(CNN)In the foothills of Siberia's Altai Mountains lies a cave that contains some of the keys to understanding the earliest humans to walk the Earth. Denisova Cave is the only place in the world where fossils have been found that belong to mysterious ancient humans called Denisovans.
We don't know what Denisovans looked like; the fossils that have been found are fragments of bone and teeth. But we do know that they overlapped with Neanderthals. one of the fossils found in the cave revealed a daughter of a Denisovan and a Neanderthal, and Denisova Cave is the only site where Neanderthal and Denisovan remains have been found together.
But thanks to new dating techniques and fossils uncovered in the cave, researchers now know more about the history of the cave and those who sought shelter in it hundreds of thousands of years ago.
Two new papers in the journal Nature describe dating of the material and fossils found in the cave to fill a key chronological gap for who occupied the cave and when.
The entrance to Denisova Cave.
The cave guards its secrets well. Excavations have been ongoing there for 40 years. Its three chambers contain literal layers of history, including animal and plant remains, charcoal fragments and Neanderthal and Denisovan fossils. Fragmented bones of four Denisovans, two Neanderthals and the daughter of both have been recovered. No modern human remains have been found in the cave.
Stone artifacts have been dated in phases from the early Middle Palaeolithic to the Upper Palaeolithic.
But radiocarbon dating is usually most effective up until 50,000 years ago. And the layers of the cave aren't pristine. Cycles of freezing and thawing, animal burrowing or even the shifting of sediment can displace bones and stone tools.
Natalia Belousova and Tom Higham took samples from the Main Chamber of Denisova Cave.
Now, new dating techniques used by two teams of researchers have established a chronological timeline for the cave, extending from 300,000 years ago to about 20,000 years ago. They believe that Denisovans lived in the cave between 287,000 and 55,000 years ago and overlapped with Neanderthal occupation of the cave between 193,000 and 97,000 years ago.
Dating techniques were applied to bone, tooth and charcoal fragments, as well as cave sediments.
"This is the first time we are able to confidently assign an age to all archaeological sequence of the cave and its contents," Tom Higham, study co-author and deputy director of the University of Oxford's Radiocarbon Accelerator Unit, said in a statement.
One of the pendants recovered from the cave.
Separately, dating of the Denisovan fossils shows that the oldest fossil was there 195,000 years ago. The youngest Denisovan fossil was between 76,000 and 52,000 years ago. And pendants and needles made of bone were dated between 49,000 to 43,000 years ago, making them the oldest artifacts found in northern Eurasia. The researchers also believe it's possible that the Denisovans made them.
Stone tools allow for the earliest suggestion that ancient humans occupied the cave 300,000 years ago, while the fossils show it's more likely that Denisovans began their occupation of the cave 200,000 years ago, with Neanderthals arriving soon after. The daughter of the Neanderthal and Denisovan reveals that the two met and interbred 100,000 years ago when the climate was warm and stable.
As more fossils are found and sediments and tools are dated, that could extend the timeline for both Neanderthals and Denisovans, the researchers said.
The cave sheltered Neanderthals and Denisovans through varying climates when the area supported warm, humid forests before much colder tundra periods, researchers say, based on the plant remains that were uncovered.
"This reliable timeline enables us to link the archaeological, environmental, fossil and DNA information together across space and time to look for patterns of change in hominin presence, behavior and their interactions with prevailing climate," Zenobia Jacobs, study author and professor at the University of Wollongong's Centre for Archaeological Science, wrote in an email. "It opens up a lot of opportunities to interrogate the archaeological record in more detail."
The dating techniques included Bayesian modeling, such as radiocarbon and uranium-series dating and optically stimulated luminescence dating (determining the last time quartz sediment was exposed to light) with genetic ages of the fossils determined by mitochondrial DNA extracted from them.
"Although there might still be some uncertainty about the detailed ages of the remains -- given the nature and complexity of the deposits and the dating methods used -- the general picture is now clear," Robin Dennell said in an article accompanying the Nature studies. Dennell, a palaeolithic archaeologist at the University of Exeter, was not associated with either study.
Due to this new long range of time associated with Denisovan occupation of the cave, the researchers have reason to believe that they lived long enough to encounter modern humans who were migrating through Asia, Jacobs said. The nearest modern human fossils were found about 621 miles from the cave.
"So, the Denisovan ancestry in living Australian Aboriginal and New Guinean people could, therefore, be the result of direct interbreeding between their ancestors and Denisovans, but we do not know where this interaction took place," Jacobs said.
An Initial Upper Palaeolithic pendant.
Though a timeline and clearer dates have been established, the new information creates more questions about the ancient humans who lived in Denisova Cave.
"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," Richard Roberts, co-author of both studies and director of the University of Wollongong's Centre for Archaeological Science, said in a statement.
Previous excavations have almost entirely being carried out in the Main and East Chambers of Denisova Cave, Jacobs said.
"So, we will continue our study in the third chamber (South Chamber), where excavations have only recently begun and are continuing at the present time. Also, we are busy working on a large number of other sites in the Altai region, to provide a regional-scale timeline for the hominin occupation and environmental history of southern Siberia," she said.