Tibetans inherited 'super athlete' gene that lets them live at high altitude from mating with extinct species of human
It is the first time a gene from another species of human has been shown unequivocally to have helped modern humans adapt to their environment, researchers said.
They found the gene came from Denisovans a human relative that became extinct 40,000-50,000 years ago, around the same time as the more well-known Neanderthals.
Tibetan nomads play billiards on an open grassland near Namtso Lake: Researchers have revealed the population evolved to thrive at high altitudes in just 3,000 years.
THE SUPER ATHLETE GENE
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.
The unusual variant of a gene involved in regulating the body’s production of hemoglobin – the molecule that carries oxygen in the blood – became widespread in Tibetans after they moved onto the high-altitude plateau several thousand years ago.This variant allowed them to survive despite low oxygen levels at elevations of 15,000 feet or more, whereas most people develop thick blood at high altitudes, leading to cardiovascular problems.
'We have very clear evidence that this version of the gene came from Denisovans,' said principal author Rasmus Nielsen, UC Berkeley professor of integrative biology.
'This shows very clearly and directly that humans evolved and adapted to new environments by getting their genes from another species.'
Nielsen and his colleagues at BGI-Shenzhen in China, the world’s largest genome sequencing center, published their research in Nature.
The gene, called EPAS1, is activated when oxygen levels in the blood drop, triggering production of more hemoglobin.
'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,' 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.'
Nielsen sketched out a possible scenario leading to this result: modern humans coming out of Africa interbred with Denisovan populations in Eurasia as they passed through that area into China, and their descendants still retain a small percentage – perhaps 0.1 percent – of Denisovan DNA.
The tiny fragment of finger bone has enabled scientists to map out the entire genetic code of the Denisovans, a little-known ancient cousin of modern humans
The group that invaded China eventually split, with one population moving into Tibet and the other, now known as Han Chinese, dominating the lower elevations.
Nielsen and his colleagues are analyzing other genomes to pin down the time of Denisovan interbreeding, which they said probably happened over a rather short period of time.
'There might be many other species from which we also got DNA, but we don’t know because we don’t have the genomes,' Nielsen said.
'The only reason we can say that this bit of DNA is Denisovan is because of this lucky accident of sequencing DNA from a little bone found in a cave in Siberia.
'We found the Denisovan species at the DNA level, but how many other species are out there that we haven’t sequenced?'
Earth's highest peak, Mount Everest, in the Tibet Autonomous Region, the People's Republic of China. Researchers have now revealed how the locals have adapted to the high altitudes
LIVING AT HIGH ALITUDES
Adaptation to low oxygen levels has allowed many peoples, from Andeans to Tibetans, to live at high altitude.
When people from lower elevations move above about 13,000 feet, where oxygen levels are about 40 percent lower than at sea level, they typically tire easily, develop headaches, produce babies with lower birth weights and have a higher infant mortality rate.
Tibetans have none of these problems, despite lower oxygen saturation in the blood and lower hemoglobin levels.
Hemoglobin, which gives blood its red color, binds and transports oxygen to the body’s tissues.
Previous research of the comparison of the genomes of 50 Tibetans and 40 Han Chinese shows that ethnic Tibetans split off from the Han less than 3,000 years ago and since then rapidly evolved a unique ability to thrive at high altitudes and low oxygen levels.
The genome-wide comparison, performed by evolutionary biologists at the University of California, Berkeley, uncovered more than 30 genes with DNA mutations that have become more prevalent in Tibetans than Han Chinese, nearly half of which are related to how the body uses oxygen.
One mutation in particular spread from fewer than 10 percent of the Han Chinese to nearly 90 percent of all Tibetans.
The new findings could steer scientists to till-now unknown genes that play a role in how the body deals with decreased oxygen, and perhaps explain some diseases, including schizophrenia and epilepsy, associated with oxygen deprivation in the womb, he said.
The widespread mutation in Tibetans is near a gene called EPAS1, a so-called 'super athlete gene' identified several years ago and named because some variants of the gene are associated with improved athletic performance.
Researchers obtained DNA from 50 Tibetans living in the Tibet Autonomous Region of China and 40 Han Chinese from Beijing.
The Tibetans lived in two villages located at elevations of 4,300 meters (14,100 feet) and 4,600 meters (15,100 feet).
Chinese researchers took blood samples from the participants and measured oxygen saturation, red blood cell concentration and hemoglobin content in their blood.
The analysis revealed that the common ancestors of Tibetans and Han Chinese split into two populations about 2,750 years ago, with the larger group moving to the Tibetan plateau.
That group eventually shrank, while the low-elevation Han population expanded dramatically. Today, the Han Chinese are the dominant ethnic group in mainland China.
The Tibetan branch either merged with the people’s already occupying the Tibetan plateau, or replaced them.
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