Science has long marveled at the endurance of high-altitude dwelling people, like the Tibetans, who can perform normally in environments with very little oxygen. According to scientists at UC Berkeley, the gene was developed long ago, when their ancestors bred with an ancient species of humans. However, it came at a price – the extinction of that very same species.

“We have very clear evidence that this version of the gene came from Denisovans,” said principal author Rasmus Nielsen, UC Berkeley professor of integrative biology. “This shows very clearly and directly that humans evolved and adapted to new environments by getting their genes from another species.”

The Denisovians are a little-known human relative that went extinct as many as 50,000 years ago, similar to Neanderthals. Like the Neanderthals, the Denisovians were likely pushed to the brink by modern humans.

The gene, called EPAS1, performs the same action in high-altitude humans as it does in world-class athletes closer to sea level: When blood oxygen levels drop, the gene kicks in and triggers the production of more hemoglobin. The variant in Tibetans, however, overcomes a serious problem – in normal humans, more hemoglobin at high altitudes leads to thicker blood, which can cause heart problems. To avoid this, the Tibetan allele tempers the hemoglobin response, raising it only slightly.

Current research estimates that around 87% of Tibetans now have the EPAS1 variant, likely a product of natural selection. It occurs in only 9% of Han Chinese (who share a common ancestor with the Tibetans), and nowhere else in the world. This leads scientists to believe that it could only have come from the Denisovians.

“We found that part of the EPAS1 gene in Tibetans is almost identical to the gene in Denisovans and very different from all other humans,” Nielsen said. “We can do a statistical analysis to show that this must have come from Denisovans. There is no other way of explaining the data.”

Now, scientists are wondering from where else our DNA might have come from, which proves difficult without genomes to sequence. Modern humans may have (and likely did) interbred with other species, but as to which ones exactly, no one’s sure.