In what's considered one of the finest examples of natural selection in action, Tibetans have acquired the ability to thrive at extremely high altitudes. Incredibly, researchers say the gene required for this adaptation was inherited from the now-extinct Denisovans.
Tibetans can thrive at altitudes that would wreck most humans. Normally, exposure to altitudes exceeding 15,000 feet causes the blood to thicken, which can lead to cardiovascular problems and even death. But Tibetans have undergone extensive physiological and genetic changes, particularly in the regulatory systems of respiration and circulation. Specifically, they have an unusual variant of a gene involved in regulating the body's production of hemoglobin, the molecule that carries oxygen in the blood. This variant allows Tibetans to survive the low levels of oxygen.
Remarkably, they acquired this capacity in as little as 3,000 years — and it would now appear that interbreeding with another human species may have had something to do with it.
A recent genetic analysis performed by researchers as the University of California at Berkeley shows that the gene came from the Denisovans, a mysterious human relative that went extinct some 40,000 to 50,000 years ago. It's the first time a gene from another human species as been shown to help modern humans adapt to their environment.
From the UoC release:
The gene, called EPAS1, is activated when oxygen levels in the blood drop, triggering production of more hemoglobin. The gene has been referred to as the "superathlete" gene because at low elevations, some variants of it help athletes quickly boost hemoglobin and thus the oxygen-carrying capacity of their blood, upping endurance. At high altitudes, however, the common variants of the gene boost hemoglobin and its carrier, red blood cells, too much, increasing the thickness of the blood and leading to hypertension and heart attacks as well as low birth weight babies and increased infant mortality. The variant, or allele, found in Tibetans raises hemoglobin and red blood cell levels only slightly at high elevations, avoiding the side effects seen in most people who relocate to elevations above 13,000 feet.
"We found that part of the EPAS1 gene in Tibetans is almost identical to the gene in Denisovans and very different from all other humans," [Rasmus] Nielsen said. "We can do a statistical analysis to show that this must have come from Denisovans. There is no other way of explaining the data."
After scanning a larger set of worldwide populations, the researchers found that the variant is only found in Denisovans and in Tibetans, and at very low frequency among Han Chinese.
The researchers theorize that modern humans coming out of Africa interbred with Denisovan populations in Eurasia as they passed through that area into China. Their descendents still retain a small percentage — about 0.1% — of Denisovan DNA. But the group that entered China eventually split, with one population moving into Tibet and the other, now known as Han Chinese, remained in the lower elevations.
The researchers are now analyzing other genomes to pin down the time of Denisovan interbreeding.
Read the entire study at Nature: "Altitude adaptation in Tibetans caused by introgression of Denisovan-like DNA". Supplementary info via University of California, Berkeley.
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