Human skin color can range from almost black (in skin with very high concentrations of the dark brown pigment melanin) to nearly colorless (appearing pinkish white due to the blood vessels under the skin). Skin color is determined primarily by the amount and type of melanin. Variations in skin color are mainly genetic in origin.
In general, people with ancestors from tropical regions and higher altitudes (who were hence exposed to greater ultraviolet radiation) have darker skin than people with ancestors from middle latitudes. This is far from a hard and fast rule, however, because many light-skinned groups have managed to survive at the equator through social adaptation. The same can be said of dark-skinned groups living at subtropical and temperate latitudes.
Melanin comes in two types: pheomelanin (red) and eumelanin (very dark brown). Both amount and type are determined by four to six genes which operate under incomplete dominance. One copy of each of those genes is inherited from each parent. Each gene comes in several alleles, resulting in the great variety of human skin tones. By absorbing ultraviolet (UV) radiation from the sun, melanin controls the amount that penetrates the skin. UV radiation is needed to manufacture vitamin D, but too much can damage the skin and degrade folate.
The evolution of the different skin tones is thought to have occurred in response to climatic conditions. The haired primate ancestors of humans, like modern great apes, had light skin. When hominids evolved relatively hairless skin (the most likely function of which was to facilitate perspiration) while living in sun-rich Africa, they co-evolved dark skin, which was needed to control the adverse effects of ultraviolet radiation on folate levels. When their descendants migrated to less sun-intensive regions in the north, low vitamin D3 levels became a problem and light skin color re-emerged. Sexual selection and diet may have played a part in the evolution of skin tone diversity as well.
The Inuit and Yupik are special cases: even though they live in an extremely sun-poor environment, they have retained their relatively dark skin, most likely because their traditional fish-based diet provides plenty of vitamin D . This is an example of the risks and benefits of sun exposure.
Geneticists estimate that a relatively small group of humans left Africa about 60,000 years ago, and that the descendants of this group went on to populate the entire non-African world. Those migrants that settled in non-African equatorial regions (such as India, New Guinea and Australia) retained most of the ancestral sequence at the MC1R locus, a gene strongly associated with determining skin color. Specifically, Harding et al. found that the haplotype sequences for Indians and New Guineans are virtually identical to those of continental sub-Saharan Africans, except for a small number of variants at silent sites.
The retention of the ancestral trait at the equator is believed due to natural selection for melanin pigment production which serves to protect the body from harmful UV rays. Notably, given that hair is a part of the skin, the retention is also analogous to that which occurred for Afro-textured hair prior to pre-Holocene admixture events among people who settled in India and Australia. However, certain evidence suggests that, unlike skin color, tightly curled hair ceased to be under strong selection once dark skin arose about one million years ago; rather, it remained as a vestigial trait among Africans, Andamanese and Melanesians and changed to straight in the north for adaptive reasons—-see hair texture). In fact, dark skin is so selectively advantageous at the equator that initially light-skinned native Americans who migrated to Mexico and/or South America experienced renewed selective pressure towards the evolution of dark skin.
According to Norton et al., the light skin observed in Europeans (with deep red and/or yellowish skin tones), non-Indian Southeast Asians, East Asians and North Africans (Maghreb) is due to independent genetic mutations in at least three loci. They concluded that light pigmentation is at least partially due to sexual selection. However, Jablonski postulates that the predominant selective factor was the need for enhanced vitamin D production in northern Eurasia (see hair texture).
There are numerous risks and benefits from sun exposure. Dark skin with large concentrations of melanin protects against ultraviolet light that can produce mutations in skin cells, which in turn may cause skin cancers. Light-skinned persons have about a tenfold greater risk of dying from skin cancer, compared with dark-skinned persons, under equal sunlight exposure. Furthermore, dark skin prevents radiation of UV-A rays from destroying the essential folic acid, derived from B vitamins. Folic acid (or folate) is needed for the synthesis of DNA in dividing cells, and folate deficiency in pregnant women is associated with birth defects.
While dark skin better preserves vitamin B (folate), it can also lead to vitamin D deficiency at higher latitudes, which in turn can cause fatal cancers affecting the colon, lung and prostate. Vitamin D deficiency is also associated with higher risk for rickets, cardiovascular disease, diabetes, and multiple sclerosis. An American study by the USDA found 87% of African Americans to be vitamin D deficient. To address this issue, some countries have programs to ensure fortification of milk with vitamin D.
The advantage of light skin at high latitudes is that it allows more sun absorption, leading to increased production of vitamin D3, necessary for calcium absorption and bone growth. The lighter skin of women at higher latitudes most likely results from the higher calcium needs of women during pregnancy and lactation.
