Ed Yong in The Atlantic:
Around 41,000 years ago, a young woman died in a cold cave, high up in Siberia’s Altai Mountains. Scientists uncovered one of her pinky bones in 2008. From it, they extracted her DNA. And from that, they deduced that she belonged to a previously unknown group of ancient hominin, whom they called the Denisovans after the cave where the finger was found.
To this date, we have no idea what a Denisovan looked like. You can still hold every known Denisovan fossil—that pinky, a toe, and two teeth—in your hand. But we know so much else about them. We know almost every letter of their genome. We know that they diverged from their close relatives, the Neanderthals, around 400,000 years ago, and that both groups diverged fromHomo sapiens around 600,000 years ago. We know that when our ancestors left Africa and spread into Asia, they encountered the Denisovans and had sex with them. We know that, as a result, Denisovan DNA lives on in people from Asia and Melanesia. One of these Denisovan genes provides modern Tibetans with a crucial adaptation that allows them to survive at high altitudes.
And now, thanks to work from Sharon Browning at the University of Washington, we know that Denisovan DNA entered the human gene pool on two occasions. Two separate groups of our horny, globe-trotting ancestors met with these mysterious hominins, and mated with them.