Tibetan Plateau Denisovans Gave Modern Tibetans Altitude Superpowers
Little is known about our extinct archaic hominin cousins, the Denisovans, who populated Asia during the Lower and Middle Paleolithic, due to limited DNA fragments and evidence. This makes reconstructing their history is a major challenge and progress takes time as more evidence of their existence emerges. Now, a new study has found that our extinct cousins reached the Tibetan plateau, thought to be one of the last places on Earth populated by humans, about 160,000 years ago! And that the genes they passed on have given the Tibetan people high altitude superpowers the study concluded.
In the context of the Tibetan Plateau Denisovans' study, high-altitude adaptability was passed on through interbreeding with Homo sapiens in the region. (JEGAS RA / Adobe Stock)
Tibetan Plateau Denisovans, Interbreeding, and Adaptation
The study findings reset the Tibetan Plateau Denisovans timeline to roughly 120,000 years earlier than previously thought. And this could provide an explanation for early human adaptations to high altitudes.
“The peopling of the Tibetan Plateau is a spectacular example of human adaptation to high altitudes as Tibetan populations have thrived for generations under strong selective pressures of the hypoxic environment,” said the authors of the study, published in the Trends in Ecology and Evolution journal. The study was published by archaeologists at the University of California, Davis, co-authored by Peiqi Zhang, a UC Davis doctoral student, and Xinjun Zhang, a UCLA postdoctoral researcher.
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Hypoxia, or oxygen starvation, is a symptom unknown to the Homo sapiens peoples of the Tibetan plateau because of their prehistoric interbreeding with Denisovans and Neanderthals. An article in The Atlantic on mitochondrial DNA has revealed that the indigenous people of Australia, Polynesia, Micronesia and Melanesia only show a 5% gene influence from Denisovans. But what about the Tibetans?
According to the latest study, interbreeding between Denisovans and Tibetan modern humans occurred twice in the hominin evolution timeline on the Tibetan Plateau. And this interbreeding passed on adaptive genes to some of the resultant children, which over many thousands of years gave the Tibetan people the capacity to be essentially immune to the negative effects of high altitudes. Obviously, most modern humans don’t’ have these adaptive genes because most people are not comfortable at high altitudes and generally experience hypoxia or oxygen starvation under such conditions.
Genetic studies have proven that all East Asians, including the Tibetans, interbred with two distinct Denisovan groups. One of these events is unique to East Asians; the other is found amongst both East and South Asians. Since all East Asians have these Denisovan genes, the recent study authors believe that this interbreeding event probably didn’t take place on a high-altitude Himalayan plateau but in a lowland region.
The new study from the University of California, Davis used genetic reconstruction techniques to arrive at their findings on Tibetan Plateau Denisovans. (vrx123 / Adobe Stock)
Answers in Archaeological and Genetic Reconstruction
The news release from the University of California, Davis describes how the researchers combined archaeological and genetic expertise and information as a mode of study. Peiqi Zhang has worked extensively on archaeological digs above 15,000 feet (4,600 meters) in Tibet, while Xinjun Zhang has worked with Denisovan and other hominin DNA. Human spread and settlement on the Tibetan Plateau prompting the burning question, "What do we know about how and when the region was populated?”
"There was a dearth of comprehensive analysis bringing both subjects together before ours, especially with an equal emphasis," Peiqi Zhang acknowledged, highlighting the difficulties in research methodology and historical reconstruction. “Archaeological and genetic studies provide essential insights into behavioral and biological human adaptations to high elevations but there is a lack of models integrating data from the two fields,” they wrote in the study.
Because of the genes passed on by the Tibetan Plateau Denisovans this Tibetan farmer doesn't experience any feeling of discomfort at high altitudes. (stveak / Adobe Stock)
Four Periods of Occupation and the EPAS Gene
The newly amended Tibetan Plateau Denisovans’ timeline indicates four major periods of occupation, starting with the Denisovans about 160,000 years ago. This was followed by three distinct periods of modern human settling, respectively dated around 40,000 years ago, 16,000 years ago, and 8,000 years ago.
The first ever Denisovan individual was identified in 2010, based on mitochondrial DNA extracted from a juvenile female finger bone discovered in the Altai Mountains of Siberia. The nuclear DNA from said find indicated close affinities with Neanderthals, our other extinct archaic cousins, who went extinct 40,000 years ago. Phys.org reports that the Denisovan Siberian female DNA also carried a haplotype highly similar to the Endothelial Pas1 (EPAS1) gene, which vastly improves oxygen circulation in the blood. Incidentally, this same gene is highly prevalent in most modern Tibetans.
"Based on archeological evidence, we know that there are gaps between these occupation periods," Peiqi Zhang said. "But the archeological work on the Tibetan Plateau is very limited. There's still a possibility of continuous human occupation since the late Ice Age, but we haven't found enough data to confirm it."
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Both researchers, along with their research team, came up with two proposed spread and settlement models as a framework for future studies and discoveries. First, random visits followed by a period of settling around the end of the Ice Age, some 9,000 years ago. Second, continuous occupation that began 30,000 to 40,000 years ago. In either model, Denisovans passed on the EPAS1 gene to modern humans somewhere between 48,000 and 46,000 years ago.
While the extinction date or period of Denisovans is unknown some studies have suggested that it happened about 20,000 years ago.
The main takeaway of this study is what special powers some modern humans inherited from Denisovans, particularly adaptation to high altitudes and cold climatic conditions. This is a revolutionary adaptive mechanism that may allow some humans to do better under climate change extremes or another Ice Age one day in the future.
Top image: We have remains of Tibetan Plateau Denisovans, who lived for long periods in this landscape. But now we know that they passed their high-altitude adaptability genes on to Tibetans and other peoples that ultimately settled in the Himalayas. Source: zah108 / Adobe Stock
By Sahir Pandey
Holder, K. 2021. Denisovans or Homo sapiens: Who were the first to settle permanently on the Tibetan Plateau? Available at: https://phys.org/news/2021-12-denisovans-homo-sapiens-permanently-tibetan.html
Kumar, N. Denisovans or people: Who was the first to populate the Tibetan plateau? Available at: https://thetimeshub.in/denisovans-or-people-who-was-the-first-to-populate-the-tibetan-plateau
Zhang, P., Zhang, X., et al. 2021. Denisovans and Homo sapiens on the Tibetan Plateau: dispersals and adaptations. Trends in Ecology & Evolution. Available at: DOI: 10.1016/j.tree.2021.11.004