If interbreeding were rare, we should not have found these individuals so easily, says Svante Pääbo, also of the Max Planck Institute for Evolutionary Anthropology. “It suggests that these groups, when they met, mixed quite freely with each other.”

This doesn’t mean Neanderthals and Denisovans were constantly interbreeding. Their genomes show they were “quite distinct populations”, says Pääbo. They controlled separate territories — the Neanderthals in Europe, the Denisovans in east Asia — and occasionally met at the boundaries. He says the Denisova cave was “a unique area where they met, and then they had no prejudices against each other”.