Reproductive Success: Only Skin Deep?

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Three distinct zones link skin color to ultraviolet radiation levels at the Earth's surface.

Skin color has long been considered an evolutionary response to ultraviolet (UV) radiation. Now, Academy anthropologist Nina Jablonski and colleague George Chaplin substantiate this relationship and link it to reproduction.

Our skin needs UV light in order to produce vitamin D3, which is essential for calcium absorption and normal growth. But too much exposure can deplete important nutrients and cause diseases such as skin cancer. Melanin, a skin pigment, acts as a natural sunscreen, absorbing UV radiation as it enters the skin. The darker the skin, the more protection.

Jablonski and Chaplin calculated how much UV radiation it takes for skin to produce vitamin D3 at different latitudes and mapped these data with NASA satellite measurements of UV levels at the Earth's surface. They found that, near the equator, skin can be dark and still produce D3. But skin needs to lighten with increasing latitude and decreasing UV levels toward the poles to synthesize the vitamin. Indeed, skin color of indigenous peoples is darkest near the equator and lightens with latitude.

UV radiation can destroy folate, a nutrient critical to embryonic development and sperm production. Thus, Jablonski believes skin color is a compromise, being light enough to allow adequate UV penetration to synthesize vitamin D3, while being dark enough to protect folate levels and ensure successful reproduction

photo of George Chaplin and Nina Jablonski
Researchers George Chaplin
and Nina Jablonski