Photo courtesy of Antoine Taveneaux, CC BY-SA 3.0
Thanks to genes they inherited from close relatives of vanished humans, Denisovans and Neanderthals, China's Tibetan communities have survived on top of the world - calling the world's most inhospitable sites home, and in 2010, scientists extracted DNA that existed 41,000 years ago from the remains of bones the size of our hands.
In 2010, Rasmus Nielsen from Berkeley-area California discovered a mutation in a gene called EPAS1 in a Chinese Tibetan group that helped them adapt to an environment with very low oxygen levels. Thanks to this mutation, they can live in locations with 40 percent less oxygen than the air most of us breathe, and they can live at altitudes of up to 4,000 meters on plateaus where most of us cannot live unaffected. To date, this remains "the best case of natural selection in the human body" - the EPAS1 mutation is present in 87% of China's Tibetan population and only 9% of China's Han population, even though the two peoples were clearly separated at least 3,000 years ago.
In addition, the team conducted a study of 40 additional Tibetan ethnic groups and 40 Han ethnic groups targeting the EPAS1 mutation, and the scientists noted that the mutation in the Tibetan group appeared distinct from the others. The difference was so striking that it was unlikely to be the result of gradual evolution within the Tibetan group. It appears to have been inherited from other humans.
Studying other complete genomes, the team eventually discovered its origin: the Denisovans. The EPAS1 mutation in the Tibetan population is almost genetically identical to that of the Denisovans. Now, this became dominant in the Tibetan population, but initially this appeared in the Denisovans.
Such a discovery is indeed quite shocking, since we still do not know anything about the external features of the Denisovans. The only fossils we have are a finger bone, a toe and two teeth. By extracting DNA from these remains, scientists have identified a species of people whose existence was never known before, deciphering their genome and understanding the parts of the modern human body that contain them. The DNA of the Denisovans shows a 5 to 7% similarity to the genome of the Atlantic Melanesian Islands population. Some populations in East Asia also have a much lower rate of proximity. And at this moment, we know that the DNA of some Denisovans also has some similarities with Tibetan populations.
Svante Paabo, the researcher who studied the Denisovan genome sequence, showed considerable excitement about this. "It's really exciting to see these Denisovan genes in some modern groups, and we only discovered in 2010 that an ancient race that was extinct is closely related to people living in some corners of the world today," he said.
"It was a complete surprise," Nielsen said. "We dared to make such an attempt several years after the Denisovans genome was published because we didn't think we could find any connection."
This discovery also adds a piece to the puzzle of human evolution - including a large number of cross-population intersections. Humans evolved in Africa, and outside of this region, all continents appear in our prehistory as pioneers in very small groups. Those pioneers adapted to life in the treeless plains of the tropics. As they continued to migrate, they experienced any challenge the world could throw at them, such as extreme temperatures and entirely new diseases.
At that time, other species already existed in the world, such as Neanderthals and Denisovans. Migrants from Africa met with these populations and produced offspring. Through these fusions, their genomes fused with the DNA of people who had long adapted to these entirely new continents. "It's a whole new way of thinking about human evolution - genetic cross-fertilization between different human races," Nielsen said.
Photo courtesy of vtibet.cn
Nielsen suggests that between 30,000 and 40,000 years ago, modern humans intersected with Denisovans in Asia. They inherited the EPAS1 mutation in the Denisovans genome, which is rarely found in populations. This allowed their descendants to better adapt in higher altitude locations, and their descendants dominated the Tibetan plateau of China. This explains why the team found the EPAS1 mutation in Denisovans in most Tibetan ethnic groups, while it is rare in Han Chinese living outside the Tibetan Autonomous Region of China.
Other scientists say that Neanderthals also brought useful genes to our genome, including parts that affect the skin, hair and immune system. "What we have learned from ancient genomes is that each (gene) may have contributed only a little to our ancestors, but these gene sequences have become treasures in our bodies," anthropologist John Hawks said in an e-mail.
"What is quite surprising is not that the Denisovans are in a high-altitude location," Hawks said. "They lived in the Altai Mountains in Siberia, which is not too high. If the Denisovans have the EPAS1 mutation in their genes, then this explains why they should also be found in more mountainous regions of China and South Asia." This gives us an idea of a possible migration route for Denisovans along southeast Asia, and we recognize that modern humans acquired such genes on their way to Australia," Hawks said.
Nielsen adds that the Denisovans don't necessarily need to adapt to altitude. Their EPAS1 mutation could help them in other ways, and it just so happens to allow the Tibetan community to live on top of the world.
When ordinary people go to the Tibetan plateau in China, their bodies need to try to adapt by making more blood red blood cells, which can transport more oxygen to the body. But the body can produce too many red blood cells, which may cause the blood to become sticky, making people more susceptible to high blood pressure and stroke. The Tibetan community does not experience such problems. Their EPAS1 mutation keeps their bodies from overproducing blood red blood cells, and it's unlikely to be used in harmful ways to help them adapt to altitude. However, cold weather may cause blood vessels to constrict and raise people's blood pressure. So perhaps the Denisovans' EPAS1 mutation may allow them to withstand extreme cold without becoming hypoxic.
"If I hope to get a definitive answer, I need to find the Denisovans and perform some physiological experiments," he said. "But I can't do that."
Ref: Huerta-Sanchez. Jin, Asan, Bianba, Peter, Vinckenbosch, Liang, Yi, He, Somel, Ni, Wang, Ou, Huasang, Luosang, Cuo, Li, Gao, Yin, Wang, Zhang, Xu. Yang, Li, Wang, Wang & Nielsen, 2014. Possible genetic infiltration of Denisovans leads to adaptation of Tibetan SF to altitude. http://dx.doi.org/10.1038/nature13408
Information source: Ed Yong
