We cannot know a precise date, but it was around 90,000 years ago that two of our very distant relatives met in Siberia. They were archaic humans, also known as hominins – humans like us but not from our specific species, Homo sapiens. One, a male, was what we call a Denisovan. The other was a Neanderthal woman. And what they did together has astonished the scientific world.
The Denisovans, also known as Denisova hominins, were identified as recently as 2010. A single piece of finger bone from a young female, dated to some 40,000 years ago, was uncovered in the Denisova Cave. This special site lies in Siberia’s Altai Mountains and has been a rich source of information about our distant ancestors.
Confirmation that the finger bone and a tooth from the Denisova Cave belonged to a formerly unknown human species came from a team of experts led by Svante Pääbo, a Swedish scientist. And the team’s DNA analysis proved that these bones were from a new species of hominin.
What’s more, researchers know that just as Denisovans once occupied this cave, so too have Neanderthals and Homo sapiens. Modern humans, in fact, lived at the site not so very long ago. Indeed, the cave takes its name from a Russian hermit called Denis who called it home during the 1700s.
As for some of the much earlier occupants, a total of four fossil remnants of individual Denisovans have been discovered in the cave: the finger bone that we’ve already heard about, a foot bone and a couple of teeth. However, although this relatively scant evidence convincingly demonstrates that the Denisovans existed, it is not enough to tell us a great deal about how these early humans lived or what they looked like.
Yet there are some things that we do know. For example, the finger bone is quite thick, unlike the fingers of humans today. And this suggests that the female was strongly built – indeed, possibly comparable to the brawny Neanderthals in terms of physique. Meanwhile, the teeth that have been found are distinctly different to those of both Homo sapiens and Neanderthals.
The earliest Neanderthal fossil – of an incomplete skull – was found in a Belgian cave in 1829, although it was not then regarded as belonging to an ancestor of modern humans. Another skull was then discovered in Gibraltar in 1848 – and yet it wasn’t until 1864 that Neanderthal remains were identified as being those of a precursor to modern humans.
In 1856, meanwhile, miners in Germany’s Neander Valley came across a partial Neanderthal skull along with other bones. And eight years later, that find was finally recognized as being from a species that was an archaic human. It was geologist William King who realized that these mysterious bones were from a different human species, and it was he who coined the name Homo neanderthalensis.
Perhaps unsurprisingly, given this history, we know far more about the Neanderthals than we do of the Denisovans. The Neanderthals were hunters, they could fashion tools from stone, and they had the ability to make fire. When it came to sustenance, it is believed that they hunted animals including reindeer, boars and mammoths while also gathering other foods such as mushrooms and pine nuts.
In terms of cultural habits, meanwhile, scientists are still debating – often heatedly – exactly how advanced the Neanderthals were. Certain discoveries have seemed to indicate that they might have buried their dead, for example, but this is hotly contested. There have also been claims that the Neanderthals created jewelry and art. Again, though, this is a highly disputed area, and no consensus has been reached to date.
In any event, scientists reckon that around 400,000 years ago the Neanderthals and the Denisovans diverged from a shared ancestor. As we’ve already seen, the two species overlapped in terms of territory, since fossils from both have been found at the Denisova Cave. It’s also believed that they would have come into contact in Eurasia.
The Neanderthals originally lived in western Eurasia, but they gradually migrated eastwards. And it was in the east of Eurasia that the Denisovans lived, meaning the Neanderthal migration could well have brought the two species into contact. Nonetheless, so far the only place where Neanderthal and Denisovan remains have been found at the same site is the Denisova Cave.
We previously mentioned that four pieces of Denisovan remains – two bones and two teeth – had been found in the Denisova Cave. But, in fact, another fragment of bone was uncovered there in 2012. This piece was part of either a leg or an arm, and it belonged to a female who had died in her early teens some 90,000 years ago.
And in August 2018 the earthshaking results of a detailed study of the DNA in the bone were announced in Nature. Astonishingly, this girl had not been a pure Denisovan. Her DNA showed that she’d actually had a Neanderthal mother and a Denisovan father. In other words, the girl had been a Denisovan and Neanderthal hybrid – the result of the two species mating.
Interestingly, scientists had speculated for some time that there might have been interbreeding between the two species. But this was the first time that definitive evidence of the theory had been identified. What’s more, the bone in question had almost been lost. When it was first discovered, the fragment hadn’t been recognized as Denisovan and so was filed away for analysis at some later date.
Thankfully, Samantha Brown, a researcher at Oxford University, had been examining bones from the Denisova Cave some years later. And Brown made the crucial identification by testing proteins contained in the relevant bone. She then passed the key fragment on to Viviane Slon, a paleogeneticist at the Max Planck Institute in Leipzig, Germany.
At first, Slon wondered whether her sample had been contaminated or if some other error was at play. But she repeated the analysis no fewer than five times – and received identical results on each occasion. The proportion of Denisovan and Neanderthal DNA in the Denisova Cave bone was an almost even split.
Speaking to National Geographic, Harvard geneticist David Reich put the discovery into context. “It’s amazing to be able to find something like this,” Reich said. “It seemed unlikely that we would be able to catch it happening in the act – an individual that’s really the product of a first-generation hybrid.”
And in fact, it turns out that Homo sapiens was quite happy to mate with other hominin species. When modern humans made their way out of Africa, they came across Neanderthals and certainly interbred with them. Indeed, most Asians and Europeans have DNA that is around 2 percent Neanderthal – irrefutable evidence that early modern humans encountered and mated with Neanderthals.
In addition to the evidence from the Denisova Cave that Neanderthals and Denisovans bred together, many modern humans also have Denisovan DNA in their make-up. And this evidence of interbreeding between ancient Denisovans and Neanderthals – both extinct for some 40,000 years – raises fascinating questions about just what it means to be human.