-7529. Archived from the original on 14 July 2015. Retrieved 17 June 2015.
- Wolchover, Natalie (22 January 2014). "A New Physics Theory of Life". Quanta Magazine. New York: Simons Foundation. Archived from the original on 13 June 2015. Retrieved 17 June 2015.
- England, Jeremy L. (28 September 2013). "Statistical physics of self-replication" (PDF). Journal of Chemical Physics. College Park, MD: American Institute of Physics. 139 (12): 121923. arXiv:1209.1179. Bibcode:2013JChPh.139l1923E. doi:10.1063/1.4818538. ISSN 0021-9606. Archived (PDF) from the original on 4 June 2015. Retrieved 18 June 2015.
- Orgel, Leslie E. (7 November 2000). "Self-organizing biochemical cycles". Proc. Natl. Acad. Sci. U.S.A. Washington, D.C.: National Academy of Sciences. 97 (23): 12503–12507. Bibcode:2000PNAS...9712503O. doi:10.1073/pnas.220406697. ISSN 0027-8424. PMC 18793. PMID 11058157.
- Chandru, Kuhan; Gilbert, Alexis; Butch, Christopher; Aono, Masashi; Cleaves, Henderson James II (21 July 2016). "The Abiotic Chemistry of Thiolated Acetate Derivatives and the Origin of Life". Scientific Reports. 6 (29883): 29883. Bibcode:2016NatSR...629883C. doi:10.1038/srep29883. PMC 4956751. PMID 27443234.
- Vallee, Yannick; Shalayel, Ibrahim; Ly, Kieu-Dung; Rao, K. V. Raghavendra; Paëpe, Gael De; Märker, Katharina; Milet, Anne (2017-11-08). "At the very beginning of life on Earth: the thiol-rich peptide (TRP) world hypothesis". International Journal of Developmental Biology. 61 (8–9). ISSN 0214-6282.
- Mulkidjanian, Armen Y. (24 August 2009). "On the origin of life in the zinc world: 1. Photosynthesizing, porous edifices built of hydrothermally precipitated zinc sulfide as cradles of life on Earth". Biology Direct. London: BioMed Central. 4: 26. doi:10.1186/1745-6150-4-26. ISSN 1745-6150.
- Wächtershäuser, Günter (December 1988). "Before Enzymes and Templates: Theory of Surface Metabolism" (PDF). Microbiological Reviews. Washington, D.C.: American Society for Microbiology. 52 (4): 452–484. ISSN 0146-0749. PMC 373159. PMID 3070320. Archived from the original on 14 September 2015.
- Mulkidjanian, Armen Y.; Galperin, Michael Y. (24 August 2009). "On the origin of life in the zinc world. 2. Validation of the hypothesis on the photosynthesizing zinc sulfide edifices as cradles of life on Earth". Biology Direct. London: BioMed Central. 4: 27. doi:10.1186/1745-6150-4-27. ISSN 1745-6150.
- Macallum, A. B. (1 April 1926). "The Paleochemistry of the body fluids and tissues". Physiological Reviews. Bethesda, MD: American Physiological Society. 6 (2): 316–357. ISSN 0031-9333. Archived from the original on 5 March 2016. Retrieved 18 June 2015.
- Mulkidjanian, Armen Y.; Bychkov, Andrew Yu.; Dibrova, Daria V.; et al. (3 April 2012). "Origin of first cells at terrestrial, anoxic geothermal fields". Proc. Natl. Acad. Sci. U.S.A. Washington, D.C.: National Academy of Sciences. 109 (14): E821–E830. Bibcode:2012PNAS..109E.821M. doi:10.1073/pnas.1117774109. ISSN 1091-6490. PMC 3325685. PMID 22331915.
- For a deeper integrative version of this hypothesis, see in particular Lankenau 2011, pp. 225–286, interconnecting the "Two RNA worlds" concept and other detailed aspects; and Davidovich, Chen; Belousoff, Matthew; Bashan, Anat; Yonath, Ada (September 2009). "The evolving ribosome: from non-coded peptide bond formation to sophisticated translation machinery". Research in Microbiology. Amsterdam, the Netherlands: Elsevier. 160 (7): 487–492. doi:10.1016/j.resmic.2009.07.004. ISSN 1769-7123. PMID 19619641.
- Colín-García, M.; A. Heredia; G. Cordero; A. Camprubí; A. Negrón-Mendoza; F. Ortega-Gutiérrez; H. Berald; S. Ramos-Bernal (2016). "Hydrothermal vents and prebiotic chemistry: a review". Boletín de la Sociedad Geológica Mexicana. 68 (3): 599–620. Archived from the original on 18 August 2017.
- Schirber, Michael (24 June 2014). "Hydrothermal Vents Could Explain Chemical Precursors to Life". NASA Astrobiology: Life in the Universe. NASA. Archived from the original on 29 November 2014. Retrieved 2015-06-19.
- Martin, William; Russell, Michael J. (29 January 2003). "On the origins of cells: a hypothesis for the evolutionary transitions from abiotic geochemistry to chemoautotrophic prokaryotes, and from prokaryotes to nucleated cells". Philosophical Transactions of the Royal Society B. London: Royal Society. 358 (1429): 59–83, discussion 83–85. doi:10.1098/rstb.2002.1183. ISSN 0962-8436. PMC 1693102. PMID 12594918.
- Ignatov, Ignat; Mosin, Oleg V. (2013). "Possible Processes for Origin of Life and Living Matter with modeling of Physiological Processes of Bacterium Bacillus Subtilis in Heavy Water as Model System". Journal of Natural Sciences Research. New York: International Institute for Science, Technology and Education. 3 (9): 65–76.
- Calvin 1969
- Schirber, Michael (1 March 2010). "First Fossil-Makers in Hot Water". Astrobiology Magazine. New York: NASA. Archived from the original on 14 July 2015. Retrieved 19 June 2015.
- Kurihara, Kensuke; Tamura, Mieko; Shohda, Koh-ichiroh; et al. (October 2011). "Self-Reproduction of supramolecular giant vesicles combined with the amplification of encapsulated DNA". Nature Chemistry. London: Nature Publishing Group. 3 (10): 775–781. Bibcode:2011NatCh...3..775K. doi:10.1038/nchem.1127. ISSN 1755-4330. PMID 21941249.
- Usher, Oli (27 April 2015). "Chemistry of seabed's hot vents could explain emergence of life" (Press release). University College London. Retrieved 2015-06-19.
- Roldan, Alberto; Hollingsworth, Nathan; Roffey, Anna; Islam, Husn-Ubayda; et al. (May 2015). "Bio-inspired CO2 conversion by iron sulfide catalysts under sustainable conditions" (PDF). Chemical Communications. London: Royal Society of Chemistry. 51 (35): 7501–7504. doi:10.1039/C5CC02078F. ISSN 1359-7345. PMID 25835242. Archived from the original on 20 June 2015. Retrieved 19 June 2015.
- Baross, J. A.; Hoffman, S. E. (1985). "Submarine hydrothermal vents and associated gradient environments as sites for the origin and evolution of life". Origins LifeEvol. B. 15 (4): 327–345. Bibcode:1985OrLi...15..327B. doi:10.1007/bf01808177.
- Russell, M. J.; Hall, A. J. (1997). "The emergence of life from iron monosulphide bubbles at a submarine hydrothermal redox and pH front". J. Geol. Soc. Lond. 154 (3): 377–402. doi:10.1144/gsjgs.154.3.0377.
- Amend, J. P.; LaRowe, D. E.; McCollom, T. M.; Shock, E. L. (2013). "The energetics of organic synthesis inside and outside the cell". Phil. Trans. R. Soc. Lond. B. 368 (1622): 20120255. doi:10.1098/rstb.2012.0255. PMC 3685458.
- Shock, E. L. & Boyd, E. S. "Geomicrobiology and microbial geochemistry:principles of geobiochemistry. Elements 11, 389 –394 (2015).
- Martin, W.; Russell, M. J. (2007). "On the origin of biochemistry at an alkaline hydrothermal vent". Phil. Trans. R. Soc. Lond. B. 362: 1887–1925.
- Nature, Vol 535, 28 July 2016. p.468
- Muller, Anthonie W. J. (7 August 1985). "Thermosynthesis by biomembranes: Energy gain from cyclic temperature changes". Journal of Theoretical Biology. Amsterdam, the Netherlands: Elsevier. 115 (3): 429–453. doi:10.1016/S0022-5193(85)80202-2. ISSN 0022-5193. PMID 3162066.
- Muller, Anthonie W. J. (1995). "Were the first organisms heat engines? A new model for biogenesis and the early evolution of biological energy conversion". Progress in Biophysics and Molecular Biology. Oxford, UK; New York: Pergamon Press. 63 (2): 193–231. doi:10.1016/0079-6107(95)00004-7. ISSN 0079-6107. PMID 7542789.
- Muller, Anthonie W. J.; Schulze-Makuch, Dirk (1 April 2006). "Sorption heat engines: Simple inanimate negative entropy generators". Physica A: Statistical Mechanics and its Applications. Utrecht, the Netherlands: Elsevier. 362 (2): 369–381. arXiv:physics/0507173. Bibcode:2006PhyA..362..369M. doi:10.1016/j.physa.2005.12.003. ISSN 0378-4371.
- Orgel 1987, pp. 9–16
- Perry, Caroline (7 February 2011). "Clay-armored bubbles may have formed first protocells" (Press release). Cambridge, MA: Harvard University. EurekAlert!. Archived from the original on 14 July 2015. Retrieved 20 June 2015.
- Dawkins 1996, pp. 148–161
- Wenhua Huang; Ferris, James P. (12 July 2006). "One-Step, Regioselective Synthesis of up to 50-mers of RNA Oligomers by Montmorillonite Catalysis". Journal of the American Chemical Society. Washington, D.C.: American Chemical Society. 128 (27): 8914–8919. doi:10.1021/ja061782k. ISSN 0002-7863. PMID 16819887.
- Moore, Caroline (16 July 2007). "Crystals as genes?". Highlights in Chemical Science. London: Royal Society of Chemistry. ISSN 2041-5818. Archived from the original on 14 July 2015. Retrieved 21 June 2015.
- Wickramasinghe, Chandra (2011). "Bacterial morphologies supporting cometary panspermia: a reappraisal". International Journal of Astrobiology. 10 (1): 25–30. Bibcode:2011IJAsB..10...25W. doi:10.1017/S1473550410000157.
- Rampelotto, P. H. (2010). Panspermia: A promising field of research. In: Astrobiology Science Conference. Abs 5224.
- Forward planetary contamination like Tersicoccus phoenicis, that has shown resistance to methods usually used in spacecraft assembly clean rooms: Madhusoodanan, Jyoti (19 May 2014). "Microbial stowaways to Mars identified". Nature. doi:10.1038/nature.2014.15249. Archived from the original on 23 May 2014. Retrieved 23 May 2014.
- Webster, Guy (6 November 2013). "Rare New Microbe Found in Two Distant Clean Rooms". NASA.gov. Archived from the original on 7 November 2013. Retrieved 6 November 2013.
- Chang, Kenneth (12 September 2016). "Visions of Life on Mars in Earth's Depths". The New York Times. Archived from the original on 12 September 2016. Retrieved 12 September 2016.
- Clark, Stuart (25 September 2002). "Tough Earth bug may be from Mars". New Scientist. London. ISSN 0262-4079. Archived from the original on 2 December 2014. Retrieved 21 June 2015.
- Horneck, Gerda; Klaus, David M.; Mancinelli, Rocco L. (March 2010). "Space Microbiology". Microbiology and Molecular Biology Reviews. Washington, D.C.: American Society for Microbiology. 74 (1): 121–156. doi:10.1128/MMBR.00016-09. ISSN 1092-2172. PMC 2832349. PMID 20197502.
- Rabbow, Elke; Horneck, Gerda; Rettberg, Petra; et al. (December 2009). "EXPOSE, an Astrobiological Exposure Facility on the International Space Station – from Proposal to Flight". Origins of Life and Evolution of Biospheres. Dordrecht, the Netherlands: Springer. 39 (6): 581–598. Bibcode:2009OLEB...39..581R. doi:10.1007/s11084-009-9173-6. ISSN 0169-6149. PMID 19629743.
- Onofri, Silvano; de la Torre, Rosa; de Vera, Jean-Pierre; et al. (May 2012). "Survival of Rock-Colonizing Organisms After 1.5 Years in Outer Space". Astrobiology. New Rochelle, NY: Mary Ann Liebert, Inc. 12 (5): 508–516. Bibcode:2012AsBio..12..508O. doi:10.1089/ast.2011.0736. ISSN 1531-1074. PMID 22680696.
- "biological abundance of elements". Encyclopedia of Science. Dundee, Scotland: David Darling Enterprises. Archived from the original on 11 March 2012. Retrieved 9 October 2008.
