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Tibetans are able to adapt to high altitudes because of a gene they picked up when their ancestors mated with a species of human they helped push to extinction, according to a new study. 

According to the study, an unusual variant of a gene involved in regulating the body's production of hemoglobin became widespread in Tibetans after they moved onto the high-altitude areas many years ago. That particular hemoglobin allowed them to survive despite low oxygen levels at 15,000 feet or more. 

"We have very clear evidence that this version of the gene came from Denisovans," a mysterious human relative that went extinct 40,000-50,000 years ago, around the same time as the more well-known Neanderthals, under pressure from modern humans, said principal author Rasmus Nielsen, UC Berkeley professor of integrative biology in the press release. "This shows very clearly and directly that humans evolved and adapted to new environments by getting their genes from another species."

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

Researchers had reported the prevalence of high-altitude version of EPAS1 in Tibetans in 2010. Back then, they based their findings on sequencing the genomes of numerous Han Chinese and Tibetans. 

"There might be many other species from which we also got DNA, but we don't know because we don't have the genomes," Nielsen said in the press release. "The only reason we can say that this bit of DNA is Denisovan is because of this lucky accident of sequencing DNA from a little bone found in a cave in Siberia. We found the Denisovan species at the DNA level, but how many other species are out there that we haven't sequenced?"

Researchers will report their findings in journal Nature.