Tuesday, July 20, 2010
Scientists from the Beijing Genomics Institute last month discovered another striking instance of human genetic change. Among Tibetans, they found, a set of genes evolved to cope with low oxygen levelsas recently as 3,000 years ago. This, if confirmed, would be the most recent known instance of human evolution....
Coloring the skin may sound simple, but nature requires at least 25 different genes to synthesize, package and distribute the melanin pigment that darkens the skin and hair. The system then had to be put into reverse when people penetrated the northern latitudes of Europe and Asia and acquired lighter skin, probably to admit more of the sunlight required to synthesize vitamin D.
Several of the 25 skin genes bear strong signatures of natural selection, but natural selection has taken different paths to lighten people’s skin in Europe and in Asia. A special version of the golden gene, so called because it turns zebrafish a rich yellow color, is found in more than 98 percent of Europeans but is very rare in East Asians. In them, a variant version of a gene called DCT may contribute to light skin. Presumably, different mutations were available in each population for natural selection to work on. The fact that the two populations took independent paths toward developing lighter skin suggests that there was not much gene flow between them.
East Asians have several genetic variants that are rare or absent in Europeans and Africans. Their hair has a thicker shaft. A version of a gene called EDAR is a major determinant of thicker hair, which may have evolved as protection against cold, say a team of geneticists led by Ryosuke Kimura of Tokai University School of Medicine in Japan.
One aspect of this pattern is that there seem to be more genes under recent selection in East Asians and Europeans than in Africans, possibly because the people who left Africa were then forced to adapt to different environments. “It’s a reasonable inference that non-Africans were becoming exposed to a wide variety of novel climates,” says Dr. Stoneking of the Max Planck Institute.