The ability of Tibetans to live on the "roof of the world" may be down to their DNA, US researchers say.
University of Utah researchers found 10 genes which help Tibetans thrive at heights where others get sick.
Two of the genes are linked to haemoglobin - the substance in blood that transports oxygen round the body.
Doctors say the research, published in Science, could lead to treatments for severe forms of altitude sickness and other illnesses.
Altitude sickness is the name given to ill-effects caused by the body's struggle to deal with a lack of oxygen at high altitude.
It helps us understand how patients cope with low oxygen levels
It can lead to brain and lung complications, which can threaten even the fittest mountaineers.
People native to high altitudes appear to be immune to such effects, through thousands of years of genetic selection.
Tibetans have evolved genes that others living at high altitudes - such as in the Andes - have not.
Professor Lynn Jorde, of the University of Utah School of Medicine, said: "For the first time, we have genes that explain that adaptation."
The study looked at DNA extracted from blood samples taken from 75 villagers living at 15,000ft (4,500m).
The Utah team, in collaboration with the Qinghai University Medical School, China, compared stretches of their genetic code with that of lowland Chinese and Japanese populations.
A handful of genes turned up, including 10 unique oxygen-processing genes.
Two appear to contribute to lower levels of haemoglobin in the blood, which may help the body fight altitude sickness.
Prof Josef Prchal of the University of Utah said the work could help in developing treatments for illnesses that affect people everywhere.
He said: "What's unique about Tibetans is they don't develop high red blood cells counts.
"If we can understand this, we can develop therapies for human disease."
Professor Hugh Montgomery is a geneticist and director of the UCL Institute for Human Health and Performance at University College London.
He said the study helped in the understanding of how patients with the likes of heart failure and lung disease cope with low oxygen in the blood.
He told the BBC: "It's important clinically because it helps us understand how patients cope with low oxygen levels.
"There are opportunities here for developing new drug therapies."
He said the work was also important scientifically, by showing how Darwin's science coupled with modern technology could be used to identify beneficial genes.
"It's a lovely example of Darwinism," he added.