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Earth has plenty of places that are difficult to live in. Hot and dry deserts like the Sahara and Atacama; cold and windy deserts like Antarctica; the constantly shifting ice packs of the Arctic. Nowadays humans live in pretty much all of them, venturing far outside the mosaic of grasses, trees and lakes that were our ancestral habitats.
But high altitudes pose a unique set of challenges. The air is thin, so it鈥檚 harder to breathe and therefore tricky to move around or work. It鈥檚 also cold, and liable to be windy, which makes farming difficult.
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On that basis, the Tibetan plateau in Asia is one of the hardest places to live permanently. It has an average elevation of 4000 metres above sea level, with some bits extending much higher. It鈥檚 also cold and rather dry. There is little rainfall, and much of the landscape is grasslands or steppes 鈥� along with large areas of permafrost. Oxygen levels are about 40 per cent lower than at sea level.
All in all, it鈥檚 understandable that archaeologists once believed the Tibetan plateau was one of the last places to be permanently settled by humans or other hominins. While hominins have been living in other parts of Asia for around 2 million years, the thinking was that they only settled the Tibetan plateau much more recently, within the past 5000 or 10,000 years.
However, over the past couple of decades that notion has been slowly but comprehensively blown apart. This realisation has been driven in large part by researchers at Chinese universities. Here鈥檚 how it happened and the new story of the settlement of the Tibetan plateau.
Roof of the world
For a long time, all the archaeological finds from the Tibetan plateau were from the past few thousand years. Based on that, the notion was that modern humans (Homo sapiens) only managed to permanently settle the higher steppes about 3600 years ago. However, it鈥檚 been clear for 20 years that people were venturing up there long before.
In 2002, David Zhang and Sheng-Hua Li at the University of Hong Kong found human hand and footprints in travertine, a kind of limestone that forms around mineral springs, near the Quesang river. They also found the remains of a fireplace. All these traces were dated to about 20,000 years ago鈥� just about the coldest point of the last glacial period.
A decade later in 2012, another hearth was found in the Qinghai Lake area in the north-eastern region of the plateau. It was 14,300 years old. The following year, stone tools were found at a third site called Xidatan 2. They were between 9200 and 6400 years old, and appeared to reflect only a brief or intermittent occupation, not permanent settlement.
The next year, a genetic study upended the whole story.
Altitude adaptation
Despite the low levels of oxygen in the air, Tibetans don鈥檛 tend to experience hypoxia. One gene that鈥檚 implicated in this is EPAS1, which is involved in the body鈥檚 response to low oxygen. Tibetans have a modified version of EPAS1, compared with people in nearby low-altitude regions.
In 2014, geneticists found that the Tibetan EPAS1 was not present in any other studied human population 鈥� but it was found in the genome of a Denisovan, an extinct group of humans that lived in Asia within the past few hundred thousand years. Humans are known to have interbred with Denisovans, and it seems the modified EPAS1 came to Tibetans from these hominins.
This immediately raised a striking possibility: did Denisovans have this mutation to help them cope with altitude? At the time, they were only known from one cave in Siberia, but the genetics hinted that they might also have lived on the Tibetan plateau.
Demonstrating this would require finding much older evidence of habitation 鈥� and of Denisovans on the plateau.
Further back in time
During the 2010s, a series of studies extended the evidence that people lived on the Tibetan plateau at several points in the past 20,000 years and offered hints as to how. A 2016 study of a site called XDW1 reported an ash layer and stone tools, dating from 11,200 years ago. According to the authors, it was the first evidence to show human activity above 4000 metres above sea level in the Tibetan plateau.
The following year came another dramatic find. At the Chusang site, about 4270 metres above sea level, human handprints and footprints had been found. Researchers determined that the site was between 7400 and 12,700 years old. Given the difficulty of accessing such a location, this suggested people were living there permanently rather than visiting. Furthermore, these people were not farmers or pastoralists 鈥� they were hunter-gatherers. There was some back-and-forth about this finding, but it seems to have held up: agriculture was not required to live permanently on the Plateau.
A 2019 study helped explain how hunter-gatherers survived. Researchers studied the 鈥�151 site鈥� in the Qinghai Lake basin, which was between 13,100 and 15,400 years old. Based on bones and other remains, the hunter-gatherers mostly targeted wild cattle and horses, bringing back highly nutritious parts like upper limb bones. The people were highly mobile, perhaps following herds on migrations.
That same year, evidence finally emerged of Denisovans on the Tibetan plateau.
Altitude pioneers
Researchers re-examined a jawbone that was originally found in 1980 in Baishiya Karst cave in the north-east of the plateau. Its shape did not match modern humans, and proteins from its teeth revealed it belonged to a Denisovan.
If that weren鈥檛 enough, radioisotope dating suggested it was at least 160,000 years old. At a stroke, the occupation history of the plateau had been extended by over 100,000 years.
A follow-up study in 2020 found several samples of Denisovan DNA in the sediments of Baishiya Karst cave, dating from 100,000, 60,000 and possibly 45,000 years ago. This indicated Denisovans either visited repeatedly or lived there permanently.
Other sites also point to hominins鈥� presence on the plateau early on. In 2018, researchers reported stone tools from a site called Nwya Devu that were 30,000 to 40,000 years old 鈥� but there were no hominin remains so it wasn鈥檛 clear who made them. Similarly, a 2021 study described yet another site, Jiangjunfu 01 in the north-eastern plateau, with lots of stone tools but no hominin remains. The tools were 90,000-120,000 years old.
Were Denisovans the first to reach and live on the plateau, or were other hominins there first? Right now we don鈥檛 know, but whoever was up there was very human.
In 2021, Zhang, now at Guangzhou University in China, and his team found more hand and footprints at the Quesang site. They were small, suggesting they were made by two children 169,000 to 226,000 years ago.聽The team described them as the 鈥渆arliest parietal art鈥�, meaning art created on surfaces like walls. That is controversial: some researchers argue that handprints left by children are play rather than art. You can make up your own mind about that.
The key point here is instead the length of time we鈥檙e now talking about. People may have been living and mucking about on the Tibetan plateau for over 200,000 years, not 3600 or even 20,000 as we thought a few years ago.
Finally, we see that they kept coming back to the same places. In July, Zhang and his colleagues published another study of the Quesang travertine. They found five handprints and 17 footprints, dated to various points between 6200 and 9200 years ago. Their interpretation is that the hot springs in the region were always attractive to hominins, perhaps because they offered a buffer against the frigid Tibetan climate. On the roof of the world, people turned to warmth from Earth鈥檚 interior to help them survive.