- Hoover, Rachel (21 February 2014). "Need to Track Organic Nano-Particles Across the Universe? NASA's Got an App for That". Ames Research Center. Mountain View, CA: NASA. Archived from the original on 6 September 2015. Retrieved 22 June 2015.
- Chang, Kenneth (18 August 2009). "From a Distant Comet, a Clue to Life". The New York Times. New York. p. A18. ISSN 0362-4331. Archived from the original on 23 June 2015. Retrieved 22 June 2015.
- Goncharuk, Vladislav V.; Zui, O. V. (February 2015). "Water and carbon dioxide as the main precursors of organic matter on Earth and in space". Journal of Water Chemistry and Technology. Dordrecht, the Netherlands: Springer on behalf of Allerton Press. 37 (1): 2–3. doi:10.3103/S1063455X15010026. ISSN 1063-455X.
- Abou Mrad, Ninette; Vinogradoff, Vassilissa; Duvernay, Fabrice; et al. (2015). "Laboratory experimental simulations: Chemical evolution of the organic matter from interstellar and cometary ice analogs" (PDF). Bulletin de la Société Royale des Sciences de Liège. Liège, Belgium: Société royale des sciences de Liège. 84: 21–32. Bibcode:2015BSRSL..84...21A. ISSN 0037-9565. Archived from the original on 13 April 2015. Retrieved 6 April 2015.
- Gallori, Enzo (June 2011). "Astrochemistry and the origin of genetic material". Rendiconti Lincei. Milan, Italy: Springer. 22 (2): 113–118. doi:10.1007/s12210-011-0118-4. ISSN 2037-4631. "Paper presented at the Symposium 'Astrochemistry: molecules in space and time' (Rome, 4–5 November 2010), sponsored by Fondazione 'Guido Donegani', Accademia Nazionale dei Lincei."
- Martins, Zita (February 2011). "Organic Chemistry of Carbonaceous Meteorites". Elements. Chantilly, VA: Mineralogical Society of America. 7 (1): 35–40. doi:10.2113/gselements.7.1.35. ISSN 1811-5209.
- Martins, Zita; Botta, Oliver; Fogel, Marilyn L.; et al. (15 June 2008). "Extraterrestrial nucleobases in the Murchison meteorite". Earth and Planetary Science Letters. Amsterdam, the Netherlands: Elsevier. 270 (1–2): 130–136. arXiv:0806.2286. Bibcode:2008E&PSL.270..130M. doi:10.1016/j.epsl.2008.03.026. ISSN 0012-821X.
- "We may all be space aliens: study". Sydney: Australian Broadcasting Corporation. Agence France-Presse. 14 June 2008. Archived from the original on 23 June 2015. Retrieved 22 June 2015.
- Callahan, Michael P.; Smith, Karen E.; Cleaves, H. James, II; et al. (23 August 2011). "Carbonaceous meteorites contain a wide range of extraterrestrial nucleobases". Proc. Natl. Acad. Sci. U.S.A. Washington, D.C.: National Academy of Sciences. 108 (34): 13995–13998. Bibcode:2011PNAS..10813995C. doi:10.1073/pnas.1106493108. ISSN 0027-8424. PMC 3161613. PMID 21836052.
- Steigerwald, John (8 August 2011). "NASA Researchers: DNA Building Blocks Can Be Made in Space". Goddard Space Flight Center. Greenbelt, MD: NASA. Archived from the original on 23 June 2015. Retrieved 23 June 2015.
- Chow, Denise (26 October 2011). "Discovery: Cosmic Dust Contains Organic Matter from Stars". Space.com. Ogden, UT: Purch. Archived from the original on 14 July 2015. Retrieved 23 June 2015.
- "Astronomers Discover Complex Organic Matter Exists Throughout the Universe". Rockville, MD: ScienceDaily, LLC. 26 October 2011. Archived from the original on 3 July 2015. Retrieved 23 June 2015. Post is reprinted from materials provided by The University of Hong Kong.
- Sun Kwok; Yong Zhang (3 November 2011). "Mixed aromatic–aliphatic organic nanoparticles as carriers of unidentified infrared emission features". Nature. London: Nature Publishing Group. 479 (7371): 80–83. Bibcode:2011Natur.479...80K. doi:10.1038/nature10542. ISSN 0028-0836. PMID 22031328.
- Clemence, Lara; Cohen, Jarrett (7 February 2005). "Space Sugar's a Sweet Find". Goddard Space Flight Center. Greenbelt, MD: NASA. Archived from the original on 5 March 2016. Retrieved 23 June 2015.
