On the mountainous Tibetan Plateau, small groups of nomadic herders still make a living two miles or more above sea level. Most of us would be poorly-equipped to deal with that altitude for long periods of time, but the Tibetans there have unique genetic adaptations that let their bodies function in the thin air.

Mysteriously, those genes seem to come from another species of human, the Denisovans, a little-understood group of hominins who died out tens of thousands of years ago. Until now