Denisova Cave in the Altai mountains of Siberia was hot property back in the day — at least to ancient humans who called it home for more than 200,000 years.

Key points: Sediments, bones and artefacts excavated from a Siberian cave showed two now-extinct species of human lived there between around 200,000 and 50,000 years ago

Sediments, bones and artefacts excavated from a Siberian cave showed two now-extinct species of human lived there between around 200,000 and 50,000 years ago Among the objects found were carved pendants made from teeth and bone spear points

Among the objects found were carved pendants made from teeth and bone spear points Whether these were made by modern humans or another species of human is up for debate

This is according to researchers who dated artefacts, fossils and sediments dug from pits in the cave's floor to piece together a record of habitation.

In a pair of papers in Nature today, they report that Denisovans — an extinct species of human whose genome was reported in 2011 — occupied the cave from around 287,000 to 50,000 years ago.

This overlapped with Neanderthals, who also resided there but for a shorter period: between around 193,000 and 91,000 years ago.

Until the timeline was put together, archaeologists did not know when Denisovans arrived at the cave, said Bert Roberts, a geochronologist at the University of Wollongong and co-author of both papers.

"They could have been there a million years ago or 100,000 years ago," Professor Roberts said.

We do not know whether Denisovans and Neanderthals were housemates living in the cave at the same time.

But the latest studies "suggest both groups lived in the region, met and — on occasion — interbred over the course of approximately 150,000 years," the researchers reported.

Fossil deep-freeze

Not much is known about the mysterious Denisovans.

Their remains have only been found in one place — Denisova Cave, hence their moniker — and even then, fossils comprise bone fragments and a few teeth from four distinct individuals and a hybrid child.

"We know annoyingly little about them," Professor Roberts said.

"We don't even know what they looked like."

This is because they were first identified not by a skeleton or skull, but by their DNA, scraped from the precious finger bone of a young girl.

Using genetic material is possible because the cave, being in the Siberian mountains, is like a big freezer, preserving DNA which would normally disintegrate in warmer, humid climates.

The entrance to Denisova Cave in the Altai mountains of Siberia. ( Supplied: Richard (Bert) Roberts )

Russian scientists have known about and excavated the cave floor for 40 years, finding bones and artefacts like tools and pendants.

But to reconstruct a timeline of habitation, they needed to date sediments.

Layers of dirt act as an archive of what was happening in the cave at the time they were laid down.

The idea is the deeper you dig, the further back in time you see.

Sediment profiles of Denisova Cave's East Chamber (left) and Main Chamber (right). ( Supplied: IAET SB RAS / Sergei Zelensky )

And because sediments in the cave span at least 300,000 years, researchers in one paper had to use a range of dating techniques.

They included radiocarbon dating, which is good to about 50,000 years old, and optically stimulated luminescence, which measures when quartz and feldspar minerals were last exposed to sunlight.

Optically stimulated luminescence, along with some nifty modelling, can take archaeologists back around 300,000 years, said Zenobia Jacobs, also from the University of Wollongong and co-author of the papers.

"So chronologies were put together using a range of techniques, but all cross-checked against each other and other independent techniques to make sure that it was all hanging together."

In a separate paper, researchers used radiocarbon dating to pin down the age of the Denisovan fossils, along with the remains of three Neanderthals and the hybrid child.

They estimated the oldest and youngest Denisovan fossils were 194,400 years old and 51,600 years old respectively.

The Neanderthals were between 90,900 and 147,300 years old and the Neanderthal/Denisovan child was between 79,300 and 118,100 years old, placing them broadly within the dates determined by sediment analysis.

The question of the artefacts

Among the Denisova Cave haul were pendants carved from teeth and bone spear points, dated between 43,000 and 49,000 years old.

So were they made by Denisovans?

The idea was floated by the researchers but Darren Curnoe, a palaeoanthropologist at the University of New South Wales who wasn't involved with the work, is not convinced.

"I think a better much better explanation is that the arrival of tools, technology and symbolic objects actually represents the arrival of modern humans," Dr Curnoe said.

Even though there are no signs yet that modern humans — Homo sapiens — lived in Denisova Cave until much later on, "they weren't too far away at about the same time", he said.

Complex behaviours like carving jewellery are typical of modern humans.

"And we now have claims in southern China there were modern humans more than 100,000 years ago," Dr Curnoe said.

"If that's correct, then modern humans were already in the neighbourhood [of Siberia] for 50,000 years or so."

Richard "Bert" Roberts (in blue) and colleagues in the East Chamber of Denisova Cave. ( Supplied: IAET SB RAS / Sergei Zelensky )

Professor Jacobs said that attributing the artefacts to Denisovans would no doubt be controversial.

"As Western scientists, we immediately assume, looking at artefacts like that, they might be made by Homo sapiens.

"But we have collaborators that feel quite strongly that evidence for Homo sapiens is not at the cave, there are no fossils or DNA, except at much later periods."

Denisovan DNA in Australia

Indigenous Australians and Papua New Guinea populations have a relatively high percentage of Denisovan DNA, which their ancestors picked up in Asia before their arrival in Australia.

Recent genetic analyses suggested interbreeding took place after the Altai Denisovans vacated the cave, said Joao Teixeira, a population geneticist at the University of Adelaide who was not involved in the recent work.

So despite there being no trace of Denisovans after around 50,000 years ago in the cave — or anywhere else, for that matter — it's highly unlikely that particular population was the last of their species.

"Not only is their geographic distribution likely more widespread than the Altai … but also the nature of Denisovan DNA seems to point to different Denisovan populations, likely reflecting geographic isolation, which would then lead to slight accumulations of genetic differences," Dr Teixeira said.