Modern humans did not begin to lighten in skin color immediately after entering Europe some 35,000 years ago. In fact, these ancestral Europeans remained brown-skinned for tens of thousands of years. This is the conclusion now emerging from studies of skin color loci.
In 2005, a team of Japanese researchers found that the depigmentation of European skin was partly due to a relatively recent allele at the SLC45A2 (AIM1) gene. They dated the allele to c. 11,000 BP and concluded that it had rapidly supplanted the original allele through positive selection (Soejima et al., 2005).
Then last year, at the Annual Meeting of the American Association of Physical Anthropologists, a molecular anthropologist at the University of Arizona, Heather Norton, presented evidence that Europeans have a similarly recent allele at another skin color gene, SLC24A5. The new allele is dated to 12,000 – 3,000 BP. As she stated during her talk: "The [evolution of] light skin occurred long after the arrival of modern humans in Europe." (Norton & Hammer, 2007).
The challenge now will be to narrow the time window. Did the change happen during the last ice age? This would be before 10,000 years ago. Or did it happen later, after hunting and gathering gave way to agriculture? This would be 7,400 to 5,900 years ago on the North European plain, where European skin is whitest (Bogucki, 1989).
Both datings are consistent with the data currently available. The corresponding explanations, however, differ greatly. If European skin whitened 7,400 to 5,900 years ago, the cause may have been a diet less rich in vitamin D and, hence, selection for lighter skin to facilitate vitamin D synthesis. This is the explanation favored by a write-up in Science:
Either way, the implication is that our European ancestors were brown-skinned for tens of thousands of years--a suggestion made 30 years ago by Stanford University geneticist L. Luca Cavalli-Sforza. He argued that the early immigrants to Europe, who were hunter-gatherers, herders, and fishers, survived on ready-made sources of vitamin D in their diet. But when farming spread in the past 6000 years, he argued, Europeans had fewer sources of vitamin D in their food and needed to absorb more sunlight to produce the vitamin in their skin. Cultural factors such as heavier clothing might also have favored increased absorption of sunlight on the few exposed areas of skin, such as hands and faces, says paleoanthropologist Nina Jablonski of PSU in State College. (Gibbons, 2007)
This explanation falls apart, however, if European skin whitened earlier, such as during the last ice age. The cause may then have been sexual selection, i.e., stronger female-female competition for male mates because of limited polygyny and high male mortality (Frost, 2006, see earlier posts here and here). Preference for lighter-skinned women is attested in a wide range of traditional, premodern societies (van den Berghe & Frost, 1986). Under conditions of intense sexual selection of women, the selective advantage of lighter-skinned women would have entailed a relative depigmentation of the entire population, both women and men.
Bogucki, P. (1989). The Neolithic Mosaic on the North European Plain. Updated paper originally delivered at the Society for American Archaeology meeting in Atlanta, Georgia.
Frost, P. (2006). European hair and eye color - A case of frequency-dependent sexual selection? Evolution and Human Behavior, 27, 85-103.
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
Norton, H.L. and 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. http://www.physanth.org/annmeet/aapa2008/AAPA2008abstracts.pdf
Soejima, M., Tachida, H., Ishida, T., Sano, A., and Koda, Y. (2005). Evidence for recent positive selection at the human AIM1 locus in a European population. Molecular Biology and Evolution, 23, 179-188.
van den Berghe, P.L., and 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.