- Than, Ker (30 August 2012). "Sugar Found In Space: A Sign of Life?". National Geographic News. Washington, D.C.: National Geographic Society. Archived from the original on 14 July 2015. Retrieved 23 June 2015.
- "Sweet! Astronomers spot sugar molecule near star". Excite. Yonkers, NY: Mindspark Interactive Network. Associated Press. 29 August 2012. Archived from the original on 14 July 2015. Retrieved 23 June 2015.
- "Building blocks of life found around young star". News & Events. Leiden, the Netherlands: Leiden University. 30 September 2012. Archived from the original on 13 December 2013. Retrieved 11 December 2013.
- Jørgensen, Jes K.; Favre, Cécile; Bisschop, Suzanne E.; et al. (20 September 2012). "Detection of the simplest sugar, glycolaldehyde, in a solar-type protostar with ALMA" (PDF). The Astrophysical Journal Letters. Bristol, England: IOP Publishing for the American Astronomical Society. 757 (1): L4. arXiv:1208.5498. Bibcode:2012ApJ...757L...4J. doi:10.1088/2041-8205/757/1/L4. ISSN 2041-8213. L4. Archived (PDF) from the original on 24 September 2015. Retrieved 23 June 2015.
- "'Life chemical' detected in comet". London: BBC News. 18 August 2009. Archived from the original on 25 May 2015. Retrieved 23 June 2015.
- Thompson, William Reid; Murray, B. G.; Khare, Bishun Narain; Sagan, Carl (30 December 1987). "Coloration and darkening of methane clathrate and other ices by charged particle irradiation: Applications to the outer solar system". Journal of Geophysical Research. Washington, D.C.: American Geophysical Union. 92 (A13): 14933–14947. Bibcode:1987JGR....9214933T. doi:10.1029/JA092iA13p14933. ISSN 0148-0227. PMID 11542127.
- Stark, Anne M. (5 June 2013). "Life on Earth shockingly comes from out of this world". Livermore, CA: Lawrence Livermore National Laboratory. Archived from the original on 16 September 2015. Retrieved 23 June 2015.
- Goldman, Nir; Tamblyn, Isaac (20 June 2013). "Prebiotic Chemistry within a Simple Impacting Icy Mixture". Journal of Physical Chemistry A. Washington, D.C.: American Chemical Society. 117 (24): 5124–5131. Bibcode:2013JPCA..117.5124G. doi:10.1021/jp402976n. ISSN 1089-5639. PMID 23639050.
- Carey, Bjorn (18 October 2005). "Life's Building Blocks 'Abundant in Space'". Space.com. Watsonville, CA: Imaginova. Archived from the original on 26 June 2015. Retrieved 23 June 2015.
- Hudgins, Douglas M.; Bauschlicher, Charles W. Jr.; Allamandola, Louis J. (10 October 2005). "Variations in the Peak Position of the 6.2 μm Interstellar Emission Feature: A Tracer of N in the Interstellar Polycyclic Aromatic Hydrocarbon Population" (PDF). The Astrophysical Journal. Bristol, England: IOP Publishing for the American Astronomical Society. 632 (1): 316–332. Bibcode:2005ApJ...632..316H. doi:10.1086/432495. ISSN 0004-637X.
- Des Marais, David J.; Allamandola, Louis J.; Sandford, Scott; et al. (2009). "Cosmic Distribution of Chemical Complexity". Ames Research Center. Mountain View, CA: NASA. Archived from the original on 27 February 2014. Retrieved 2015-06-24. See the Ames Research Center 2009 annual team report to the NASA Astrobiology Institute here "Archived copy". Archived from the original on 1 March 2013. Retrieved 2015-06-24. .
- García-Hernández, Domingo. A.; Manchado, Arturo; García-Lario, Pedro; et al. (20 November 2010). "Formation of Fullerenes in H-Containing Planetary Nebulae". The Astrophysical Journal Letters. Bristol, England: IOP Publishing for the American Astronomical Society. 724 (1): L39–L43. arXiv:1009.4357. Bibcode:2010ApJ...724L..39G. doi:10.1088/2041-8205/724/1/L39. ISSN 2041-8213.
- Atkinson, Nancy (27 October 2010). "Buckyballs Could Be Plentiful in the Universe". Universe Today. Courtenay, British Columbia: Fraser Cain. Archived from the original on 14 July 2015. Retrieved 24 June 2015.
