Only Skin Deep
world map

What causes variation in skin color? Geography and UVR, not race. Jablonski and Chaplin predicted that skin color would correlate closely with UVR, based on geographic latitude, annual UVR exposure and other environmental factors. Their predictions closely match actual distribution of skin colors.

Credit: © George Chaplin

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Only Skin Deep (continued from previous page)

Scientists also know that a certain amount of ultraviolet radiation (UVR) from the sun helps the human body use vitamin D to absorb the calcium necessary for strong bones. However, too much UVR can strip away folate (folic acid), an essential nutrient to the development of healthy fetuses. Skin pigmentation developed as the body’s way of balancing its need for vitamin D and folate. Those closer to the equator had darker skin to prevent folate deficiency.

Jablonski continues, "As some groups moved into regions farther from the equator where UVR levels are lower, natural selection favored lighter skin, which allowed enough vitamin D-forming UVR to penetrate their skin." Jablonski and her colleague George Chaplin sought to prove their hypothesis that human skin color correlates closely to annual levels of UV radiation levels based on geography and other environmental factors. They measured skin reflectance (a way to quantify skin color by measuring the amount of light it reflects) in people around the world. Their actual observations of skin reflectance closely parallel their predictions.

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