Albinism is a condition characterized by the total or partial absence of melanin, resulting in very light skin, eyes and/or hair and impaired vision. It is caused by an inability to convert tyrosine to melanin and has a genetic basis.
Differences in skin tone are the most readily perceptible phenotypical distinction of human populations. Virtually every society has tended to assign some valuation to skin color differences, especially when these have corresponded with existing political and economic differentiations. However, according to classical scholar Frank Snowden, the Egyptians and Greeks assigned relatively neutral connotations to skin color variation because conquest rather than skin color was the major determinant of slave status. Moreover, as variations in skin tone typically correspond with other social and cultural characteristics, more general trends in perceptions and stereotypes tend to be propagated throughout history and across cultures.
Sexual preference for paleness in women by men has been found in cultures throughout the world. In recent years, several research projects have suggested a general preference for lighter-skinned women by African-American men. In his foreword to Peter Frost's 2005 Fair Women, Dark Men, U. of Washington sociologist Pierre L. van den Berghe summarizes:
However, there is an abundance of negative stereotypes as well. In Western societies, persons with dark coloring, particularly those of African origin, were viewed negatively for centuries. Such stereotypes almost always worked to the advantage of the dominant (white) culture. While there is evidence that these prejudices are fading, the wide level of variance between positive and negative stereotypes for the same skin tone indicates both the power and the arbitrariness of such issues.
By contrast, in other cultures, particularly those without much exposure to modern society and the social biases created towards skin color, there exist a negative cultural reaction to pale skin and it is looked upon unfavorably. A number of indigenous African groups such as the Masai, would abandon their children born with conditions such as albinism and there exists sexual preference for darker skin. Pale skin has been associated with being cursed or evil spirits associated with witch craft in such societies.
The tone of human skin can vary from a dark brown to a nearly colorless pigmentation, which may appear reddish due to the blood in the skin. Europeans generally have lighter skin, hair, and eyes than any other group, although this is not always the case. Africans generally have darker skin, hair, and eyes, although this too is not universal. For practical purposes, such as exposure time for sun tanning, six skin types are distinguished following Fitzpatrick (1975), listed in order of decreasing lightness:
|type||also called||tanning behavior||hair and eye color||von Luschan scale|
|I||very light, or "nordic" or "celtic" ||Often burns, occasionally tans.||Tends to have freckles, red, brown, auburn, chestnut, or blond hair, blue, hazel, green or grey eyes.||1-5|
|II||light, or light-skinned European||Usually burns, sometimes tans||Tends to have light or dark hair, blue, green, hazel, brown or grey eyes.||6-10|
|III||light intermediate, or dark-skinned European ||Rarely burns, usually tans.||Usually has brown hair and blue, green, hazel, brown, or, rarely, dark brown eyes.||11-15|
|IV||dark intermediate, also "Mediterranean" or "olive skin"||Rarely burns, often tans.||Tends to have black to dark brown hair and blue, green, hazel, brown or dark brown eyes.||16-21|
|V||dark or "brown" type||Naturally brown skin||Black hair and brown or hazel eyes.||22-28|
|VI||very dark, or "black" type||Naturally black-brown skin||Black hair and dark brown eyes, with minor variations.||29-36|
In attempting to discover the mechanisms that have generated such a wide variation in human skin tone, Jablonski and Chaplin discovered that there is a high correlation between the tone of human skin of indigenous peoples and the average annual ultraviolet (UV) radiation available for skin exposure where the indigenous peoples live. Accordingly, Jablonski and Chaplin plotted the skin tone (W) of indigenous peoples who have stayed in the same geographical area for the last 500 years versus the annual UV available for skin exposure (AUV) for over 200 indigenous persons and found that skin tone lightness W is related to the annual UV available for skin exposure AUV according to
where the skin tone lightness W is measured as the percentage of light reflected from the upper inner arm at which location on humans there should be minimal tanning of human skin due to personal exposure to the sun; a lighter skinned human would reflect more light and would have a higher W number. Judging from the above linear fit to the empirical data, the theoretical lightness maximum of human skin would reflect only 70 per cent of incident light for a hypothetical indigenous human-like population that lived where there was zero annual UV available for skin exposure (AUV = 0 in the above formula). Jablonski and Chaplin evaluated average annual UV available for skin exposure AUV from satellite measurements that took into consideration the measured daily variation in the thickness of the ozone layer that blocked UV hitting the Earth, measured daily variation in opacity of cloud cover, and daily change in angle at which the sunlight containing UV radiation strikes the Earth and passes through different thicknesses of Earth's atmosphere at different latitudes for each of the different human indigenous peoples' home areas from 1979 to 1992.