- Marlaire, Ruth, ed. (3 March 2015). "NASA Ames Reproduces the Building Blocks of Life in Laboratory". Ames Research Center. Moffett Field, CA: NASA. Archived from the original on 5 March 2015. Retrieved 5 March 2015.
- Ferus, Martin; Nesvorný, David; Šponer, Jiří; Kubelík, Petr; Michalčíková, Regina; Shestivská, Violetta; Šponer, Judit E.; Civiš, Svatopluk (2015). "High-energy chemistry of formamide: A unified mechanism of nucleobase formation". Proc. Natl. Acad. Sci. U.S.A. 112 (3): 657–662. Bibcode:2015PNAS..112..657F. doi:10.1073/pnas.1412072111. PMC 4311869.
- Lancet, Doron (30 December 2014). "Systems Prebiology-Studies of the origin of Life". The Lancet Lab. Rehovot, Israel: Department of Molecular Genetics; Weizmann Institute of Science. Archived from the original on 26 June 2015. Retrieved 26 June 2015.
- Segré, Daniel; Ben-Eli, Dafna; Deamer, David W.; Lancet, Doron (February 2001). "The Lipid World" (PDF). Origins of Life and Evolution of the Biosphere. Kluwer Academic Publishers. 31 (1–2): 119–145. doi:10.1023/A:1006746807104. ISSN 0169-6149. PMID 11296516. Archived (PDF) from the original on 26 June 2015. Retrieved 11 September 2008.
- Eigen, Manfred; Schuster, Peter (November 1977). "The Hypercycle. A Principle of Natural Self-Organization. Part A: Emergence of the Hypercycle" (PDF). Naturwissenschaften. Berlin: Springer-Verlag. 64 (11): 541–65. Bibcode:1977NW.....64..541E. doi:10.1007/bf00450633. ISSN 0028-1042. PMID 593400. Archived from the original (PDF) on 3 March 2016. Retrieved 13 June 2015.
- Eigen, Manfred; Schuster, Peter (1978). "The Hypercycle. A Principle of Natural Self-Organization. Part B: The Abstract Hypercycle" (PDF). Naturwissenschaften. Berlin: Springer-Verlag. 65: 7–41. Bibcode:1978NW.....65....7E. doi:10.1007/bf00420631. ISSN 0028-1042. Archived from the original (PDF) on 3 March 2016. Retrieved 13 June 2015.
- Eigen, Manfred; Schuster, Peter (July 1978). "The Hypercycle. A Principle of Natural Self-Organization. Part C: The Realistic Hypercycle" (PDF). Naturwissenschaften. Berlin: Springer-Verlag. 65 (7): 341–369. Bibcode:1978NW.....65..341E. doi:10.1007/bf00439699. ISSN 0028-1042. Archived from the original (PDF) on 16 June 2016. Retrieved 13 June 2015.
- Markovitch, Omer; Lancet, Doron (Summer 2012). "Excess Mutual Catalysis Is Required for Effective Evolvability" (PDF). Artificial Life. Cambridge, MA: MIT Press. 18 (3): 243–266. doi:10.1162/artl_a_00064. ISSN 1064-5462. PMID 22662913. Retrieved 2015-06-26.
- Tessera, Marc (2011). "Origin of Evolution versus Origin of Life: A Shift of Paradigm". International Journal of Molecular Sciences. Basel, Switzerland: MDPI. 12 (6): 3445–3458. doi:10.3390/ijms12063445. ISSN 1422-0067. PMC 3131571. PMID 21747687. Special Issue: "Origin of Life 2011"
- Brown, Michael R. W.; Kornberg, Arthur (16 November 2004). "Inorganic polyphosphate in the origin and survival of species". Proc. Natl. Acad. Sci. U.S.A. Washington, D.C.: National Academy of Sciences. 101 (46): 16085–16087. Bibcode:2004PNAS..10116085B. doi:10.1073/pnas.0406909101. ISSN 0027-8424. PMC 528972. PMID 15520374.
- Clark, David P. (3 August 1999). "The Origin of Life". Microbiology 425: Biochemistry and Physiology of Microorganism (Lecture). Carbondale, IL: College of Science; Southern Illinois University Carbondale. Archived from the original on 2000-10-02. Retrieved 2015-06-26.
