Until recently, researchers assumed that after human ancestors shed most body hair, sometime before 2 million years ago, they quickly evolved dark skin for protection from skin cancer and other harmful effects of UV radiation. Then, when humans migrated out of Africa and headed to the far north, they evolved lighter skin as an adaptation to limited sunlight. (Pale skin synthesizes more vitamin D when light is scarce.)
Previous research on skin-color genes fit that picture. For example, a “depigmentation gene” called SLC24A5 linked to pale skin swept through European populations in the past 6000 years. But Tishkoff ’s team found that the story of skin color evolution isn’t so black and white. Her team, including African researchers, used a light meter to measure skin reflectance in 2092 people in Ethiopia, Tanzania, and Botswana. They found the darkest skin in the Nilo-Saharan pastoralist populations of eastern Africa, such as the Mursi and Surma, and the lightest skin in the San of southern Africa, as well as many shades in between, as in the Agaw people of Ethiopia.
At the same time, they collected blood samples for genetic studies. They sequenced more than 4 million single nucleotide polymorphisms (SNPs)—places where a single letter of the genetic code varies across the genomes of 1570 of these Africans. They found four key areas of the genome where specific SNPs correlate with skin color.
The first surprise was that SLC24A5, which swept Europe, is also common in East Africa—found in as many as half the members of some Ethiopian groups. This variant arose 30,000 years ago and was probably brought to eastern Africa by people migrating from the Middle East, Tishkoff says. But though many East Africans have this gene, they don’t have white skin, probably because it is just one of several genes that shape their skin color