Data on albino individuals from other warm climates, such as in Central America, reveal that they are also prone to developing skin cancer. While dark skin doesn't always prevent skin cancer, high concentrations of melanin -- the pigment that gives skin color -- can serve as a natural defense against UV rays.
The melanin-associated gene probably arose by accident in human ancestors, Greaves explained, with natural selection favoring it by increased survival of those who had it, and who would have then passed it on to their children.
Non-albino people with light skin today therefore probably had ancestors that began as pale skinned, evolved darker skin, and then evolved light skin again.
"We assume that all hominin migrants from Africa over the past 100,000 years would have been dark skinned," Greaves said. "What happened to those migrant populations' skin color later depended upon geography and UVR (ultraviolet radiation) exposures. Those migrating into Europe underwent selection in favor of paler skin -- probably to gain more Vitamin D (essential for healthy bones and teeth)."
He continued, "Those migrants who tracked the tropics or equator into southern India, New Guinea and Australasia maintained their original dark skin," since they were under conditions of higher UV radiation.
Both Robin Weiss, emeritus professor of viral oncology at University College London, and Randolph Nesse, director of the Center for Evolution, Medicine and Public Health at Arizona State University’s School of Life Sciences, described the new study as "compelling."
Weiss told Discovery News that the findings "show how important darker skin pigmentation is to protect against skin cancer early enough in adult life to have an influence on human evolution."