Scientists have found evidence that an ancient species of human called Denisovans lived at high altitudes in Tibet.
The ability to survive in such extreme environments had previously been associated only with our species - Homo sapiens.
The ancient ancestor seems to have passed on a gene that helps modern people cope at high elevations.
Details of the study are published in the journal Nature.
The Denisovans were a mysterious human species living in Asia before modern humans like us expanded across the world tens of thousands of years ago.
Until recently, the only fossils came from a few fragments of bone and teeth from a single site in Siberia - Denisova Cave.
But DNA had shown that they were a distinct branch of the human family.
Now, scientists have identified the first Denisovan fossil from another site. It's a mandible (lower jawbone) discovered in 1980 at Baishiya Karst Cave, 3,280m up on the Tibetan Plateau.
A technique called uranium-series dating was used on carbonate deposits on the bone. This yielded a date of 160,000 years ago for the mandible.
Co-author Jean Jacques Hublin, from the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, said finding evidence of an ancient - or archaic - species of human living at such high elevations was a surprise.
"When we deal with 'archaic hominins' - Neanderthals, Denisovans, early forms of Homo sapiens - it's clear that these hominins were limited in their capabilities to dwell in extreme environments.
"If you look at the situation in Europe, we have a lot of Neanderthal sites and people have been studying these sites for a century-and-a-half now.
"The highest sites we have are at 2,000m altitude. There are not many, and they are clearly sites where these Neanderthals used to go in summer, probably for special hunts. But otherwise, we don't have these types of sites."
Of the Denisovans on the Tibetan Plateau, he said: "It's a plateau... and there are obviously enough resources for people to live there and not just come occasionally."
While the researchers could not find any traces of DNA preserved in this fossil, they managed to extract proteins from one of the molars, which they then analysed applying something called ancient protein analysis.
"Our protein analysis shows that the Xiahe mandible belonged to a hominin population that was closely related to the Denisovans from Denisova Cave," said co-author Frido Welker, from the University of Copenhagen, Denmark.
The discovery may explain why individuals studied at Denisova Cave had a gene variant known to protect against hypoxia (oxygen deficiency) at high altitudes. This had been a puzzle because the Siberian cave is located just 700m above sea level.
Present-day Sherpas, Tibetans and neighbouring populations have the same gene variant, which was probably acquired when Homo sapiens mixed with the Denisovans thousands of years ago.
In fact, the gene variant appears to have undergone positive natural selection (which can result in mutations reaching high frequencies in populations because they confer an advantage).
"We can only speculate that living in this kind of environment, any mutation that was favourable to breathing an atmosphere impoverished in oxygen would be retained by natural selection," said Prof Hublin.
"And it's a rather likely scenario to explain how this mutation made its way to present-day Tibetans."
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