"It's a fairly simple and beautiful explanation for one of the most obvious characteristics that distinguishes humans," Jablonski says.
Variations in skin color occur because over millennia, as populations of humans have migrated, they have needed to adapt to the amount of ultraviolet radiation (UVR) from the sun falling on different regions of Earth. The degree of skin pigmentation is related to two conflicting physiological needs: the need to protect the body against the breakdown of some essential compounds that are sensitive to UVR such as the B vitamin folate, while allowing just enough UVR into the skin to initiate the synthesis of the "sunshine vitamin," vitamin D-3. Both folate and vitamin D-3 are essential for healthy reproduction.
Folate is required for the normal development of the nervous system in the early embryo. That's why pregnant women are urged to take folate supplements to prevent serious birth defects such as spina bifida. Vitamin D-3 maintains healthy blood levels of calcium and phosphorus, thus promoting bone growth.
Too much UVR can not only cause skin cancer, but more importantly breaks down folate, thereby threatening a person's chance at having a healthy baby.
In other words, Jablonski says, "Skin color is an evolutionary balancing act between protecting yourself from excess UVR, while allowing a little UVR into your skin to produce vitamin D."
While others have suggested a relationship between skin color and sun exposure, Jablonski is the first to directly link sun exposure to the reproductive mechanism regulating skin color. That mechanism involves adjusting levels of melanin, the brown pigment in the skin that acts as a natural sunscreen, protecting against UV.
If high levels of melanin are so crucial in humans, why aren't we all dark-skinned? "There is a broad range of human skin tones because humans live in a broad range of environments, from those that receive a lot of sun and UV radiation, mostly near the equator, to those that receive very little, near the poles," Jablonski says. "Some people have a lot of melanin because it protected their ancestors from the harmful effects of UV radiation; other people have less melanin because they live where there is less UV radiation, and their skin must be lighter in order to permit the available UVR to be used in vitamin D production."
But what about places like Alaska, which is close to the North pole, but where the indigenous peoples are dark? Geologically speaking, these populations have not lived in the region very long. But more importantly, their traditional diet is rich in fish and marine mammal blubber. They've consumed so much vitamin D-3 that they have not required a reduction in pigmentation that would otherwise benefit people living 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. "For years people couldn't understand why dark Indians and Pakistanis living in northern England suffered from vitamin D-deficiency diseases. Now it has become clear that the natural sunscreen in their skins wouldn't allow them to synthesize enough vitamin D from the sunlight."
"One of the interesting findings that our study has thrown up is that in every indigenous population we've examined, females are slightly more lightly colored than males, probably because they need to synthesize more vitamin D in their skin during critical periods of their life history, such as pregnancy and lactation," she adds.
Jablonski arrived at her theory after she and her spouse George Chaplin, also with the California Academy of Sciences, studied data from NASA's Total Ozone Mapping Spectrometer, which was launched in 1978 to measure how much ultraviolet radiation reaches Earth's surface. Jablonski and Chaplin compared that data with published data on skin color in indigenous populations from more than 50 countries. The unmistakable correlation showed 