Sunday, June 2, 2013
People of European ancestry have an unusually diverse palette of hair and eye colors
This diversity is commonly ascribed to their unusually white skin. Ancestral Europeans became lighter-skinned, and this genetic change therefore caused other changes to hair and eye pigmentation. Actually, the genetic changes are different in each case. European skin turned white through a replacement of alleles, primarily at TYRP1, SLC24A5, and SLC45A2. European hair and eyes diversified in color through a proliferation of new alleles, primarily at MC1R for hair color and in the HERC2-OCA2 region for eye color. It now appears that this diversification has occurred at other gene loci as well. A region downstream from EDNRB is associated with differences in hair color and two other loci, VASH2 and POLS, are associated with differences in eye color. TPCN2 is associated with differences in hair color and ASIP is associated with red hair. People of European descent display the widest variation in pigmentation traits, such as iris (eye) and hair coloration, in the world. In particular, eye color variation is nearly restricted to people of (at least partial) European descent. Eye color categories here often concern blue, brown and intermediate (green, etc.). In the rest of the world, people tend to have brown eye color, which is considered to be the ancestral human trait in agreement with the Out-of-Africa hypothesis of modern humans. The current variation in eye color is thought to have originated via a genetic founder event involving non-brown irises in early European history. It is furthermore assumed that eye color variation in Europe has been shaped by positive selection via sexual selection i.e., mate choice preference. Alternatively it has been proposed that eye color variation evolved via a correlation with skin color and its environmental adaptation e.g. maximizing vitamin D conversion in low levels of UV radiation, or as a combination of both. One suggested geographic region for the origin of blue eye color in Europe is the southern Baltic, as indicated by concentric rings of decreasing frequency of the blue eye color trait spreading from the southern Baltic region, resulting in a strong north–south gradient in blue eye color frequency across Europe. It is doubtful whether a lack of vitamin D at northern latitudes played a role in the whitening of European skin, let alone in the diversifying of European hair and eye color. An obvious feature of the northward dispersal of humans is a quasi-geographic reduction in pigmentation. Coloration varies greatly among northerners. Native Inuit display medium-to-dark, rather than light pigmentation, and both northern and central-dwelling Asians display medium pigmentation. Recent population genetic data show that the reduction in skin pigmentation occurred sporadically and incompletely in northern and Asian populations. Moreover, while modern humans reached Central Europe around 40,000 years ago, they reached northern Europe only after the last ice sheets receded less than 11,000 years ago. It is only these humans that display light pigmentation, and recent molecular genetic studies suggest that the very light pigmentation of northern Europeans did not develop until 11,000 to 19,000 years ago. This time period began long after the entry of modern humans into Europe, the implication being that ancestral Europeans were brown-skinned for tens of thousands of years. The question is why did this color change take place?