Jablonski and Chaplin proposed an explanation for the observed variation of untanned human skin with annual UV exposure. By Jablonski and Chaplin's explanation, there are two competing forces affecting human skin tone:
Jablonski and Chaplin note that when human indigenous peoples have migrated, they have carried with them a sufficient gene pool so that within a thousand years, the skin of their descendants living today has turned dark or turned light to adapt to fit the formula given above—with the notable exception of dark-skinned peoples moving north, such as to populate the seacoast of Greenland, to live where they have a year-round supply of food rich in vitamin D, such as fish, so that there was no necessity for their skin to lighten to let enough UV under their skin to synthesize the vitamin D that humans need for healthy bones.
In considering the tone of human skin in the long span of human evolution, Jablonski and Chaplin note that there is no empirical evidence to suggest that the hominid ancestors six million years ago had a skin tone different from the skin tone of today's chimpanzees—namely light-skinned under black hair. But as humans evolved to lose their body hair a parallel evolution permitted human populations to turn their base skin tone dark or light to adjust to the competing demands of 1) increasing eumelanin to protect from UV that was too intense and 2) reducing eumelanin so that enough UV would penetrate to synthesize enough vitamin D. By this explanation, prior to Homo sapiens colonization of extra-African territories, humans had dark skin given that they lived for extended periods of time where the sunlight is intense. As some humans migrated north, over time they developed light skin.
Several genes have been invoked to explain variations of skin tones in humans, including SLC45A2, ASIP, TYR, and OCA2. A recently discovered gene, SLC24A5 has been shown to account for a substantial fraction of the difference in the average of 30 or so melanin units between Europeans and Africans.
Accordingly, the MC1R gene specifies the amino acid sequence in the receptor protein that relays through the cell membrane the hormone signal from the pituitary gland to produce the melanin that makes human skin very dark. Many variations in the amino acid sequence of this receptor protein result in lighter or darker skin.
The human MC1R gene consists of a string of 954 nucleotides, where each nucleotide is one of the four bases Adenine (A), Guanine (G), Thymine (T), or Cytosine (C). But 261 of the nucleotides in the MC1R gene can change with no effect on the amino acid sequence in the receptor protein produced from the gene. For example, the nucleotide triplets GGT, GGC, GGA, and GGG are all synonymous and all produce the amino acid Glycine, so a mutation in the third position in the triplet GGT is a "silent mutation" and has no effect on the amino acid produced from the triplet. Harding et al (2000, p. 1355) analyzed the amino acid sequences in the receptor proteins from 106 individuals from Africa and 524 individuals from outside Africa to find why the tone of the sampled Africans' skin was dark.
Harding found that there were zero differences among the Africans for the amino acid sequences in their receptor proteins, so the skin of each individual from Africa was dark. In contrast, among certain (European) non-African individuals, there were 18 different amino acid sites in which the receptor proteins differed, and each amino acid that differed from the African receptor protein resulted in skin lighter than the skin of the African (and other equatorial) individuals. Nonetheless, the variations in the 261 silent sites in the MC1R were similar between the Africans and non-Africans, so the basic mutation rates among the Africans and non-Africans were the same. Also, close examination of the haplotype variation among the non-Europeans (including East Asians) suggested that, among most non-European non-Africans, the most common variants were in the silent mutation positions. Thus, at least at this locus, most non-Europeans share the ancestral function. The fact that relatively light skinned east Asians varied little genetically from dark skinned Africans at this locus supports the conclusion that skin color is a complex trait determined by several genes. Thus light skin among east Asians occurs by way of a different genetic mechanism than that among Europeans.
With regard to Europeans, the next question to ask would be: why were there zero differences and no divergences in the amino acid sequences of the receptor protein among the Africans (and other equatorial groups) while there were 18 differences among the populations in Ireland, England, and Sweden? Harding et al (2000, pp. 1359–1360) concluded that the intense sun in Africa created an evolutionary constraint that severely reduced the survival rates of progeny with any difference in the 693 sites of the MC1R gene that resulted in even one small change in the amino acid sequence of the receptor protein—because any variation from the African receptor protein produced significantly lighter skin that gave less protection from the intense African sun. In contrast, in Sweden, for example, the sun was so weak that no mutation in the receptor protein reduced the survival probability among progeny.
