Neanderthals and Denisovans might have lived side by side for tens of thousands of years, scientists report in two papers in Nature^1,2.

The long-awaited studies are based on the analysis of bones, artefacts and sediments from Denisova Cave in southern Siberia, which is dotted with ancient-human remains. They provide the first detailed history of the site’s 300,000-year occupation by different groups of ancient humans.

“We can now tell the whole story of the entire cave, not just bits and pieces,” says Zenobia Jacobs, a geochronologist at the University of Wollongong, Australia, who co-led one of the studies.

Ancient-human hotspot

Soviet archaeologists began unravelling the story of Denisova Cave, at the foot of the Altai Mountains, in the early 1980s. Since then, scientists have found the fragmentary remains of nearly a dozen ancient humans at the site. The cave became world famous in 2010, after an analysis of the DNA from a tiny hominin finger bone found that the creature was distinct from both modern humans and Neanderthals³. It belonged to a previously unknown hominin group, later named Denisovans.

Additional sequencing of the DNA in bone remains from the cave found that Denisovans were a sister group to Neanderthals, and might once have lived across Asia — where they interbred with the ancestors of some humans now living there⁴.

Last year, the site produced another spectacular discovery: DNA analysis of a long bone fragment revealed the first ever known ‘hybrid’ of two ancient-human groups, a woman — nicknamed Denny — whose mother was a Neanderthal and father a Denisovan⁵.

Dating tangle

Most of the cave’s remains are older than the 50,000-year limit of the radiocarbon dating technique that’s used on organic materials, and efforts to use other methods to date the sediments in which the remains are buried have been hampered by the lack of a good map of the cave’s geological layers. Many scientists worry that disturbances in the cave, such as animal burrows, have scrambled its contents such that remains and artefacts no longer sit in sediments of similar age.

To surmount those challenges, researchers led by Jacobs and Wollongong geochronologist Richard Roberts used a dating technique that determines when individual grains of soil were last exposed to light¹. This allowed them to identify regions of the cave in which the soil had been disturbed so that adjacent grains returned wildly different dates. They could then omit those areas when dating sediments in the same geological layer as hominin remains and tools.

The first signs that any ancient-human species had occupied the cave are stone tools — excavated beginning in the 1980s — that were dated to around 300,000 years old. But the researchers could not work out whether Denisovans or Neanderthals made them. The cave’s Denisovan remains (including some DNA that leached into the soil) date to between 200,000 years ago and 55,000 years ago, whereas the oldest Neanderthal remains are around 190,000 years old and the youngest date to some 100,000 years ago.

The researchers cannot find out precisely when the groups lived together, or whether they ever shared the cave. But the existence of the hybrid individual — who lived around 100,000 years ago — means that the groups must have lived close enough to each other to meet at that time. Furthermore, Denny’s father harboured a sliver of Neanderthal ancestry, suggesting that his ancestors had previously interbred with Neanderthals.

Who was here?

Homo sapiens might also have lived in the cave, the researchers suggest. Bone pendants and tools — similar to those made by early modern humans in Europe — from the cave’s younger layers date to between 49,000 and 43,000 years old, reports a team led by archaeologists Katerina Douka at the Max Planck Institute for the Science of Human History in Jena, Germany, and Tom Higham at the University of Oxford, UK, in the second Nature paper².

The researchers dated one hominin bone to around 46,000-50,000 years ago, but could not retrieve any DNA to investigate which species it belonged to.

No other H. sapiens remains from this period, known as the Initial Upper Palaeolithic, have been found in Denisova cave or the wider Altai region. For this reason, the Russian archaeologists who spearhead the site’s excavation have argued that Denisovans made the artefacts, which are more sophisticated than the site’s older stone tools. But Higham would like to see more proof before linking the artefacts to any group. “It’s possible Denisovans could have made the Upper Palaeolithic. It’s possible the Russians are right. At the moment, with the evidence we have, we can’t really be sure,” he says.

Hybrids similar to Denny are another suspect, says Robin Dennell, an archaeologist at the University of Exeter, UK, and author of an accompanying essay on the studies⁶.

It is also possible that whoever made the artefacts was influenced by contact with H. sapiens, he says. “I would be very surprised if the Initial Upper Palaeolithic at Denisova was made by Denisovans or Neanderthals with no input from our species.”