The Homo Desinovan, a newly discovered human species

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 oa a Desnisovan 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 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.

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.

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.

Neanderthals might no be the hunched cavemen we thought they were the study says:

"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.

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.