Tishkoff says that her results complicate the traditional evolutionary story of human skin. In this view, humanity began with dark skin in Africa to protect against the harmful effects of the sun’s ultraviolet radiation. As people migrated to other continents, some groups evolved lighter skin, to more effectively produce vitamin D in areas where sunlight is scarce.
But most of the variants that Tishkoff’s team identified, for both light and dark skin, have an ancient African origin. They likely arose in hominids like Homo erectus long before the dawn of our own species, and have coexisted in balance for hundreds and thousands of years. In many cases, the older variant is responsible for lighter skin, not darker. That’s consistent with an idea from Nina Jablonski, an anthropologist from Pennsylvania State University, who thinks that the ancient ancestors of humans—much like other primates—had pale skin. “As our ancestors moved out of the forest and into the savannah, they lost their hair and evolved darker skin,” says Nick Crawford, a researcher in Tishkoff’s lab.
But that wasn’t an all-encompassing change. Different groups of people adapted to their own particular environments, not just around the world, but within Africa, too. “Africa is not some homogenous place where everyone has dark skin,” Tishkoff says. “There’s huge variation.” For example, her team’s measurements showed that the Nilotic peoples in eastern Africa have some of the darkest skin around, while the San of southern Africa have light skin, comparable to some East Asians.
This physical diversity is mirrored in these groups’ genes. The first gene identified as affecting human skin color—MC1R—is very diverse in European populations but remarkably similar across African ones. Based on that pattern, says Tishkoff, some geneticists have concluded that the evolutionary pressure for dark skin in Africa is so strong that any genetic variants that altered skin color were ruthlessly weeded out by natural selection. “That’s not true,” says Tishkoff—but it’s what happens when you only study skin color in Western countries. “When you look at this African-centered perspective, there’s a lot of variation.”
For example, a gene called MFSD12 has variants that are linked to darker skin; these are common in dark-skinned people from East Africa, but rare among the lighter-skinned San. MFSD12 also shows how the search for pigmentation genes can reveal new insights about the basic biology of our skin. Two years ago, the gene didn’t even have a name, but it was linked to vitiligo—a condition where people develop white patches on dark skin. By deleting the gene in fish and mice, Tishkoff’s colleagues confirmed that it controls the balance between light and dark pigments.
Another gene called SLC24A5 has a variant that has traditionally been seen as “European,” because it is so starkly associated with lighter skin in Western European populations. But Tishkoff’s team showed that the variant entered the East African gene pool from the Middle East several millennia ago and well before the era of colonization. Today, it is common in Ethiopian and Tanzanian groups, but rare in other areas.