Fossils and DNA traces demonstrated Denisovans were present in the cave from at least 200,000 to 50,000 years ago, and Neanderthals, a closely related extinct human species, were present there between 200,000 and 80,000 years ago, the new research found. Overlooking the Anui River, where herds of animals came to drink, it offered an unparalleled vantage for spotting game and other humans, as well as refuge from Siberian storms for generations of Neanderthals, their Denisovan cousins, and modern humans.

It's not completely out of the blue that Neanderthals and Denisovans mingled. "Today, people in places like East Asia and New Guinea still carry fragments of Denisovan DNA".

The Denisovans were a hominin species whose fossils are known only from a few fragments of bone and teeth unearthed in the Denisova Cave.

Scientists previous year described a Denisova Cave bone fragment of a girl whose mother was a Neanderthal and father a Denisovan, evidence of interbreeding.

"Everyone said, 'These Denisovans, we have no idea how old they are, '" said Katerina Douka, an archaeologist at the Max Planck Institute for the Science of Human History in Germany. Now, two new studies reveal a chronology for the cave's inhabitants.

As more fossils are found and sediments and tools are dated, that could extend the timeline for both Neanderthals and Denisovans, the researchers said.

Optical dating has been applied to archaeological sites around the world, with the minerals quartz and potassium feldspar most often used.

"This new chronology for Denisova Cave provides a timeline for the wealth of data generated by our colleagues in Siberia on the archaeological and environmental history of the cave over the past three glacial-interglacial cycles", Professor Jacobs said.

The other study, led by Dr. Katerina Douka from the Max Planck Institute for the Science of Human History, obtained radiocarbon ages from bone, tooth and charcoal fragments recovered from the upper layers of the site, and developed a statistical model to integrate all dating information for the cave.

Researchers Michael Shunkov, Maxim Kozlikin and Vladimir Uliyanov convene in the south chamber of Denisova Cave.

Even so, questions remain about the dated material in the cave. For instance, "do the human fossils derive from human occupations or from, say, carnivore activity, and have they been transported far from their original depositional location?" asked study researcher Chris Stringer, a research leader of human origins at London's Natural History Museum.

Carbon dating, which uses the half-life of radioactive carbon isotopes to show the age of organic matter, is only reliable on specimens up to roughly 50,000 years old. 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.

Upper Palaeolithic artifacts from Denisova Cave, which date from 50,000 to 35,000 years ago. The scale bar equals 1 cm.

In addition, it's possible that modern humans made some of the artifacts in the cave.

"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", said University of Oxford's Professor Tom Higham.

"With direct dates of between 43,000 and 49,000 years ago, they are the earliest such artifacts known from all of northern Eurasia".

But Stringer said he would put his money on early modern humans.

The model combined optical, radiocarbon and uranium-series ages with information on the stratigraphy of the deposits and genetic ages for the Denisovan and Neanderthal fossils relative to each other-the latter based on the number of substitutions in the mitochondrial DNA sequences, which were analysed at the Max Planck Institute for Evolutionary Anthropology in Germany.

The Denisova Cave last made headlines in August with the discovery of Denny, a half-Neanderthal half-Denisovan hybrid who was the first evidence of inter-breeding among those two early human species.