Indeed, for the individuals from Ireland, England, and Sweden, the mutation variations among the 693 gene sites that caused changes in amino acid sequence were the same as those in the 261 gene sites at which silent mutations still produced the same amino acid sequence. Thus, Harding concluded that the intense sun in Africa selectively increased mortality rates among the progeny of individuals who had a mutation in the MC1R gene that made the skin lighter. However, the mutation rate toward lighter skin in the progeny of those Africans who had moved North to areas with weaker sun was comparable to that of the people whose ancient ancestors grew up in Sweden. Hence, Harding concluded, the lightness of human skin was a direct result of random mutations in the MC1R gene that were non-lethal at the latitudes of Sweden. Even the mutations that produce red hair with little ability to tan were non-lethal in the northern latitudes.
Rogers, Iltis & Wooding (2004) examined Harding's data on the variation of MC1R nucleotide sequences for people of different ancestry to determine the most probable progression of the skin tone of human ancestors over the last five million years. Comparing the MC1R nucleotide sequences for chimpanzees and humans in various regions of the Earth, Rogers concluded that the common ancestors of all humans had light skin tone under dark hair—similar to the skin tone and hair color pattern of today's chimpanzees. That is, five million years ago the human ancestors' dark hair protected their light skin from the intense African sun so that there was no evolutionary constraint that killed off the progeny of those who had mutations in the MC1R nucleotide sequences that made their skin light. Sweet (2002) argues that cave paintings suggest Europeans may have been dark as recently as 13,000 years ago. The painters depicted themselves as having darker complexions than the animals they hunted.
However, over 1.2 million years ago, judging from the numbers and spread of variations among human and chimpanzee MC1R nucleotide sequences, the human ancestors in Africa began to lose their hair and they came under increasing evolutionary pressures that killed off the progeny of individuals who retained the inherited lightness of skin. Folate breakdown in sun-exposed skin is inhibited by the presence of melanin, and folate is essential for human fetal development. It is likely that folate conservation played an important role in the selection of dark skin in the ancient African ancestors of modern humans. By 1.2 million years ago, all people having descendants today had exactly the receptor protein of today's Africans; their skin was dark, and the intense sun killed off the progeny with any lighter skin that resulted from mutational variation in the receptor protein.
However, the progeny of those humans who migrated North away from the intense African sun had another evolutionary constraint: vitamin D availability. Human requirements for vitamin D (cholecalciferol) are in part met through photoconversion of a precursor to vitamin D3. As humans migrated north from the equator, they were exposed to less intense sunlight, in part because of the need for greater use of clothing to protect against the colder climate. Thus, under these conditions, evolutionary pressures would tend to select for lighter-skinned humans as there was less photodestruction of folate and a greater need for photogeneration of cholecalciferol. Tracking back the statistical patterns in variations in DNA among all known people sampled who are alive on the Earth today, it appears that
Approximately 10% of the variance in skin color occurs within groups, and ~90% occurs between groups. This distribution of skin color and its geographic patterning — with people whose ancestors lived predominantly near the equator having darker skin than those with ancestors who lived predominantly in higher latitudes — indicate that this attribute has been under strong selective pressure. Darker skin appears to be strongly selected for in equatorial regions to prevent sunburn, skin cancer, the photolysis of folate, and damage to sweat glands.
A leading hypothesis for the selection of lighter skin in higher latitudes is that it enables the body to form greater amounts of vitamin D, which helps prevent rickets. Evidence for this includes the finding that a substantial portion of the differences of skin color between Europeans and Africans resides in a single gene, SLC24A5 the threonine-111 allele of which was found in 98.7 to 100% among several European samples, while the alanine-111 form was found in 93 to 100% of samples of Africans, East Asians and Indigenous Americans. However, the vitamin D hypothesis is not universally accepted, and lighter skin in high latitudes may correspond simply to an absence of selection for dark skin. Melanin which serves as the pigment, is located in the epidermis of the skin, and is based on hereditary gene expression.
Because skin color has been under strong selective pressure, similar skin colors can result from convergent adaptation rather than from genetic relatedness. Sub-Saharan Africans, populations from southern India, and Indigenous Australians have similar skin pigmentation, but genetically they are no more similar than are other widely separated groups. Furthermore, in some parts of the world in which people from different regions have mixed extensively, the connection between skin color and ancestry has been substantially weakened . In Brazil, for example, skin color is not closely associated with the percentage of recent African ancestors a person has, as estimated from an analysis of genetic variants differing in frequency among continent groups .
Considerable speculation has surrounded the possible adaptive value of other physical features characteristic of groups, such as the constellation of facial features observed in many eastern and northeastern Asians. However, any given physical characteristic generally is found in multiple groups, and demonstrating that environmental selective pressures shaped specific physical features will be difficult, since such features may have resulted from sexual selection for individuals with certain appearances or from genetic drift.