Early expansions of hominins out of Africa
Several expansions of Homo (archaic humans) out of Africa and throughout Eurasia took place in the course of the Lower Paleolithic, and into the beginning Middle Paleolithic, between about 2 and 0.2 million years ago (Ma). These expansions also known, collectively, as Out of Africa I, in contrast to the expansion of Homo sapiens (anatomically modern humans) into Eurasia, which may have begun shortly after 0.2 million years ago, dubbed "Out of Africa II".
The earliest presence of Homo (or indeed any hominin) outside of Africa, dates to close to 2 million years ago. A 2018 study claims evidence for human presence Shangchen, central China, as early as 2.12 Ma based on magnetostratigraphic dating of the lowest layer containing stone artefacts. The oldest known human remains found outside of Africa were found in Dmanisi, Georgia (Dmanisi skull 4), dated to 1.8 Ma and classified as Homo erectus georgicus.
Until the early 1980s, early humans were thought to have been restricted to the African continent in the Early Pleistocene, or until about 0.8 Ma; Hominin migrations outside East Africa were apparently rare in the Early Pleistocene, leaving a fragmentary record of events.
Evolution in Africa
Australopithecina emerge about 5.6 million years ago, in East Africa (Afar Depression). Gracile australopithecines (Australopithecus afarensis) emerge in the same region, around 4 million years ago. The earliest known retouched tools were found in Lomekwi, Kenya, and date back to 3.3 Ma, in the late Pliocene. They might be the product of Australopithecus garhi or Paranthropus aethiopicus, the two known hominins contemporary with the tools.
Homo is assumed to have emerged by around 2 million years ago, Homo habilis being found at about 2.1 million years ago at Lake Turkana, Kenya. The delineation of the "human" genus, Homo, from Australopithecus is somewhat contentious, for which reason the superordinate term "hominin" is often used to include both. "Hominin" technically includes pre-human species as old as 10 million years ago (the separation of Homininae into Hominini and Gorillini).
Possibly pre-human hominin expansion out of Africa has been associated with Graecopithecus and Ouranopithecus, found in Greece and Anatolia and dated to c. 8 million years ago, but these are probably Homininae but not Hominini. Possibly related are the Trachilos footprints found in Crete, dated to close to 6 million years ago.
"Early hominin expansion out of Africa" is closely related to the emergence of Homo erectus just after 2 million years ago. The two species would have lived face to face in East Africa for nearly half a million years.
The oldest Homo erectus fossils appear almost contemporaneously, shortly after two million years ago, both in Africa and in the Caucasus. The ancestors of Indonesian Homo floresiensis may have left Africa even earlier.
The earliest well-dated Eurasian H. erectus site is Dmanisi in Georgia, securely dated to 1.8 Ma. A skull found at Dmanisi is evidence for caring for the old. The skull shows that this Homo erectus was advanced in age and had lost all but one tooth years before death, and it is perhaps unlikely that this hominid would have survived alone. It is not certain, however, that this is sufficient proof for caring – a partially paralysed chimpanzee at the Gombe reserve survived for years without help. The earliest known evidence for African H. erectus, dubbed Homo ergaster, is a single occipital bone (KNM-ER 2598), described as "H. erectus-like", and dated to about 1.9 Ma (contemporary with Homo rudolfensis). This is followed by a fossil gap, the next available fossil being KNM-ER 3733, a skull dated to 1.6 Ma. Early Pleistocene sites in North Africa, the geographical intermediate of East Africa and Georgia, are in poor stratigraphic context. The earliest of the dated is Ain Hanech in northern Algeria (c. 1.8 – 1.2 Ma), an Oldowan grade layer. These sites attest that early Homo erectus have crossed the North African tracts, which are usually hot and dry.:2 There is little time between Homo erectus’ apparent arrival in South Caucasus around 1.8 Ma, and its probable arrival in East and Southeast Asia. There is evidence of H. erectus in Yuanmou, China, dating to 1.7 Ma and in Sangiran, on Java, Indonesia, from 1.66 Ma.
Ferring et al. (2011) suggest that it was still Homo habilis that reached West Asia, and that early H. erectus developed there. H. erectus would then have dispersed from West Asia, to East Asia (Peking Man) Southeast Asia (Java Man), back to Africa (Homo ergaster), and to Europe (Tautavel Man).
