More on the human skin color gene

Nicholas Wade:

A gene that is responsible for the pale skin of Europeans and the dark skin of Africans has been discovered by scientists at Pennsylvania State University.

The gene comes in two versions, one of which is found in 99 percent of Europeans and the other in 93 to 100 percent of Africans, the researchers report in today's issue of Science.

The gene is unusual because with most human genes, different versions are generally shared, though one version may be more common in one race than another. One exception is the Duffy null allele, a version of a gene that prevents malaria, that is found almost exclusively in one race, sub-Saharan Africans.

The new gene falls into the same category as the Duffy gene, and it may shed light on the evolutionary pressures to which Europeans were subjected as their ancestors, who were presumably dark skinned, moved into the northern latitudes some 40,000 years ago.

Humans acquired dark skins in Africa about 1.5 million years ago to shield their newly hairless bodies from the sun. Its ultra-violet rays destroy folic acid, a shortage of which leads to birth defects.

But when the modern humans who left Africa began to live in northern latitudes, they needed more sunlight to penetrate the skin, to permit the chemical reaction that produces vitamin D.

The new gene was first identified not in humans but in a mutant zebra fish, a small striped fish common in aquariums. The mutant fish are known as golden, because their stripes, usually black, are much paler and their bodies more yellow.

Dr. Keith C. Cheng, an author of the report, and his colleagues showed that the golden version of the fish gene altered production of melanosomes, the tiny black particles of pigments that give skin its color.

The researchers then found that in humans, who have their own form of the gene, the version common in Africans allowed larger melanosomes, which tend to clump together, whereas the version in Europeans produced smaller and more dispersed melanosomes.

Asians have the same version of the gene as Africans, so they presumably acquired their light skin through the action of some other gene that affects skin color, said Dr. Cheng.

Mark D. Shriver, another author of the article, said his laboratory was trying to assess when the European version of the gene became so common, as well its geographical origin.

The intense selective pressure that drove the version to become universal in Europeans may have included sexual selection.

"In Africa people are much darker than they need to be for UV protection, so to me that screams sexual selection," Dr. Shriver said. Black skin, in other words, may have been favored by men and women in sexual partners, just as pale skin may have been preferred in sexual partners among Europeans and Asians.

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