About 2 million years ago, permanent dark skin color imparted by the pigment-melanin-began to evolve in humans to regulate the body's reaction to ultraviolet rays from the sun, said Nina Jablonski, Distinguished Professor of Anthropology.
Melanin helped humans maintain the delicate balance between too much sunlight and not enough sunlight. The pigment allowed enough ultraviolet radiation to produce vitamin D, a vitamin that helps the body absorb calcium, while protecting the skin from the intense ultraviolet radiation in the equator. Too much sunlight can cause the destruction of folate, which is also critical to cell division.
As some humans moved away from the equator to places where the sun's rays are not nearly as intense, they lost pigmentation, said Jablonski.
Unlike their ancestors, modern humans are more mobile. A person with darker skin may move to regions with less intense sunlight, and those with less pigmentation may move to areas that are closer to the equator.
In addition to moving regularly, most people now live in cities with limited exposure to the sun. Nearly 60 percent of the people in the world live in cities now, said Jablonski.
Most people who live in cities also work indoors, further reducing their ability to make enough vitamin D in their skin.
Health problems are compounded when people do not receive enough sunlight, or when they have a mismatch between their skin pigmentation and ultraviolet radiation.
"This can lead to a vitamin D catastrophe for many people," Jablonski said.
Jablonski said that there are ways to increase vitamin D without increasing the risk of skin cancer through exposure to the sun.
"By far, the safest way and the cheapest way is to use vitamin D supplements, which are widely available in stores," said Jablonski.
The findings are presented at the annual meeting of the American Association for the Advancement of Science in Boston.