(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 (Harding et al. 2000 p 1355). 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 fact 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 regards 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 reduced severely the survival 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 of 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 was the same as the mutation variations 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 killed off 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 African individuals who had moved North to areas with weaker sun was comparable to the mutation rate of the folks whose ancient ancestors grew up in Sweden. Hence, Harding concluded that 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 5 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 based on cave paintings, 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 that retained the inherited lightness of their skin. Folate breakdown in sun-exposed skin is inhibited by the presence of melanin and 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 (
Rogers, Iltis & Wooding 2004, p. 107).
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
- From ~1.2 million years ago for at least ~1.35 million years, the ancestors of all people alive were as dark as today's Africans.
- The descendants of any pre-historic people who migrate North from the equator will mutate to become light over time because the evolutionary constraint keeping Africans' skin dark decreases generally the further North a people migrates[19]. This also occurs as a result of selection for light skin due to the need to produce vitamin D by way of the penetration of sunlight into the skin (the exception being if dietary sources of vitamin D are available—see the Inuit).
- The genetic mutations leading to light skin among East Asians are different from those of Europeans, suggesting that, following the migration out of Africa, the two groups became distinct populations that experienced a similar selective pressure due to settlement in northern latitudes.