Tibetans live in a region that averages more than 4,000 meters above sea level. (Not for nothing is it called the roof of the world.) How did they come to be able to cope with their extreme environs? Some researchers in China and the United States think they might know, and their findings were published Wednesday in Nature. By sequencing DNA from a group of Tibetans and comparing the code to other gene databases, the researchers have discovered that Tibetans are inheritors of an ancient trait that helps regulate the oxygenation in their blood. But surprisingly, this trait did not arise in Homo sapiens. Rather, it came from another group of humans, the Denisovans—mysterious, little-known hominid cousins that died out some 40,000 years ago.

The new study on Tibetans demonstrates for the first time an evolutionary advantage conferred directly by Denisovans, an adaptation that seems to be singular to the Tibetan people. For people whose ancestors lived in milder altitudes, experiencing a dearth of oxygen at great heights causes the level of hemoglobin, the protein that carries oxygen in blood, to increase in attempt to compensate. But this raises the likelihood of cardiac events in the short term, and it is unhelpful for reproduction, as it increases the risk of preeclampsia (hypertension during pregnancy). Tibetans don’t have the same reaction to elevation: They have greater fitness and higher fertility even when there is little to breathe. This, along with other respiratory adaptations, allows them to thrive where others cannot.

Denisovans and Neanderthals are called extinct human “species”—a term that used to demark a clear line between two organisms incapable of interbreeding to produce fertile offspring. But the definition is no longer so clear. We know that these hominin cousins did couple with our Homo sapien ancestors—and some of us have inherited from them valuable modern traits. How we define “humans” past and present is a subject to contemplate—as fitting for scientists as for pilgrims to think about on their journeys across Tibetan plains.