A lost tribe once walked the Earth alongside Neanderthals and humans. Their genetic legacy still lingers within us. But we know next to nothing about this mysterious race. And our hopes of finding out more comes down to a single Siberian cave.
We have a tooth. A fingerbone. Some very old trinkets. And some distinctive DNA.
That's all we have – so far – of the long lost hominine tribe dubbed the Denisovans.
They roamed across Asia about the same time as the Neanderthals.
And understanding them means learning more about ourselves.
While traces of their bloodline have been found in crossbreeds in Asia, only one place is known to have been occupied by the Denisovans, Neanderthals and early modern humans.
And that's the Denisova cave in Russia from where they were given their name.
Now a study involving researchers from the University of Wollongong has been published in the science journal Nature. It casts new light on those who inhabited this cave, the different worlds they lived in – and the legacy they have passed down through the generations.
WHO WERE THE DENISOVANS
The Denisova Cave is hidden among the foothills of Siberia's Altai Mountains.
Its significance as a shelter of ancient peoples has been known for the past 40 years.
But it leapt to public prominence in 2010 when DNA recovered from a single fingerbone revealed the existence of a previously unsuspected type of hominin.
It sparked a worldwide search for further evidence of this enigmatic species, and the genetic legacy it left behind.
But most of what we know comes from this single place.
In 2018, a simple tiara made from woolly mammoth ivory was found buried in its depths.
It had a hole ground in its rounded end, where a cord was used to tie it to the back of the head. It's been dated as having been made some 45,000 to 50,000 years ago. It was made to keep hair out of its wearer's eyes. And it was discarded once it was broken.
Was it made by the Denisovans?
"Here we are likely to deal with another, more ancient culture, because there was not a single piece of bone belonging to a Homo sapiens found in the cave," the Novosibirsk Institute of Archeology and Ethnography Alexander Fedorchenko told the Siberian Times.
It's a pressing question. And one not easily answered.
There are other intriguing objects found within the cave. Some 30 pieces of mammoth ivory have been recovered from the cave. Among the most unique objects are the fragments of a glistening green chlorite stone bracelet, beads made of ostrich eggs – and an ancient reddish-brown processed hematite 'crayon'.
THE TROUBLE WITH TIME
Among the more recent finds – again in the Denisova Cave itself – was a 90,000-year-old bone fragment from a 13-year-old child. It proved to be the daughter of a Neanderthal mother and Denisovan father.
So could the Denisovans have mated directly with modern humans? Or has their DNA been passed down through Neanderthal hybrids?
We know homo sapiens appeared in other parts of Asia some 50,000 years ago.
So it depends on whether or not evidence of Denisovan occupation overlaps this time frame.
It seems to do so: some of the objects associated with them could date from as recently as 45,000 years ago.
"For the same reason, another open question is whether Denisovans or modern humans made the oldest bone points and personal ornaments [tooth pendants] found in the cave "says study co-author University of Oxford Professor Tom Higham. "With direct dates of between 43,000 and 49,000 years ago, they are the earliest such artefacts known from all of northern Eurasia."
The new Nature study is the result of a detailed investigation of researchers from Russia, Australia, Canada, Germany and the United Kingdom.
It has put the layers of dust found in the cave through a fine tooth comb.
It reveals that the site was occupied almost continuously for the past 300,000 years, whether it was one of the three ice ages that happened during this time or an intervening era of fertile warmth.
Different occupants at different times left behind different artefacts.
Among those are those who have been specifically identified as belonging to the mysterious Denisovans.
Their fossils and remnant DNA date from 200,000 to 50,000 years ago.
Along with them were Neanderthals, seeking shelter in the cavernous cave between 200,000 and 100,000 years ago.
It took some effort to pin down these timelines.
Some 50 radiocarbon tests were performed to date specific objects and sediment layers.
Dr. Katerina Douka from the Max Planck Institute for the Science of Human History, Germany, used radiocarbon technology to date the relatively recent bone, tooth and charcoal fragments in the upper levels of the cave.
She said the new age estimates of the fossils "include all the dating evidence available for these small and isolated fossils, which could easily have been displaced after deposition."
Then there were more than 100 ages obtained by optical dating of sediments (which reveals the last time a buried item was exposed to sunlight) to build a chronology for Denisova Cave.
This has been combined with genetic analysis of Denisova and Altai Mountain Neanderthals to cast a statistical outline of what happened in the remote region.
PUSHING THE LIMITS
University of Wollongong Professor Zenobia Jacobs contributed to the optical dating of cave sediments. The technique was used because much of the site was too old for reliable radiocarbon dating (most objects lose their natural radioactivity after about 50,000 years).
Fortunately, it has been found to be buried grains of quartz or felspar containing datable traces of when they were last exposed to sunlight. It can pinpoint dates up to some 350,000 years ago.
"We had to invent some new methods to date the deepest and oldest deposits and construct a robust chronology for the sediments in Denisova Cave," said Associate Professor Bo Li, geochronologist at the University of Wollongong.
"This new chronology for Denisova Cave provides a timeline for the wealth of data generated by our Russian colleagues on the archaeological and environmental history of the cave over the past three glacial-interglacial cycles," Professor Jacobs said in a statement.
University of Wollongong Professor Richard Roberts says the study has revealed more about the Siberian cave's inhabitants. But there is still a lot of needing to be learned.
"While these new studies have lifted the veil on some of the mysteries of Denisova Cave, other intriguing questions remain to be answered by further research and future discoveries," he says.
Much of what we know is made by connecting disparate 'dots' in the archaeological record.
Their bones tell us what they may have looked like.
Their teeth tell us where they came from, and what they ate.
Tools and ornaments hold clues about their culture.
But DNA is much more intimate.
Analysis of genetic coding can reveal the 'ghosts' of long-extinct human ancestors.
It also reveals exactly how 'cozy' each species was with each other.
We know humans mated with Neanderthals.
We know Neanderthals mated with Denisovans.
We now believe there was a fourth partner in this ancient partner swapping game.
Whether a glitch in the data or a ghost in the genome, this tantalizing trace has recently emerged from studies of the remains of humans living in the Middle East between 14,000 and 3400 years ago.
It's another hominid branch.
They've been dubbed the Basal Eurasians.
"If you like, it's a third branch," told Chris Stringer of the Natural History Museum in London New Scientist. It's a branch separate from those who stayed in Africa, as well as those who spread across Eurasia. No fossil evidence has yet been identified.
And similar studies indicate the previously unsuspected presence of Neanderthals in Africa as recently as 30,000 years ago.
What's certain is that the future of human history is facing some significant rewrites.
If the science is up to it.