The Tibetan Plateau is one of the harshest environments on Earth. It encompasses nearly a million square kilometers of land in the air, covered with cold, lean air and hard to breathe. Today, Chinese researchers have made a remarkable discovery on the roof of the world: the oldest signs of human activity in this demanding landscape.
Researchers led by Xiaoling Zhang, an archaeologist at the Institute of Vertebrate Paleontology and Paleoanthropology in China, discovered more than 3,600 stone artifacts in a part of the Tibetan central plateau called Nwya Devu. The site is rich in black slate – it's not the ideal raw material for stone tools, but the best available for miles around. Whatever the toolmakers, they took advantage of what they had to make flake stone flakes up to eight inches long.
Most of the tools were found buried in the earth, 30,000 to 40,000 years ago, the sunlight was visible, making Nwya Devu the oldest sign of human activity well dated Tibetan plateau. And at 15,000 feet above sea level, Nwya Devu is also the highest archaeological site in the world, dating back more than 10,000 years.
The discovery, published in Science On Thursday, highlight the resilience of modern humans as they scattered outside Africa and the world. The discovery could also help to understand how the DNA of Denisovans – a mysterious human species that lived in Siberia – could have helped modern Tibetans prosper.
"This is really the first case that shows that there were human populations in the highlands," says archaeologist Jeff Brantingham of the University of California, Los Angeles, who studies the Tibetan plateau without take care of nothing. this study.
Sunlight puts the clock
Previously, archaeologists knew that hunter-gatherers lived on the sidelines of the Tibetan Plateau about 15,000 years ago. Many experts have argued that no one was permanently living on the Tibetan Central Plateau until about six or seven thousand years ago, once Tibetans had mastered the raising of yaks and sheep and barley growing. Meanwhile, however, some researchers have been hoping for older evidence.
"My name is on the publications that conclude [a late peopling of the plateau]and I believed it, but I never really liked it, "says study co-author John Olsen, an archaeologist at the University of Arizona.
The main problem was that superficial evidence dating back more than 15,000 years was scarce. Some sites had intriguing stone tools, but they were found scattered on the surface. To reliably date old tools, researchers had to find out which ones were buried and which had remained intact since the time of their creators.
Enter Nwya Devu. In 2013, Zhang's team began working on the site, digging a total of 20 test pits. The researchers finally found a layer of earth bearing stone tools.
To date, Zhang's team has relied on the fact that some crystals present in these sediments act a bit like stopwatches, accumulating the dose of natural radiation that they absorb. Sunlight restoring this chronometer to zero, so by measuring the amount of radiation absorbed by the sediments in the dark, Zhang's team concluded that the soils – and the tools that they contain – were exposed to the sun 30,000 to 40,000 years ago.
Olsen assumes that the toolmakers used Nwya Devu as a seasonal workshop and camping site. Groups of hunter-gatherers could have camped on the site for several weeks, perhaps scheduling their visits to herds of migratory birds, which still stop today to recover on the lakes located a few kilometers from the site.
Who made the tools?
No human remains nor DNA has yet been found in Nwya Devu, leaving questions about the identity of the former toolmakers.
"The authors have used the word" Tibetan "a lot, and they act as if the people they were looking at were actually Tibetans – that's not the case," said the explorer. National Geographic, Mark Aldenderfer, archaeologist at the University of California. Merced. "We do not know who these people were."
That said, the discovery could still help explain how researchers interpret the genetic history of modern Tibetans. By comparing the DNA of different people, geneticists can go back in time and have a blurred glimpse of how genetically distinct populations were mixed and mixed. Using this approach, two recent studies have concluded that most modern Tibetan ancestries date back to a population that diverged from the Han Chinese about 9,000 years ago.
However, the DNA also tells a longer and more confusing story. Both studies show signs of differentiation between Tibetans and Han Chinese dating back 40,000 to 50,000 years, which may indicate a first wave of people on the Tibetan plateau. Since Nwya Devu dates back to about the time of these genetically inferred population movements, the site allows locating the ancient peoples and the routes they have taken to get to the plateau.
The site's tools do not look like those found in eastern China, but they seem almost identical to the tools recovered in Mongolia and Xinjiang, an autonomous territory located in northwestern China. Similar tools have also been found in the western Tibetan plateau, Aldenderfer said.
This is really the first solid case to show that there were human populations in the highlands.
University of California at Los Angeles
One of the genetic studies also offers an intriguing clue to how people have managed to survive at such altitudes. The study revealed that about 30,000 to 60,000 years ago, a group of people living in the area held high amounts of DNA from Neanderthals and Denisovans, some of our species sisters now missing. This group eventually brought some ascendancy to modern Tibetans, passing on Denisovan's DNA.
In particular, modern Tibetans have a variant of Denisovan's gene EPAS1 much more often than we would expect by chance. This variant is thought to help Tibetans survive at high altitudes by helping their blood to absorb more oxygen.
Future excavations in Nwya Devu or elsewhere on the shelf would help clarify the situation, especially if the researchers found human remains or were able to extract fragments of the DNA from the tool makers in the soil itself.
"Neither biologists nor archaeologists can tell this story without the help of the other," says Olsen.