Merrit Kennedy at NPR:
About 50,000 years ago, ancient humans in what is now West Africa apparently procreated with another group of ancient humans that scientists didn’t know existed.
There aren’t any bones or ancient DNA to prove it, but researchers say the evidence is in the genes of modern West Africans. They analyzed genetic material from hundreds of people from Nigeria and Sierra Leone and found signals of what they call “ghost” DNA from an unknown ancestor.
Our own species — Homo sapiens — lived alongside other groups that split off from the same genetic family tree at different times. And there’s plenty of evidence from other parts of the world that early humans had sex with other hominins, like Neanderthals.
That’s why Neanderthal genes are present in humans today, in people of European and Asian descent. Homo sapiens also mated with another group, the Denisovans, and those genes are found in people from Oceania.
The findings on ghost DNA, published in the journal Science Advances, further complicate the picture of how Homo sapiens — or modern humans — evolved away from other human relatives.