Differences in skin color are intriguing, but few of us have a clear idea what causes the variations. Scientists have long known that human skin color varies with the amount of exposure to the sun's ultraviolet (UV) radiation, but until recently they had not identified the process of natural selection that actually influences this phenomenon. Now the work of scientists Nina Jablonski and George Chaplin, of the California Academy of Sciences, casts a new light on race concepts by relating skin-color variation to evolution and reproduction.
Folate and the Sunshine Vitamin
Melanin, the brown pigment in the skin, acts as a natural sunscreen. It protects against UV, and populations in the tropics are darker skinned since there is more sunlight where they live. UV ages the skin, causes skin cancer, and—most significant to Jablonski and Chaplin's work— breaks down folate, essential vitamin B needed for cell division and producing new DNA.
Pregnant women in particular require large amounts of folate to support rapid cell division in the embryo. Women of reproductive age are advised to take folate supplements to prevent serious birth defects such as spina bifida. So if a higher melanin level is so beneficial, why isn't everyone dark-skinned?
In their analysis of human evolutionary history, Jablonski and Chaplin concluded that modern humans most likely evolved in the tropics, where they were exposed to high UV levels. But as they moved into regions away from the equator, where UV levels are lower, humans became fairer so as to allow enough UV radiation to penetrate their skin and produce vitamin D, the "sunshine vitamin," also obtained from eating fish and marine mammals. Vitamin D is essential for maintaining healthy blood levels of calcium and phosphorous, and thus promoting bone growth.
Skin color, according to Jablonski and Chaplin, basically becomes a balancing act between the evolutionary demands of photo-protection and the need to create vitamin D in the skin.
But things aren't always what they ought to be. That is the case with Eskimos and other inhabitants of northern Alaska and northern Canada. "Looking at Alaska, one would think that the native people should be pale as ghosts," Jablonski says. One of the reasons they're not is that these populations have not lived in the region very long in terms of geological time. But more importantly, their traditional diet is rich in fish and other seafood. They've consumed huge doses of vitamin D, so they haven't had to undergo the same reduction in pigmentation that would otherwise be required at such high latitudes. "What's really interesting is that if these people don't eat their aboriginal diets of fish and marine mammals, they suffer tremendously high rates of vitamin D-deficiency diseases such as rickets in children and osteoporosis in adults," Jablonski says.
A similar problem occurs when dark-skinned people move to northern latitudes. "For years people couldn't understand why dark Indians and Pakistanis living in northern England suffered from vitamin D-deficiency diseases," Jablonski says. "Now it has become clear that the natural sunscreen in their skins wouldn't allow them to synthesize enough vitamin D from the sunlight." Cultural factors exacerbated the problem, such as the wearing of veils by some Muslim women. "It's a real detective story," she adds.
Race: A Manmade Grouping
One of the important implications of Jablonski and Chaplin's work is that it underlines the concept of race as purely a social construct, with no scientific grounds. DNA research has shown that genetically all humans, regardless of skin color and other surface distinctions, are basically the same. In an April 2001 article titled, "The Genetic Archaeology of Race," published in the Atlantic Monthly, Steve Olson writes "the genetic variants affecting skin color and facial features are essentially meaningless —they probably involve a few hundred of the billions of nucleotides in a person's DNA. Yet societies have built elaborate systems of privilege and control on these insignificant genetic differences."
Jablonski and Chaplin view their work as relevant to how we get along with each other. According to Jablonski, many people are "happy and relieved" when they hear about this research. "All of a sudden their own coloration isn't something that was just handed to them," she says. "It isn't a social stigma. It's something that evolved in their ancestors for a good set of biological reasons. And it takes the wind out of racism and bigotry. It's a fairly simple and beautiful explanation for one of the most obvious characteristics that distinguishes humans."