After generations of natural selection, the light skin alleles became uniform among European populations, and dark skin alleles were maintained in sunnier, hotter regions. But in the original modern human source population, everyone had those genes — in fact, we probably inherited them from our hominin ancestors.
“Any population that left [Africa] almost certainly had the light and the dark alleles because they’re so old,” says Tishkoff, who explains that with the genes her team identified, many of the ancestral variants were, in fact, the light-skinned variant. “One of the reasons for thinking that is that our closest ancestors are chimps,” she says.
Dark skin isn’t necessary when you have body hair, she explains, but around the time of Homo habilis, who left the forest and went into the savannah, there would have been natural selection to lose the body hair and increase the number of sweat glands, and if they lost the body hair there would be a selection to have darker pigmented skin because of more skin exposure.
There’s a lot we still don’t know about the biology of skin color, but the more we learn about it, the less important skin color seems to be. If you think of every human’s genome as a novel written in just four nucleotide letters — G, A, T, and C — the genetic differences between people with different skin tones don’t amount to much more than alternate spellings of certain words.