Differences in human skin color are commonly explained as an adaptive response to solar UV radiation and latitude. The further away from the equator you are, the weaker will be solar UV and the less your skin will need melanin to prevent sunburn and skin cancer.
A variant of this explanation involves vitamin D, which the body needs to make strong bones and which the skin produces with the help of UV-B. The further away from the equator you are, the lighter your skin will be to let enough UV-B into its tissues for vitamin D production. Or so the explanation goes.
To test this hypothesis, Osborne et al. (2008) measured skin color and bone strength in a hundred white and Asian adolescent girls from Hawaii. Skin color was measured at the forehead and the inner arm. Bone strength was measured by section modulus (Z) and bone mineral content (BMC) at the proximal femur. A multiple regression was then performed to investigate the influences of skin color, physical activity, age, ethnicity, developmental age, calcium intake, and lean body mass on Z and BMC. Result: no significant relationship between skin color and bone strength.
Is there, in fact, any hard evidence that humans vary in skin color because they need to maintain the same level of vitamin D production in the face of varying levels of UV-B? Robins (1991, pp. 204-205) found the data to be unconvincing when he reviewed the literature. In particular, there seems to be little relationship between skin color and blood levels of 25-OHD—one of the main circulating metabolites of vitamin D:
The vulnerability of British Asians to rickets and osteomalacia has been ascribed in part to their darker skin colour, but this idea is not upheld by observations that British residents of West Indian (Afro-Caribbean) origin, who have deeper skin pigmentation than the Asians, very rarely manifest clinical rickets … Moreover, artificial irradiation of Asian, Caucasoid and Negroid subjects with UV-B produced similar increases in blood 25-OHD levels irrespective of skin pigmentation … A study under natural conditions in Birmingham, England, revealed comparable increases in 25-OHD levels after the summer sunshine from March to October in groups of Asians, West Indians and Caucasoids … This absence of a blunted 25-OHD response to sunlight in the dark-skinned West Indians at high northerly latitudes (England lies farther north than the entire United States of America except for Alaska) proves that skin colour is not a major contributor to vitamin D deficiency in northern climes.
The higher incidence of rickets in British Asians probably has less to do with their dark color than with their systematic avoidance of sunlight (to remain as light-skinned as possible).
Skin color and natural selection via solar UV
Solar UV seems to be a weak agent of natural selection, be it through sunburn, skin cancer, or vitamin D deficiency. Brace et al. (1999) studied skin color variation in Amerindians, who have inhabited their continents for 12,000-15,000 years, and in Australian Aborigines, who have inhabited theirs for some 50,000 years. Assuming that latitudinal skin-color variation in both groups tracks natural selection by solar UV, their calculations show that this selection would have taken over 100,000 years to create the skin-color difference between black Africans and northern Chinese and ~ 200,000 years to create the one between black Africans and northern Europeans (Brace et al., 1999). Yet modern humans began to spread out of Africa only about 50,000 years ago. Clearly, something other than solar UV has also influenced human variation in skin color … and one may wonder whether lack of solar UV has played any role, via natural selection, in the extreme whitening of some human populations.
Indeed, people seem to do just fine with a light brown color from the Arctic Circle to the equator. Skeletal remains from pre-contact Amerindian sites show little evidence of rickets or other signs of vitamin D deficiency—even at latitudes where Amerindian skin is much darker than European skin (Robins, 1991, p. 206).
Why, then, are Europeans so fair-skinned when ground-level UV radiation is equally weak across Europe, northern Asia, and North America at all latitudes above 47º N? (Jablonski & Chaplin, 2000). Proponents of the vitamin D hypothesis will point to the Inuit and say that non-Europeans get enough vitamin D at high northerly latitudes from fatty fish. So they don’t need light skin. In actual fact, if we look at the indigenous peoples of northern Asia and North America above 47º N, most of them live far inland and get little vitamin D from their diet. For instance, although the Athapaskans of Canada and Alaska live as far north as the Inuit and are even somewhat darker-skinned, their diet consists largely of meat from land animals (caribou, deer, ptarmigan, etc.). The same may be said for the native peoples of Siberia.
Conversely, fish consumption is high among the coastal peoples of northwestern Europe. Skeletal remains of Danes living 6,000-7,000 years ago have the same carbon isotope profile as those of Greenland Inuit, whose diet is 70-95% of marine origin (Tauber, 1981). So why are Danes so light-skinned despite a diet that has long included fatty fish?
