A new DNA study of Tibetans has identified the genome behind their ability to live in high-altitude, low-oxygen environments. It was found that this gene came from an unexpected source - from a mysterious group of ancient people, whom scientists named Denisovans (Denisovan man). The research was carried out in collaboration between Chinese, Danish and American scientists and was published in the journal Nature.
In the 1990s, there was a consensus on the origin of the human species, Homo sapiens, to which we all belong. This species is young, developed in Africa about 200 thousand years ago, spread throughout the world and reached Europe, perhaps only 60 thousand years ago.
But we are not the first species of the genus Homo to live in Europe. Fossils from 300,000 to 30,000 years old indicate the presence of Neanderthals and Homo Neanderthals. Many believed that modern Europeans and Asians left Africa and drove out Neanderthals, that it was a complete replacement, without interbreeding. Scientists discovered the sequence of one small piece of Neanderthal DNA found in the mitochondria of a cell, and concluded that Neanderthals were significantly different from all modern humans.
But in 2010, new research refuted this hypothesis. Genome analysis of Neanderthals showed that modern Europeans and Asians, not Africans, inherited 1–4% of their DNA from Neanderthals. Obviously there was crossbreeding between species.
Another surprise for the scientific world - fragmentary fossils of a man who lived about 50,000 years ago were found in Denisova Cave in Siberia. Scientists obtained high quality DNA and sequenced the Denisovan human genome. These ancient people were genetically different from the Neanderthals, just like modern people. Suddenly, three different groups of people emerged, simultaneously living 50,000 years ago. Did Denisov's man interbreed with modern people? Apparently, yes, since a small part of the DNA of modern New Guineans showed a great similarity to the DNA of the Denisovan man.
Researchers have compared the DNA of Denisovan humans with that of modern humans to find evidence of adaptations formed by natural selection. The goal of some of the studies was to uncover the genetic causes of important biological differences between modern humans and our ancestors. Other researchers have found local adaptations in specific human populations that have allowed them to cope with unusual environmental conditions.
The latest study has identified genetic variants that may have led the Tibetans to adapt to live at 4,000 meters above sea level, where the oxygen concentration is more than half that at sea level. Scientists have found that a single gene, called EPAS1, produces a protein that regulates many of the genes involved in the body's processing of oxygen. The gene found in the Tibetans was also found in the Sherpas and Mongols, who also live in high altitude conditions.
EPAS1 showed signs of natural selection in the Tibetans, as an initially rare variant became common among the population. Scientists suggest that, as in other cases of adaptation to local conditions in the human body, the variant of the gene adapted to the low oxygen content appeared as a result of a random mutation in modern humans (Homo Sapiens) of Central Asia.
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New research has shown unexpected results. By sequencing this gene in many Tibetans and Han people for comparison, the authors found that this gene is very different from the rest of their chromosomes.
But the Tibetan gene had too many differences from the Han gene, but a variant of the Tibetan gene was found in the ancient Denisovans.
Thus, in this case, local adaptation became possible due to the acquisition of the gene of a pre-existing human species - the Denisovan. In Denisovans, this gene probably provided adaptation to conditions with low oxygen content for hundreds of thousands of years. This gene then spread in one modern human group - among the Tibetans, whose environment required it.