1. To absorb calcium and phosphorus from food passing through the gut, humans need vitamin D. This vitamin is either produced in the skin through the action of UV-B light or obtained from certain food sources, notably fatty fish.
2. Humans are often vitamin-D deficient, even in tropical regions where UV-B exposure is intense and continual. This deficiency has led to high frequencies of rickets in many populations, particularly western Europeans and North Americans during the great rickets epidemic from c. 1600 to the mid-20th century. This epidemic occurred in areas where human skin was already producing sub-optimal levels of vitamin D because of the naturally weak sunlight at northern latitudes. These levels then fell even further wherever the Industrial Revolution had reduced sun exposure through air pollution, tall buildings, and indoor factory life.
3. If ancestral humans were often sub-optimal for vitamin D, natural selection should have favored lighter skin color, as a way to produce more of this vitamin by allowing more UV-B into the skin. Such selection, however, would have been counterbalanced in the tropical zone by selection for darker skin, to prevent sunburn and skin cancer.
4. This equilibrium would have ceased once ancestral humans had left the tropical zone. On the one hand, selection for darker skin would have relaxed, there being less sunburn and skin cancer. On the other, selection for lighter skin would have increased, there being less UV-B for vitamin-D production.
Ancestral humans thus began to lighten in skin color once they had entered Europe’s northern latitudes. This selection pressure eventually drove European skin color almost to the limit of depigmentation.
Were ancestral Europeans deficient for vitamin D?
There are several problems with the vitamin-D hypothesis. First, if lack of this vitamin created the selection pressure that led to white European skin, why are Europeans genetically polymorphic in their ability to maintain blood levels of vitamin D? At least two alleles reduce the effectiveness of the vitamin-D binding protein, and their homozygotes account for 9% and 18% of French Canadians (Sinotte et al., 2009). If lack of this vitamin had been so chronic, natural selection would have surely weeded out these alleles. And why does European skin limit vitamin-D production after only 20 minutes of UV-B exposure? (Holick, 1995). Why is such a limiting mechanism necessary?
There is also little evidence that ancestral Europeans suffered from vitamin-D deficiency. Before the 17th century, we have only sporadic evidence of rickets in skeletal remains and even these cases may be false positives, as Wells (1975) notes:
It is likely that these low frequencies of rickets should be even lower because some of the authors quoted above have based their diagnoses on such features as plagiocrany (asymmetry of the skull), which may occur merely from cradling habits and other causes (Wells, 1967a) or on irregularities of the teeth, which probably result from many adverse factors in foetal life as well as in infancy.
On this point, Chaplin and Jablonski (2009) affirm: “Despite taphonomic biases, it [rickets] has been recognized in early archeological and Neolithic materials at the rate of 1-2.7% (a reasonably high selective value).” In fact, Wells (1975) reports no cases from Paleolithic Europe and only sporadic cases from Neolithic Europe. The range of 1-2.7% seems to apply to “a gradual, albeit slow, increase of the disease during the European Middle Ages” (Wells, 1975). Wells (1975) cites a series of Hungarian remains that indicate an increase in frequency from 0.7 to 2.5% between the 10th and 13th centuries. As Wells notes, even this low incidence is probably inflated by false positives.
Why is skin white only among Europeans?
The vitamin-D hypothesis raises a second problem. Why is white skin an outlier among the skin tones of indigenous human populations north of 45° N? Skin is much darker among people who are native to these latitudes in Asia and North America and who receive similar levels of UV-B at ground level. Murray (1934) attributes their darker skin to a diet rich in vitamin D:
One of the chief difficulties up to now in accounting for the origin of the white or unpigmented race has been the existence of the darkly pigmented Eskimo in these same dark sunless Arctic regions which we have been discussing as the probable original habitat of the white race. The unravelling of the causes of rickets has fully explained this anomaly. The Eskimo though deeply pigmented and living in a dark habitat, nevertheless is notoriously free from rickets. This is due to his subsisting almost exclusively on a fish oil and meat diet. Cod liver oil, as has been stated, is fully as efficient as sunlight in preventing rickets. Now the daily diet of the Eskimo calculated in antirachitic units of cod liver oil equals several times the minimum amount of cod liver oil needed to prevent rickets. Because of his diet of antirachitic fats, it has been unnecessary for the Eskimo to evolve a white skin in the sunless frigid zone. He has not needed to have his skin bleached by countless centuries of evolution to admit more antirachitic sunlight. He probably has the same pigmented skin with which he arrived in the far north ages ago.
This argument fails to explain why skin is equally dark among inland natives of northern Asia and North America who consume little fatty fish and yet show no signs of rickets. One might also point out that fatty fish has long been a major food source for the coastal inhabitants of northwestern Europe. According to carbon isotope analysis of 7,000-6,000 year old human remains from Denmark, the diet must have been 70-95% of marine origin (Tauber, 1981). Yet Danes are very pale-skinned.
Some have suggested that sufficient vitamin D could have been obtained from the meat of land animals, if eaten in sufficient quantities (Sweet, 2002). This has led to a revised version of the vitamin-D hypothesis: ancestral Europeans lightened in color when they made the transition from hunting and gathering to agriculture 8,000 to 5,000 years ago, and not when they first arrived some 35,000 years ago.
Do we know when Europeans became white? This change has been roughly dated at two gene loci. At SLC45A2 (AIM1), Soejima et al. (2005) have come up with a date of ~ 11,000 BP. At SLC24A5, Norton and Hammer (2007) suggest a date somewhere between 12,000 and 3,000 BP. These are rough estimates but it looks like Europeans did not turn white until long after their arrival in Europe. As a Science journalist commented: “the implication is that our European ancestors were brown-skinned for tens of thousands of years” (Gibbons, 2007). Thus, the original version of the vitamin-D hypothesis no longer seems plausible.
Of course, the revised vitamin-D hypothesis is still plausible, i.e., Europeans became pale-skinned after giving up hunting and gathering for agriculture. But this scenario does raise problems. For one thing, it would mean that many Europeans turned white at the threshold of history. In the case of Norway, agriculture did not arrive until 2400 BC and fatty fish, rich in vitamin D, have always been a mainstay of the diet (Prescott, 1996).
References
Chaplin, G., & Jablonski, N.G. (2009). Vitamin D and the evolution of human depigmentation, American Journal of Physical Anthropology, early view
Gibbons, A. (2007). American Association Of Physical Anthropologists Meeting: European Skin Turned Pale Only Recently, Gene Suggests. Science 20 April 2007:Vol. 316. no. 5823, p. 364 DOI: 10.1126/science.316.5823.364a http://www.sciencemag.org/cgi/content/summary/316/5823/364a
Holick, M.F. (1995). Noncalcemic actions of 1,25-dihydroxyvitamin D3 and clinical applications, Bone, 17, 107S-111S.
Loomis, W.F. (1967). Skin-pigment regulation of vitamin-D biosynthesis in Man, Science, 157, 501-506.
Murray, F.G. (1934). Pigmentation, sunlight, and nutritional disease, American Anthropologist, 36, 438-445.
Norton, H.L. & Hammer, M.F. (2007). Sequence variation in the pigmentation candidate gene SLC24A5 and evidence for independent evolution of light skin in European and East Asian populations. Program of the 77th Annual Meeting of the American Association of Physical Anthropologists, p. 179.
Prescott, C. (1996). Was there really a Neolithic in Norway? Antiquity, 70, 77-87.
Robins, A.H. (2009). The evolution of light skin color: role of vitamin D disputed, American Journal of Physical Anthropology, early view.
Sinotte, M., Diorio, C., Bérubé, S., Pollak, M., & Brisson, J. (2009). Genetic polymorphisms of the vitamin D binding protein and plasma concentrations of 25-hydroxyvitamin D in premenopausal women, American Journal of Clinical Nutrition, 89, 634-640.
Soejima, M., Tachida, H., Ishida, T., Sano, A., & Koda, Y. (2005). Evidence for recent positive selection at the human AIM1 locus in a European population. Molecular Biology and Evolution, 23, 179-188.
Sweet, F.W. (2002). The paleo-etiology of human skin tone. http://backintyme.com/essays/?p=4
Tauber, H. (1981). 13C evidence for dietary habits of prehistoric man in Denmark, Nature, 292, 332-333.
Wells, C. (1975). Prehistoric and historical changes in nutritional diseases and associated conditions, Progress in Food and Nutrition Science, 1(11), 729-779.
