The Selective Power of Ultraviolet

Why do humans have so many skin tones? The theory is that as hominoids became less hairy, dark skin color evolved as protection against the tropical sun. Then, as early humans migrated northward, a shift to fair skin enabled people far from the equator to get enough Vitamin D. Now a group of researchers has shown the connection using direct measures of the sun's ultraviolet radiation.

Anthropologists Nina Jablonski and George Chaplin of the California Academy of Sciences in San Francisco used data from NASA satellites collected between 1978 and 1993 to compute average annual UV exposure in different areas of the world. They correlated this with data on skin reflectance--the higher the reflectance the whiter the skin--from ethnic groups in more than 50 countries. Ever lighter skin matches up quite neatly with decreasing UV levels, they report in the July issue of the Journal of Human Evolution.

This correlation is "an important finding since it bolsters the case for UV radiation as a link to selection," says anthropologist John Relethford of the State University of New York, Oneonta. The authors say dark skin protects against the damaging UV radiation, and the skin pigment melanin helps prevent the breakdown of folate, a metabolite of folic acid, an essential nutrient. Folate, which is easily degraded by sunlight, is necessary for embryo development and spermatogenesis. As populations ranged farther from the equator's steady supply of sunlight, according to this theory, lighter skin was more of an advantage because it transmits enough UV light for the synthesis of Vitamin D, which is necessary for skeletal and immune system development.

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