How Humans Went From Being One Shade to Many

Our primate ancestors that first lost most of their body hair were likely pale skinned, according to a new study that concludes our human forebears probably evolved darker skin later to safeguard against skin cancer and other problems that can result from too much sun exposure.

The study, published in the latest Proceedings of the Royal Society B, helps explain both the historical origins and biological significance of skin coloration in humans.

Author Mel Greaves, a professor at The Institute of Cancer Research in the U.K., told Discovery News that "the likelihood is that the common ancestor of hominins and chimps had pale skin."

Greaves explained that chimpanzees and many other primates, under their fur, have pale skin with pigment-producing cells restricted to hair follicles. Sometime between 2 and 3 million years ago, our ancestors in East Africa experienced a dramatic loss of body hair. Primates that are not directly on the human family tree kept their copious amounts of hair/fur, while our ancestors lost much of it.

As for why the hair loss occurred, Greaves said that it was "almost certainly to facilitate heat loss by sweating in physically very active hunters, especially in the more open, dry and hot Savannah."

Indigenous humans from East Africa and throughout sub-Saharan Africa today all have black skin, however, and DNA reveals that these individuals evolved a gene, MC1R, associated with skin pigment production. Many scientists over the years, including Charles Darwin, theorized that black skin was acquired early in human evolution as an adaptation to limit UV radiation damage from sun exposure.

To test that theory, Greaves studied African albinos, meaning people who have a congenital absence of pigment in their skin, hair and eyes. He found that they were highly susceptible to developing skin cancer.

"Almost all albinos in equatorial Africa develop skin cancer in their 20's," Greaves told Discovery News. "A few -- maybe 10 percent -- escape, and these are mainly females with a more indoor lifestyle."

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.

Read more at Discovery News