The genetic make-up of Tibetans is likely the result of a mix of Nepali Sherpas and Han Chinese, according to American scientists.
They say that two ancestral gene pools - one belonging to a group that migrated 30,000 years ago and adapted to high altitudes, and another that migrated more recently from low altitudes - gave rise to Tibetans.
The study led by scientists from the University of Chicago also showed that the transfer of beneficial mutations between the two groups is the key reason why Tibetans are so well adapted to life above 4,000 metres.
Anna Di Rienzo, professor of human genetics at the university and one of the authors of the study, said: "Modern Tibetans appear to descend from populations related to modern Sherpa and Han Chinese. Tibetans carry a roughly even mix of two ancestral genomes."
The research was published in Nature Communications.
A similar study three years ago also suggested the majority of the Tibetan gene pool may have diverged from Han Chinese about 3,000 years ago, but the new study provides more insights.
The team looked at data from 69 Nepali Sherpas and 96 dwellers of the Tibetan Plateau and Yunnan .
"We studied three samples of Tibetans: one from Lhasa, one from Qinghai province and one from Yunnan province," Di Rienzo said.
"We concluded that the three samples analysed originated from an ancient population adapted to high altitude that mixed with migrants from low altitude. Through this mixing, the migrants acquired the genetic adaptations from the high-altitude residents."
Genetic admixture is the result of breeding between two originally separated populations.
In the case of Tibetans, researchers found high-altitude ancestry in the Tibetan genome, indicating that low-altitude migrants received adaptive traits from the highlanders.
But genetic identity is separate from ethnic identity.
"It is important to emphasise that the events we infer are quite ancient, many thousands of years ago. Therefore, we are talking about populations that are 'ancestral' to the contemporary ones, whether they are Han Chinese or Sherpas. It is entirely possible that the ethnic identity of these contemporary groups had not yet formed as such," Di Rienzo said.