If it wasn't for an extinct relative of modern humans known as the Denisovans, some researchers suspect our own species might never have made their home on the highest and largest plateau in the world.
The Tibetan Plateau, sometimes called the Himalayan Plateau, is nicknamed 'the roof of the world' because it sits, on average, 4,000 meters (13,000 feet) above sea level.
This vast sweep of elevated land, which covers most of Tibet, along with parts of China, India, Pakistan, and several other countries in the region, is usually considered one of the last places that Homo sapiens settled permanently. Studies suggest there have been periods of occupation by various ancestors taking place over the past 160,000 years, but gaps in the record are hard to interpret.
Have there always been people up on the roof of the world, or is each period a resettlement by a new community?
A geneticist and an archaeologist have now suggested another timeline that works just as well with the limited evidence we have on hand.
The researchers incorporated both archaeological and genetic evidence to develop two, contrasting models of occupation: one continuous and one divided up over time. Crucially, the two models can be tested, potentially telling us one day how far back modern populations stretch.
In the discontinuous model, humans visited on and off for tens of thousands of years, until finally staying put around 9,000 years ago.
Alternatively, current evidence could also support permanent colonization that began on the plateau between 30,000 and 40,000 years ago. If so, the long genetic lineage might have passed on some helpful tricks for living up where the air is thin.
According to recent DNA analyses, a single crossbreeding event between Denisovans and H. sapiens in East Asia, no sooner than 46,000 years ago, might have infused our species with the genes they needed to make their home in such a low oxygen environment.
"Although we don't know if [Denisovans] were adapted to the high altitude, the transmission of some of their genes to us [could] be the game changer thousands of years later for our species to get adapted to hypoxia," says anthropologist Nicolas Zwyns from the University of California, Davis.
"That to me is a fantastic story."
Whether that's a true story, however, is not yet clear.
Archaeological evidence on its own suggests Denisovans first appeared on the Tibetan plateau about 160,000 years ago. But it's still not known whether these early humans made their home here all year round or just visited on occasion.
The same is true of our own species. The first archeological evidence of H. sapiens on the plateau reaches back 40,000 years, but continuous occupation may not have occurred here until after the last glacial period roughly 11,000 years ago.
Given significant patches in the archaeological timeline, the truth will likely only be figured out if we incorporate genetic data, too.
Today, most modern Tibetans have DNA containing a special variation in the Endothelial Pas1 (EPAS1) gene, which helps humans withstand the lack of oxygen found at high altitudes by increasing oxygen transport in the blood.
In 2010, a Denisovan finger bone found in the mountains north of the Tibetan plateau showed a comparable genetic quirk. So did Denisovans living on the plateau have a similar haplotype?
The short answer is: Maybe. We just don't have enough Denisovan remains to confirm.
According to the authors of the current paper, recent genetic research has shown all East Asians, including Tibetans, hold the same patterns of Denisovan DNA.
This suggests genes across the region were derived from the same interbreeding event, which was specific to East Asians, and probably occurred between 46,000 and 48,000 years ago.
Only after this intermixing did H. sapiens make it to the top of the world, possibly as a result of the genes they acquired from Denisovans in the lowlands.
But how long would it have taken for those high-altitude genes to be positively selected for in the East Asian population?
Research on the EPAS1 gene haplotype in modern Tibetans suggests the quirk was positively selected for anytime between 2,800 years ago and 18,300 years ago.
But the genetic divergence of modern Tibetans and Han Chinese seems to have occurred 30,000 years ago, which might indicate earlier selectivity.
Until we know more, the authors of the current paper argue we shouldn't rule out the possibility that H. sapiens permanently lived on the Tibetan Plateau as far back as 40,000 years ago.
"Currently the low-resolution data does not allow a complete validation/rejection of either hypothesis," the authors write.
"However, the models could establish an interpretative framework with clearly archaeological and genetic predictions for further studies."
The study was published in Trends in Ecology & Evolution.
Editor's note (13 December 2021): An earlier version of this article incorrectly identified some of the countries located on the Tibetan plateau. This error has now been corrected.