Researchers suggest genetic adaptation that allowed humans to live high in the Tibetan mountains originated about 30,000 years ago.
Population mixing caused the genes to be passed onto recent migrants, who live at lower altitudes, a University of Chicago Medical Center news release reported.
High elevations can be problematic for life because of low oxygen levels, but Tibetans have adapted to these conditions. Low hemoglobin concentrations allow Tibetans to experience less complications such as certain blood clots that those from lower elevations.
"The Tibetan genome appears to arise from a mixture of two ancestral gene pools, "Anna Di Rienzo, PhD, professor of human genetics at the University of Chicago and corresponding author of the study said in the news release." One migrated early to high altitude and adapted to this environment. The other, which migrated more recently from low altitudes, acquired the advantageous alleles from the resident high-altitude population by interbreeding and forming what we refer to today as Tibetans."
Tibetans also have EGLN1 and EPAS1 gene variants which is involved in oxygen homeostasis. The team looked at "genome-wide data from 69 Nepalese Sherpa" to make their findings. The genes were compared with those from unrelated individuals.
The team found modern Tibetans were descended from Sherpa and Han Chinese. "Tibetans carry a roughly even mixture of two ancestral genomes: one a high-altitude component shared with Sherpa and the other a low-altitude component shared with lowlander East Asians," the news release reported
Modern Sherpa have very low levels of the low-altitude genome while highland genetics are low in lower-elevation dwellers; this suggests Tibetan ancestors exchanged genes in a process called "genetic admixture."
"This is a good example of evolution as a tinkerer," Cynthia Beall, PhD, professor of anthropology at Case Western Reserve University and co-author on the study, said in the news release. "We see other examples of admixtures. Outside of Africa, most of us have Neanderthal genes-about [two to five] percent of our genome-and people today have some immune system genes from another ancient group called the Denisovans."
The team found Tibetans also were found to share high-elevation genes with the Sherpa; they found these traits were "disproportionally enhanced" after admixture; indicating natural selection.
"The chromosomal locations that are so important for Tibetans to live at high elevations are locations that have an excess of genetic ancestry from their high-altitude ancestral gene pool," Di Rienzo said. "This is a new tool we can use to identify advantageous alleles in Tibetans and other populations in the world that experienced this type of admixture and selection."