It appears H. erectus took longer to move into Europe, the earliest site being Barranco León in southeastern Spain dated to 1.4 Ma. and a controversial Pirro Nord in Southern Italy, allegedly from 1.7 – 1.3 Ma.
In any case, by 1 Ma, Homo erectus had settled in most of the Old World. It is hard to say, however, whether settlement was continuous in Western Europe, or if successive waves repopulated the territory in glacial interludes. Early Acheulean tools at Ubeidiya from 1.4 Ma is some evidence for a continuous settlement in the West, as successive waves out of Africa after then would likely have brought Acheulean technology to Western Europe.
Presence of Lower Paleolithic human remains in Indonesian islands is good evidence for seafaring by Homo erectus late in the Early Pleistocene. Bednarik suggests that navigation had appeared by 1 Ma, possibly to exploit offshore fishing grounds. He has reproduced a primitive dirigible raft to demonstrate the feasibility of faring across the Lombok Strait on such a device, which he believes to have been done before 850 ka. The strait has maintained a width of at least 20 km for the whole of the Pleistocene. Such an achievement by Homo erectus in the Early Pleistocene offers some strength to the suggested water routes out of Africa, as the Gibraltar, Sicilian, and Bab-el-Mandeb exit routes are harder to consider if watercraft are deemed beyond the capacities of Homo erectus.
Routes out of Africa
The Sinai Peninsula is the simplest African exit route, being since the Pliocene the only land bridge between the two continents of the Old World. Unless one argues for watercraft on behalf of Homo erectus, it is surely the only way out. However, it was hard to access until the Middle Pleistocene. The Nile was a much smaller river and followed a different course.
There are two Eurasian entryways that take advantage of the Sinai. First, the Levantine corridor, which moves north along the Eastern Mediterranean. Second, down the eastern bank of the Red Sea. Archaeological efforts in Arabia are limited, and attention is usually given to the Levantine corridor.
The Bab-el-Mandeb is a 30 km strait between East Africa and the Arabian Peninsula, with a small island, Perim, 3 km off the Arabian bank. The strait has a major appeal in the study of Eurasian expansion in that it brings East Africa in direct proximity with Eurasia. It does not require hopping from one water body to the next across the North African desert.
The land connection with Arabia disappeared in the Pliocene, and though it may have briefly reformed, the evaporation of the Red Sea and associated increase in salinity would have left traces in the fossil record after just 200 years and evaporite deposits after 600 years. Neither have been detected. A strong current flows from the Red Sea into the Indian Ocean and crossing would have been difficult without a land connection.
Strait of Gibraltar
The Strait of Gibraltar is the Atlantic entryway to the Mediterranean, where Spanish and Moroccan banks are only 14 km apart. A decrease in sea levels in the Pleistocene due to glaciation would not have brought this down to less than 10 km. Deep currents push westwards, and surface water flows strongly back into the Mediterranean.
Entrance into Eurasia across the strait of Gibraltar could explain the hominin remains at Barranco León in southeastern Spain (1.4 Ma) and Sima del Elefante in northern Spain (1.2 Ma). But the site of Pirro Nord in southern Italy, allegedly from 1.3 – 1.7 Ma, suggests a possible arrival from the East. Resolution is insufficient to settle the matter.
Strait of Sicily
The modern Strait of Sicily separates Tunisia and Sicily by 145 km, but is shallow and would have been much narrower in glacial maxima. We have a poor understanding of plate tectonics of this area for the greater part of the Pleistocene. But while plate tectonics could have made the strait narrower than predicted by the lowering of sea levels alone, contrast of Pleistocene fauna strongly argues against an actual land bridge.:3 Since the strait is only 400 km away from the North African hominin site of Ain Hanech in Algeria (1.8 Ma or 1.2 Ma) it remains a plausible route for Early Pleistocene expansion into Eurasia. But there is close to no evidence for a hominin passage. Alimen based most of her argument in favour of such a migration on Bianchini's discovery of Sicilian Oldowan grade tools. Radiometric dates, however, have not been produced, and the artefacts might as well be from the Middle Pleistocene.
