Scientists already know that early humans mated with Neanderthals and Denisovans, distant relatives on the family tree. Now a new study suggests that another such group existed, one that has yet to be identified, reports the Guardian. In their study in the journal Science Advances, researchers say they found evidence of what they refer to as an "archaic ghost population." As NPR notes, evidence of this unnamed cousin didn't come from the discovery of a fossil in a cave. Instead, scientists made the find while studying the DNA of people in West Africa. "All have ancestry from this unknown archaic population," says lead researcher Sriram Sankararaman of UCLA, per the Guardian.
The theory is that ancestors of this branch of the tree split from the ancestors of modern humans about 1 million years ago. Researchers speculate that about 20,000 of them lived in what is now West Africa, and they mated with homo sapiens somewhere around 50,000 years ago. The result of that pairing is what turned up in the DNA study. "We don't have a clear identity for this archaic group," Sankararaman says, per NPR. "That's why we use the term 'ghost.'" The New York Times notes that the ultimate confirmation of this theory would be the discovery of a fossil with a DNA match to the "ghost" ancestors. The problem is that DNA degrades quickly in the tropical climate, though scientists are holding out hope. "I can't wait for that to happen," says Sankararaman. (Read more homo sapiens stories.)