So surprising was the find that Viviane Slon didn’t believe her results at first. “My first reaction was, ‘What did I do wrong?’” says Slon, a researcher at the Max Planck Institute for Evolutionary Biology. Ancient DNA is notoriously finicky. Because the old genetic material is so degraded and fragmented, it is easy to get tantalizing but false results. She repeated her experiments, again and again, extracting DNA six separate times. “It’s really when we saw this over and over again we realized, in fact, it was mixed Neanderthal and Denisovan ancestry,” she says.

Neanderthals and Denisovans split off from each other some 400,000 years ago, making them far more distinct than any two groups of modern humans living today. Yet both appeared to have lived in or around the Denisova Cave. In 2010, excavators also found a Neanderthal toe bone in the cave. This new bone fragment—from the daughter of a Neanderthal and a Denisovan—suggests the two groups not only inhabited the same place but at the same time.

It wasn’t just Neanderthals: Ancient humans had sex with other hominids.

Russian scientists first excavated this sliver of bone in 2012. It was one of more than 2,000 fragments that Slon’s collaborators at Oxford analyzed using a protein called collagen. The collagen in this one, they realized, was of human-like origin, so they sent it to the ancient-DNA lab at Max Planck for extraction. The inch-long fragment is too small to even tell which bone it came from. Nevertheless, it yielded a wealth of genomic information.

The daughter herself was a mix of Neanderthal and Denisovan. Her mother’s half of the genome most resembled DNA from a Neanderthal found in Croatia. It did not particularly match DNA from the Neanderthal actually found right in the Denisova cave in 2010, suggesting that Neanderthals migrated west to east in multiple waves. Her father’s Denisovan half of the genome actually had a touch of Neanderthal DNA—suggesting he too had a Neanderthal ancestor hundreds of generations ago. And somehow, 50,000 years ago or more, her mother and father met. The proof is in her DNA.

The discovery has stunned scientists, but it also has them questioning whether it is so stunning at all. Svante Pääbo, the director of the Max Planck Institute for Evolutionary Anthropology, recalls sequencing a 40,000-year-old human in Romania, which turned out to have a Neanderthal ancestor just four to six generations back. Interbreeding is so rare, he thought at the time, that the discovery of such a recent ancestor must just be a fluke. But after sequencing just six individuals from the Denisova cave, they have already found a direct hybrid offspring. Maybe it was not so uncommon after all.