Causes for dispersal
Climate change and hominin flexibility
For a given species in a given environment, available resources will limit the number of individuals that can survive indefinitely. This is the carrying capacity. Upon reaching this threshold, individuals may find it easier to gather resources in the poorer yet less exploited peripheral environment than in the preferred habitat. Homo habilis could have developed some baseline behavioural flexibility prior to its expansion into the peripheries (such as encroaching into the predatory guild). This flexibility could then have been positively selected and amplified, leading to Homo erectus' adaptation to the peripheral open habitats. A new and environmentally flexible hominin population could have come back to the old niche and replace the ancestral population. Moreover, some step-wise shrinking of the woodland and the associated reduction of hominin carrying capacity in the woods around 1.8 Ma, 1.2 Ma, and 0.6 Ma would have stressed the carrying capacity's pressure for adapting to the open grounds.
Lithic analysis implies that Oldowan hominins were not predators. However, Homo erectus appears to have followed animal migrations to the north during wetter periods, likely as a source of scavenged food. The sabre-tooth cat Megantereon was an apex predator of the Early and Middle Pleistocene (before MIS 12). It became extinct in Africa c. 1.5 Ma, but had already moved out through the Sinai, and is among the faunal remains of the Levantine hominin site of Ubeidiya, c. 1.4 Ma. It could not break bone marrow and its kills were likely an important food source for hominins, especially in glacial periods.
In colder Eurasian times, the hominin diet would have to be principally meat-based and Acheulean hunters must have competed with cats.
Coevolved zoonotic diseases
Bar-Yosef and Belfer-Cohen suggest that the success of hominins within Eurasia once out of Africa is in part due to the absence of zoonotic diseases outside their original habitat. Zoonotic diseases are those that are transmitted from animals to humans. While a disease specific to hominins must keep its human host alive long enough to transmit itself, zoonotic diseases will not necessarily do so as they can complete their life cycle without humans. Still, these infections are well accustomed to human presence, having evolved alongside them. The higher an African ape's population density, the better a disease fares. 55% of chimps at the Gombe reserve die of disease, most of them zoonotic. The majority of these diseases are still restricted to hot and damp African environments. Once hominins had moved out into drier and colder habitats of higher latitudes, one major limiting factor in population growth was out of the equation.
While Homo habilis was certainly bipedal, its long arms are indicative of an arboreal adaptation. Homo erectus had longer legs and shorter arms, revealing a transition to obligate terrestriality, though it remains unclear how this change in relative leg length might have been an advantage. Sheer body size, on the other hand, seems to have allowed for better walking energy efficiency and endurance. A larger Homo erectus would also dehydrate more slowly and could thus cover greater distances before facing thermoregulatory limitations. The ability for prolonged walking at a normal pace would have been a decisive factor for effective colonisation of Eurasia.
Thermoregulation and dehydration are major problems that need to be dealt with to move into the open grasslands. In particular, vascularisation of the brain is crucial in maintaining it in the narrow frame of tolerable temperatures.
Bones of the higher cranium grow in response to expansion of cerebral mass, in such a way that brain tissue and blood vessels mold the inner brain case. Endocranial casts of fossil skulls allow approximating brain vascularisation. Dean Falk noticed that a single large vessel, the occipital marginal sinus, was responsible for irrigating most of the brain in early australopiths (Australopithecus afarensis, Paranthropus robustus and boisei). The vessel grew smaller with time to be progressively replaced by a network of small veins in later hominins, starting with Homo habilis and continuing well into Eurasia. She interprets the change as an adaptation to cool the brain, which she uses to advance her "radiator theory" for accelerated encephalisation from Homo habilis onwards. To Falk, bipedalism, which predates large brains, favoured a rewiring of cerebral blood vessels into a gravity-assisted irrigation network, itself allowing the cool down needed for encephalisation.
Endocranial casts of Homo habilis and Homo erectus differ in the organisation of the frontal lobe, in particular in the prefrontal cortex where higher mental functions of consciousness and abstraction occur. By themselves, mental capacities have likely played a role in the success of Eurasian colonisation. They would have allowed for greater social complexity, predation and sharing prey, and an overall higher quality diet.
According to Wheeler, loss of functional body hair would have helped prevent hyperthermia, since hair will obstruct air flow over the skin and restrict cooling by evaporation. He further suggests that body cooling due to hair loss has relieved a thermal constraint on brain size (but in a response to Falk's radiator hypothesis, Ralph Holloway maintains that there is no evidence for a temperature constraint on brain size). It remains unclear whether hair loss was part of human adaptation or preadaptation to Eurasia.
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