Natural Selection in Dark Skin 

Preventing UV light from penetrating the skin is important in places where UV light is intense, because the radiation can damage the body in a number of ways. One way is by causing DNA damage that can lead to skin cancer. Skin cancer rates are far higher in people with lighter skin. The disease generally develops relatively late in life, however often after reproduction has taken place. As a result, it has been argued that skin cancer would not be a very effective agent of natural selection. However another way UV light can damage the body is by destroying folic acid. Folic acid is a nutrient that is needed for good health and successful reproduction. Inadequate folic acid may cause anemia. Also, folic acid is also crucial for the development of sperm in men. An hour of exposure to intense sunlight is enough to cut folate levels in half for someone with light skin. Protecting the body's folic acid would be a potent selective force for dark skin, because it would help insure reproductive success.

Natural Selection in Light Skin

Protection from UV light can readily explain why dark skin evolved in early humans living in tropical Africa. But what explains the evolution of light skin in people who moved out of the tropics and into regions where there was less intense solar radiation? The most likely explanation concerns another nutrient, vitamin D. Vitamin D is needed by the body so that calcium can be absorbed by the bones, keeping them strong. Vitamin D is manufactured in the skin when it is exposed to UV light. Where UV radiation is intense, as it is near the equator, enough UV light penetrates even very dark skin for adequate vitamin D synthesis to occur. At higher latitudes where UV radiation is much less intense, however, too little light may penetrate dark skin for adequate vitamin D to be synthesized. At low levels of UV light, it may take a dark skinned person two hours to produce the same amount of vitamin D as someone with light skin produces in 15 minutes. Vitamin D deficiency, in turn, may cause health problems, such as osteoporosis and rickets.