In 1980, a Buddhist monk went to Baishiya Karst cave to pray. Located at the foot of a cliff on the edge of China’s Gansu Tibetan Plateau, the cave’s opening, pierced through a vertical expanse of gray stone, is such a perfect embodiment of a monastic site that it almost veers at the parody. It looks remarkably like a single orbit, looking across the river basin below. The painting practically calls for a grizzled sage perched at the entrance, pondering to one depth or another.
We do not know what this anonymous monk was looking for when he painfully climbed the path leading to the cave. But he came down with something far more concrete than the usual spiritual insights gained on such a pilgrimage.
At some point during his stay, the monk found a strange mandible – a single length of jawbone studded with a handful of enormous molars. This specimen was passed to the sixth Gungtang Lama – a “living Buddha” – who in turn gave it to researchers at Lanzhou University. The relic languished in storage for years. Its discoverer did not imagine that the jawbone was the first evidence of a new group of humans: the Denisovans. It would take nearly four decades for the significance of the discovery to become clear. But the first hints of its true meaning appeared in 2010, hundreds of miles away.
Small discoveries, big impact
For decades, researchers believed that Homo sapiens emerging from Africa were the only ancestors of modern humans. Everything changed with the revelation that Neanderthals had also contributed, thanks to a team of biologists who sequenced their genome in 2009 . Then, in 2010, the genetic analysis of a tooth and a tiny finger bone, both found years ago among the muddled sediments of Denisova Cave in Siberia’s Altai Mountains, have revealed a third human lineage – one that has persisted in some modern human groups. These new humans were named after the cave where they were found.
According to Bence Viola, a paleoanthropologist at the University of Toronto and co-author of the 2010 study Nature document revealing the findings, these unfortunate Denisovans were likely eaten, hunted, or scavenged by cave hyenas. “They usually regurgitate the bones. Or poop them. One of these two ways is probably how the bones entered the cave,” Viola explains.
The same is likely true of the other human remains discovered at the site, which also include Neanderthals and modern humans. H. sapiens. Among these discoveries, two other teeth (one deciduous molar and one permanent molar), a probable arm bone fragment and two pieces of the parietal bone from the back of the skull were confirmed to be of Denisovan origin.
Sediment analysis of the cave suggest that the Denisovans lived there 300,000 to 130,000 years ago, and then again 100,000 years ago. Neanderthals settled around 170,000 years ago, suggesting a possible overlap with the Denisovans. Modern humans, on the other hand, only appeared around 45,000 years ago. These are of course rough estimates based on mitochondrial DNA (mtDNA) dating – spectacularly illuminating but inaccurate science – as well as the presence of stone tools and animal bones bearing their marks. .
“Some of the oldest artifacts, which are relatively unspecific to the Middle Palaeolithic [stone tools], were probably made by Denisovans,” suggests Viola. However, the layers of sediment deposited in the cave have been disturbed over time, making it difficult to distinguish them from those that might have been made by Neanderthals. And the paths leading to the convergence of these three groups at this remote location are even more speculative.
Although there are many competing theories of modern human development, at least one thing is certain: the linear progression of H. sapiens outside of Africa and their direct evolution into modern humans is totally implausible. Researchers are still trying to piece together the origins of Neanderthals and Denisovans — and their subsequent interaction with modern humans.
The narrative they’ve built together so far begins with migrating from a late version of homo erectus, another early hominid species, from Africa to Eurasia circa 700,000 years ago. “This branch would have been something much closer to humans today than H. erectus was,” says paleoanthropologist John Hawks of the University of Wisconsin-Madison, who has studied Denisovan’s origins and their relationship to modern humans. “We should probably call them something else, but I don’t think there’s a good name for them,” Viola adds. Until recently it was thought to be a separate species called Homo heidelbergensis, but Hawks warns that specimens that have been classified as such are now more likely early Neanderthals.
Regardless of their taxonomic status, these migrants split into Neanderthals and Denisovans shortly after leaving, probably around 600,000 years ago. “The hominins west of Altai are Neanderthals. And as far as we’ve found, the hominids east of the cave are Denisovan,” says Hawks. The split was probably the result of geographical factors such as the Himalayan mountains and the Eurasian ice sheet expansion.
Although they also met elsewhere, the Denisova Cave provided the only direct archaeological evidence that two groups met following their divergence and intersected. Perhaps most notable is this arm bone fragment – his owner was a young girl born to a Neanderthal mother and a Denisovan father.
Neanderthals and Denisovans then mated with Homo sapiens, who traveled out of Africa and the rest of the world as long ago as 180,000 years ago. Early H. erectus was already present in Asia about 1.8 million years ago, and some of their descendants – called “superarchaic– also contributed to the modern human lineage. “The Denisovan genome carries between 2 and 10 percent of the DNA of a more archaic group of hominins,” says Viola.
This raises the question of whether this array of hominins were in fact different species, or simply populations with distinctive characteristics. Species designation in living creatures is spongy; generally, different species cannot interbreed, although many exceptions have been discovered. (Take the liger, for example, a lion-tiger hybrid found only in captivity.) Ultimately, these semantic debates matter little in terms of the Denisovans’ overall impact. Whether they are crosses of different species or simply distinct populations, their genetic heritage remains clear.
A lasting legacy
Most extraordinary among their impact is the 2019 discovery that current residents of the Tibetan Plateau draw their capacity to survive in the high altitude, low oxygen environments of their Denisovan ancestors. The source of this discovery? An analysis of mandible proteins revealed by the monk at Baishiya Cave – the only concrete evidence of the eastward extent of the Denisovan population.
In contrast, the remaining evidence of their presence across Asia is purely genetic. Denisovan DNA is also apparent in contemporary island populations of Southeast Asia, Australia, and even North America, although to a lesser extent. the DNA percentages are highest in the Philippines, where 5% of the modern Ayta Magbukon indigenous people’s genome is Denisovan, and in Papua New Guinea, where 4% of the country’s modern highland genome is Denisovan.
This suggests that the Denisovans were a fairly advanced people, able to navigate. They probably have crossed the Wallace line, a boundary that separates the fauna of Asia and Australia, then interbred with modern humans who later did the same. The relatively small populations of both groups in these isolated island regions may explain why Denisovan DNA persisted there, and not in mainland Asia, where it was likely overwhelmed by a larger influx of modern humans. Yet no fossil evidence has been found in the island nations of Southeast Asia – possibly because the climate there is not conducive to its formation.
But those who study the Denisovans hope that an array of promising fossils from the continent might help complete the sketch we have constructed of their movements and influence. A assortment of bones of the Xujiayao site and the so-called dragon-man skull described in 2021, both from northern China, are promising candidates. “Our first assumption is going to be that anything that is not clearly H. erectus in East Asia is Denisovan,” says Hawks. Yet until this material is analyzed, the Denisovans will remain genetic ghosts, summoned by fragmented fossilized remains and the blood of their distant descendants.
Learn more about the Denisovans in the collection below: