For a brief period in our species’ history, we shared our world with other sapient humans, closely related to us but distinct. We don’t know much about how our ancestors interacted with these other now-extinct hominins, but we know that at least some of those interactions were pretty intimate, because many modern humans now carry traces of DNA from Neanderthals and another ancient hominin group called Denisovans.

Most modern people of European and Asian descent carry between one- and three-percent Neanderthal DNA, and most people of Asian and Oceanian descent carry about one- to five-percent Denisovan DNA. Because Neanderthals and Denisovans arose outside Africa, the ancestors of modern African people would never have encountered them, although researchers have suggested that a so-far unidentified hominin species in Africa mingled with our ancestors there, so all of us may carry traces of that distant relative as well.

These weren’t isolated incidents. The genetic legacy that many of us now carry is probably the mark of years of sustained contact between two groups. Consistent with that, it now turns out that humans may have had contact with Denisovans not just at one place and time, but two.