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.
This article appeared in the South China Morning Post print edition as Tibetans ‘a mix of Sherpas and Han’ 
In conclusion, the array of data presented supports the Tibeto-Burman affinities of the Himalayan populations examined in the present study. Whereas Tibet and Nepal share high levels of haplogroup O3a5-M134 (especially O3a5a-M117), commonly found in East Asia, they display an absence of Southeast Asia–specific markers (O1-M119 and O2-M268). In combination with the relatively younger age generated for O3a5-M134, East and Central Asian ancestry for the Himalayan groups is likely. However, subsequent gene flow from the Indian subcontinent into Nepal is signaled by the presence of haplogroups R and H, as well as from the results of the admixture analysis, in which Indian contribution is >40% in both Newar and Kathmandu. In contrast, Indian influence in Tamang is null.
The Tibetan gene pool reflects significant contributions from East and/or Southeast Asia. The elevated presence of haplogroup D-M174 chromosomes in Tibet and Japan indicates prolonged geographic isolation of these regions, since it is sparsely distributed in the rest of East Asia. The age of Tibetan D1-M15 and D2-M55 lineages are considerably younger than the Paleolithic Japanese D2-M55, indicative of a recent bottleneck. Haplogroups Q-M242, C-M216, and N-M231 in Himalayan populations point to gene flow from Mongolia and Siberia, underscoring the geographic accessibility of the Tibetan plateau for human dispersals from the north. The Himalayas served as a strong barrier to gene flow from the south into the Tibetan plateau, although the same is not true for population movements occurring in the opposite direction. This preferential migrational direction may be associated with the physiological stress imposed on emigrants from lower altitudes. 
Since it has been agreed that Tibetan genetics belong to the Han and the Nepalese, it is more than likely that it began with Han and gradually intermarried with the Nepalese/Serpas. The Nepalese themselves are of mixed genetics with an input of Hindu blood and other tribal Burmese-Thai ancestry. Such intermingling of the different genetic traits is common in that area of Asia. Hence, for Tibetans to be considered as a part of the Chinese family of tribes is quite logical from their point of historical perspective
 Tibetans are genetic mix of Sherpa and Han: http://www.scmp.com/news/china/article/1426564/tibetans-are-genetic-mix-sherpas-and-han-us-study-claims
 Himalayan genetics: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1852741/