Skin color and sexual selection via male choice
Latitudinal variation in human skin color is largely an artefact of very dark skin in sub-Saharan agricultural peoples and very light skin in northern and eastern Europeans. Elsewhere, the correlation with latitude is much weaker. Indeed, human skin color seems to be more highly correlated with the incidence of polygyny than with latitude (Manning et al., 2004).
This second correlation is especially evident in sub-Saharan Africa, where high-polygyny agriculturalists are visibly darker than low-polygyny hunter-gatherers (i.e., Khoisans, pygmies) although both are equally indigenous. Year-round agriculture allows women to become primary food producers, thereby freeing men to take more wives. Thus, fewer women remain unmated and men are less able to translate their mate-choice criteria into actual mate choice. Such criteria include a preference, widely attested in the African ethnographic literature, for so-called ‘red’ or ‘yellow’ women — this being part of a general cross-cultural preference for lighter-skinned women (van den Berghe & Frost, 1986). Less mate choice means weaker sexual selection for light skin in women and, hence, less counterbalancing of natural selection for dark skin in either sex to protect against sunburn and skin cancer. Result: a net increase in selection for dark skin.
Just as weaker sexual selection may explain the unusually dark skin of sub-Saharan agricultural peoples, stronger sexual selection may explain the unusually light skin of northern and eastern Europeans, as well as other highly visible color traits.
Among early modern humans, sexual selection of women varied in intensity along a north-south axis. First, the incidence of polygyny decreased with distance from the equator. The longer the winter, the more it cost a man to provision a second wife and her children, since women could not gather food in winter. Second, the male death rate increased with distance from the equator. Because the land could not support as many game animals per unit of land area, hunting distance increased proportionately and hunters more often encountered mishaps (drowning, falls, cold exposure, etc.) or ran out of food, especially if other food sources were scarce.
Sexual selection of women was strongest where the ratio of unmated women to unmated men was highest. This would have been in the ‘continental Arctic’, a steppe-tundra environment where women depended the most on men for food and where hunting distances were the longest (i.e., long-distance hunting of highly mobile herds with no alternate food sources). Today, this environment is confined to the northern fringes of Eurasia and North America. As late as 10,000 years ago, it reached much further south. This was particularly so in Europe, where the Scandinavian icecap had pushed the continental Arctic down to the plains of northern and eastern Europe (Frost, 2006).
The same area now corresponds to a zone where skin is almost at the physiological limit of depigmentation and where hair and eye color have diversified into a broad palette of vivid hues. This ‘European exception’ constitutes a major deviation from geographic variation in hair, eye, and skin color (Cavalli-Sforza et al., 1994, pp. 266-267).
Brace, C.L., Henneberg, M., & Relethford, J.H. (1999). Skin color as an index of timing in human evolution. American Journal of Physical Anthropology, 108 (supp. 28), 95-96.
Cavalli-Sforza, L.L., Menozzi, P., & Piazza, A. (1994). The History and Geography of Human Genes. Princeton: Princeton University Press.
Frost, P. (2006). European hair and eye color – A case of frequency-dependent sexual selection? Evolution and Human Behavior, 27, 85-103
Jablonski, N.G. & G. Chaplin. (2000). The evolution of human skin coloration, Journal of Human Evolution, 39, 57-106.
Manning, J.T., Bundred, P.E., & Mather, F.M. (2004). Second to fourth digit ratio, sexual selection, and skin colour. Evolution and Human Behavior, 25, 38-50.
Osborne, D.L., C.M. Weaver, L.D. McCabe, G.M. McCabe, R. Novotony, C. Boushey, & D.A. Savaiano. (2008). Assessing the relationship between skin pigmentation and measures of bone strength in adolescent females living in Hawaii. American Journal of Physical Anthropology, 135(S46), 167.
Robins, A.H. (1991). Biological perspectives on human pigmentation. Cambridge Studies in Biological Anthropology, Cambridge: Cambridge University Press.
Tauber, H. (1981). 13C evidence for dietary habits of prehistoric man in Denmark. Nature, 292, 332-333.
van den Berghe, P.L., & Frost, P. (1986). Skin color preference, sexual dimorphism and sexual selection: A case of gene-culture co-evolution? Ethnic and Racial Studies, 9, 87-113.