Denisovans (pronounced de-knee-soh-vans) are a group of ancient humans. Our first tantalising glimpse of the Denisovans came in 2010 with genetic analysis of a tooth and tiny finger bone found in a cave called Denisova in the Altai mountains of Siberia. The fossil record of these mysterious humans remains extraordinarily sparse – comprising just four fragments of bone and teeth and a jawbone found in Tibet.
However, their DNA has revealed that about 765,000 years ago, they shared a common ancestor with the Neanderthals and our species, Homo sapiens. Once this ancestral population had split, our branch of the human family tree stayed in Africa while the Neanderthal/Denisovan one moved into Eurasia. By roughly 430,000 years ago, the Eurasian branch had itself split, ultimately giving rise to the Neanderthals in western Eurasia and the Denisovans in the east.
It isn’t clear why the Denisovans and Neanderthals diverged, but a new idea suggests that as the Arctic ice sheet expanded southwards to the Black Sea, cutting Europe off from Asia, it divided the early humans into the east and westerly groups mentioned above.
These splits weren’t permanent, though. After tens of thousands of years of independent evolution, members of the three populations met and interbred. As a result, five per cent of the Denisovan genome lives on – not in the inhabitants of Siberia but in people living thousands of kilometres away in southeast Asia, especially in Papua New Guinea.
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In fact, we now know that Denisovans bred with modern humans in at least two places: in east Asia, and further south-east in Indonesia or Australasia. What’s more, most Tibetans carry a stretch of Denisovan DNA that helps them cope with limited oxygen at altitude. This suggests interbreeding between H. sapiens and Denisovans is what allowed humans to colonise Earth’s highest plateau.
Far from home
We have hard evidence of the Denisovans’ existence in two places: Denisova cave in Siberia, and Baishiya Karst cave on the Tibetan plateau. Most anthropologists think the Siberian cave was their northern limit, because further north was too cold.
Genetic results suggest that they also lived much further south than Tibet, however, roaming huge areas of Asia as far south as Indonesia. At the time the Denisovans were alive, Indonesia was attached to mainland Asia because of low sea levels. Lands further south were still cut off though, and there is no reason to think they made it to Australia.
To the east, there are multiple potential Denisovan fossils from China. These include several bones from Xujiayao in northern China and two craniums from Xuchang in central China. Like a jawbone found off the coast of Taiwan, none of these quite fits the profile of a known species, so are good candidates to be Denisovans. To the west, an arm bone from Sel’Ungur cave in Kyrgyzstan could be Denisovan too, but attempts to extract ancient DNA from it have proved unsuccessful.
There is little evidence to indicate when and why the Denisovans died out. The most recent interbreeding episode with Homo sapiens may have been just 30,000 years ago. It is possible that there was so much interbreeding that they faded into the wider early human population. Alternatively, on arriving in Denisovan habitat, H. sapiens may have outcompeted or killed their cousins, or brought lethal diseases with them.
Climatic events could have been crucial too. “If we accept Denisovans lived from Siberia to Indonesia, you have very different climatic and environmental circumstances in which they lived,” says Bence Viola at the University of Toronto, Canada. “So I feel that there likely is not a single answer.”
A distinct species?
A 2018 genetic analysis of a sliver of bone revealed it belonged to a teenage girl who had a Neanderthal mother and a Denisovan father. “Denny”, as she was nicknamed, is the only first-generation hybrid hominin ever found.
This discovery – along with the revelation that living humans contain genes from both Denisovans and Neanderthals – has fuelled a debate about how to classify these groups of Stone Age peoples. In fact, the researchers who first identified Denisovans have never claimed them to be a distinct species. Now there is a growing debate about whether H. sapiens, Neanderthals and Denisovans are in fact all one species.
Studies carefully avoid calling them a species. Whereas Neanderthals have the species name Homo neanderthalensis, Denisovans are referred to as a “population”. This partly reflects the fact that we know so much less about them, but there are other reasons.
Jean-Jacques Hublin at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, points out that species names are convenient labels we use to make sense of the world, and there is no hard dividing line between us and Denisovans. “We are connected by a chain of inter-fecundity that goes back 65 million years to a small sort of squirrel-like primate that lived in a tree,” he says. Moreover, modern humans, Neanderthals and Denisovans could all interbreed, which by some definitions means none is a separate species.
A further complication is that there seem to have been two distinct groups of Denisovans: those whose genes are found in mainland Asia, and those whose genes are found in Melanesia. Lumping them together may be an oversimplification.
