Some writers argue that European skin became white to offset a decline in dietary vitamin D. Pre-agricultural diets, however, were rich in vitamin D only among coastal Europeans who consumed fatty fish.
The ‘vitamin D hypothesis’ is often invoked to explain differences in skin color among human populations (Loomis 1967; Murray 1934). As modern humans spread out of Africa, they entered northern regions, like Europe, where sunlight is weaker and less conducive to synthesis of vitamin D by the skin. Thus, to maintain the same level of vitamin-D synthesis, there was strong natural selection to depigment the skin and let the sun’s rays penetrate it more easily.
This might explain the whiteness of Europeans, but what about the darker skin of northern Asians and native North Americans? They too live at high latitudes. And they too receive much less sunlight than do tropical peoples.
The standard reply is that they get enough vitamin D from their diet, specifically fatty fish, so natural selection has not lightened their skin to the same degree:
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. […] 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 (Murray 1934).
Actually, this explanation holds true only for a minority of northern natives, i.e., the Inuit, the Aleuts, and some other coastal peoples. Fatty fish is absent from the diet of non-coastal peoples, i.e., most Algonkians, Athapaskans, and indigenous Siberians. Yet the latter are if anything darker-skinned than the Inuit.
Conversely, fatty fish has long been a staple of Scandinavians, who nonetheless are very white-skinned. And the word ‘long’ is no exaggeration: skeletal remains of Danes living 7,000-6,000 years ago have the same carbon isotope profile as those of Greenland Inuit, whose diet is 70-95% of marine origin (Tauber 1981).
But the vitamin-D hypothesis has another shortcoming: Europeans did not turn white until long after their ancestors came to Europe some 35,000 years ago. If we examine the various alleles that lighten European skin color, the time of origin seems to be relatively late. At the SLC45A2 (AIM1) gene, the date is ~ 11,000 BP (Soejima et al. 2005). At SLC24A5, the date falls between ~ 12,000 and 3,000 BP (Norton & Hammer 2007). As a Science journalist commented: “the implication is that our European ancestors were brown-skinned for tens of thousands of years” (Gibbons, 2007).
This has led to an updated version of the vitamin-D hypothesis. It has two postulates:
1. Selection for white skin began long after modern humans entered Europe.
2. The cause was not the weak sunlight of Europe’s high latitudes, but rather less intake of vitamin D from food sources. This happened when a diet of terrestrial game, fish, and wild plants gave way to one based on grains and livestock—in short, the advent of agriculture (Khan & Khan 2010; Sweet 2002).
If we pursue this line of reasoning, Europeans must have turned white almost at the dawn of history. We know that agriculture spread into southeastern Europe from the Middle East around 9,000 years ago. By 7500 BP it had reached a line stretching from the Netherlands through Central Europe and to the Black Sea. Thus, the extreme skin depigmentation of northern Europeans would have occurred over the last seven millennia or so. Actually, the time frame is even narrower, since white-skinned Europeans appear in ancient Egyptian art from the second millennium B.C.
So we’re left with around 3,000 years, at most. Is this pace of phenotypic change consistent with selection due to weak sunlight? Not according to current opinion. Brace et al. (1999) studied how skin color varies among Amerindians, who have inhabited North and South America for 12,000-15,000 years, and among Aborigines, who have inhabited Australia for some 50,000 years. If latitudinal variation in skin color tracks natural selection due to the intensity of sunlight, calculations show that this kind of 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.
But there’s another problem. How do we know that ancestral Europeans did ingest much less vitamin D when agriculture replaced hunting/fishing/gathering? This question is met only with affirmations, e.g. “Because of the lack of meat and fish in the diet of the new farmers, vitamin D intake would have been drastically reduced” (Khan & Khan 2010). No one seems to have actually quantified vitamin-D intake before and after the advent of agriculture.
To gain a rough idea, we can consult Loomis (1967) for a listing of vitamin-D content by food source:
Food source – Vitamin D content (I.U./gram)
Halibut liver oil – 2,000-4,000
Cod liver oil – 60-300
Milk – 0.1
Butter – 0.0-4.0
Cream – 0.5
Egg yolk – 1.5-5.0
Calf liver – 0.0
Olive oil – 0.0
Sweet (2002) provides a longer list:
Food source – Vitamin D content (I.U.)
Cod liver oil, 1 tbs – 1,360
Salmon, 3.5 oz. – 360
Mackerel, 3.5 oz. – 345
Herring, 3.5 oz. – 315
Sardines, 3.5 oz. – 270
Eel, 3.5 oz. – 200
Shrimp, 3.5 oz. – 150
Beef liver, 3.5 oz. – 30
Egg, 1 whole – 25
Beef, pork, chicken, 3.5 oz. – 20
Cheese, 1 oz. – 4
Unfortified milk – 0
Unfortified cereal – 0
Clearly, fatty fish has a lot of vitamin D. But the same cannot be said for terrestrial animals, like calves. Furthermore, the figures from the first list were initially published in 1938, when most cattle were kept outdoors and exposed to sunlight. These were also American cattle. They lived farther south than the game animals that ancestral Europeans once hunted and whose flesh probably had a lower vitamin-D content.
Thus, before agriculture, Europeans got substantial vitamin D from their diet only in coastal regions, like Scandinavia, where people ate fatty fish. Europeans who lived inland—the majority—did not have this dietary source. One might counter that the issue is not vitamin-D content per se but rather substances, like phytic acids in cereals, that deplete the body’s supply of calcium and phosphorus. The advent of agriculture would have artificially increased the body’s need for vitamin D.
Perhaps. Ultimately, this debate will end only when we know the precise time frame when Europeans became white. We already know that this time frame considerably postdates the arrival of modern humans in Europe (c. 35,000 BP). If it significantly predates the arrival of agriculture (after 9,000 BP), the vitamin-D hypothesis will be out of the running, even in its updated form.
This puzzle will then be placed within a larger one. Why do Europeans possess such unusual color traits that involve not only the skin but also the hair and the eyes? How did they evolve so rapidly a white skin and a diverse palette of eye and hair colors? If humans were any other animal, such traits would be readily put down to sexual selection.
Brace, C.L., Henneberg, M., and Relethford, J.H. (1999). Skin color as an index of timing in human evolution. American Journal of Physical Anthropology, 108 (supp. 28), 95-96.
Gibbons, A. (2007). American Association Of Physical Anthropologists Meeting: European Skin Turned Pale Only Recently, Gene Suggests. Science 20 April 2007, 316. no. 5823, p. 364 DOI: 10.1126/science.316.5823.364a http://www.sciencemag.org/cgi/content/summary/316/5823/364a
Khan, R. and B.S.R. Khan. (2010). Diet, disease and pigment variation in humans, Medical Hypotheses, early view.
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.
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 (visited on July 10, 2008).
Tauber, H. (1981). 13C evidence for dietary habits of prehistoric man in Denmark. Nature, 292, 332-333.