- Pasek, Matthew A. (22 January 2008). "Rethinking early Earth phosphorus geochemistry". Proc. Natl. Acad. Sci. U.S.A. Washington, D.C.: National Academy of Sciences. 105 (3): 853–858. Bibcode:2008PNAS..105..853P. doi:10.1073/pnas.0708205105. ISSN 0027-8424. PMC 2242691. PMID 18195373.
- Witt, Adolf N.; Vijh, Uma P.; Gordon, Karl D. (2003). "Discovery of Blue Fluorescence by Polycyclic Aromatic Hydrocarbon Molecules in the Red Rectangle". Bulletin of the American Astronomical Society. Washington, D.C.: American Astronomical Society. 35: 1381. Bibcode:2003AAS...20311017W. Archived from the original on 2003-12-19. Retrieved 2015-06-26. American Astronomical Society Meeting 203, #110.17, January 2004.
- "NASA Cooks Up Icy Organics to Mimic Life's Origins". Space.com. Ogden, UT: Purch. 20 September 2012. Archived from the original on 25 June 2015. Retrieved 26 June 2015.
- Gudipati, Murthy S.; Rui Yang (1 September 2012). "In-situ Probing of Radiation-induced Processing of Organics in Astrophysical Ice Analogs—Novel Laser Desorption Laser Ionization Time-of-flight Mass Spectroscopic Studies". The Astrophysical Journal Letters. Bristol, England: IOP Publishing for the American Astronomical Society. 756 (1): L24. Bibcode:2012ApJ...756L..24G. doi:10.1088/2041-8205/756/1/L24. ISSN 2041-8213. L24.
- "NASA Ames PAH IR Spectroscopic Database". NASA. Archived from the original on 29 June 2015. Retrieved 17 June 2015.
- Dartnell, Lewis (12 January 2008). "Did life begin on a radioactive beach?". New Scientist. London (2638): 8. ISSN 0262-4079. Archived from the original on 27 June 2015. Retrieved 26 June 2015.
- Adam, Zachary (2007). "Actinides and Life's Origins". Astrobiology. New Rochelle, NY: Mary Ann Liebert, Inc. 7 (6): 852–872. Bibcode:2007AsBio...7..852A. doi:10.1089/ast.2006.0066. ISSN 1531-1074. PMID 18163867.
- Parnell, John (December 2004). "Mineral Radioactivity in Sands as a Mechanism for Fixation of Organic Carbon on the Early Earth". Origins of Life and Evolution of Biospheres. Kluwer Academic Publishers. 34 (6): 533–547. Bibcode:2004OLEB...34..533P. doi:10.1023/B:ORIG.0000043132.23966.a1. ISSN 0169-6149. PMID 15570707.
- Boltzmann, L. (1886) The Second Law of Thermodynamics, in: Ludwig Boltzmann: Theoretical physics and Selected writings, edited by: McGinness, B., D. Reidel, Dordrecht, The Netherlands, 1974.
- Schrödinger, Erwin (1944) What is Life? The Physical Aspect of the Living Cell. Cambridge University Press
- Onsager, L. (1931) Reciprocal Relations in Irreversible Processes I and II, Phys. Rev. 37, 405; 38, 2265 (1931)
- Prigogine, I. (1967) An Introduction to the Thermodynamics of Irreversible Processes, Wiley, New York
- Dewar, R; Juretić, D.; Županović, P. (2006). "The functional design of the rotary enzyme ATP synthase is consistent with maximum entropy production". Chem. Phys. Lett. 430: 177–182. Bibcode:2006CPL...430..177D. doi:10.1016/j.cplett.2006.08.095.
- Unrean, P., Srienc, F. (2011) Metabolic networks evolve towards states of maximum entropy production, Metabolic Engineering 13, 666-673.
- Zotin, A. I. (1984) Bioenergetic trends of evolutionary progress of organisms, in: Thermodynamics and regulation of biological processes, edited by: Lamprecht, I. and Zotin, A. I., De Gruyter, Berlin, 451-458.
- Schneider, E.D.; Kay, J.J. (1994). "Life as a Manifestation of the Second Law of Thermodynamics". Mathl. Comput. Modelling. 19 (6–8): 25–48. doi:10.1016/0895-7177(94)90188-0.
- Michaelian, K (2005). "Thermodynamic stability of ecosystems". J. Theor. Biol. 237 (3): 323–335. Bibcode:2004APS..MAR.P9015M. doi:10.1016/j.jtbi.2005.04.019.