Over the past decade, the Denisova Cave in Siberia has yielded some of the most fascinating fossils ever found. To the naked eye, they are not much to look at — a few teeth, bits of bone.

But the fossils contain DNA dating back tens of thousands of years. That genetic material shows that Denisovans were a distinct branch of human evolution, a lost lineage.

At some point in the distant past, the Denisovans disappeared — but not before interbreeding with modern humans. Today, people in places like East Asia and New Guinea still carry fragments of Denisovan DNA.

One of the biggest obstacles to understanding the Denisovans is their age. Standard methods for dating these fossils have left scientists perplexed.

“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.

Over the past six years, Dr. Douka and other experts have been creating a sort of history of the Denisova Cave. They have dated 103 layers of sediment on the cave floor, as well as 50 items found in them, including bones, pieces of charcoal and tools.

The scientists unveiled this chronology in a pair of papers published on Wednesday. That timeline shows that humans occupied the cave for perhaps as long as 300,000 years. And it raises some intriguing hints that Denisovans may have been capable of sophisticated thought, on par with modern humans.

In an accompanying commentary, Robin Dennell of the University of Exeter in England wrote that Dr. Douka and her colleagues have created “a rigorous and compelling timeline.”

In the 1970s, Russian scientists began digging into that sediment, finding fossils of animals like hyenas and bears, fragments of humanlike bones and thousands of stone tools, as well as bracelets, beads and other ornaments.

In 2010, researchers at the Max Planck Institute of Evolutionary Anthropology announced they had found DNA in teeth and bones from the cave. In addition to Denisovan DNA, they found a few bone fragments that contained Neanderthal DNA.

By comparing the mutations in this DNA, the scientists got a better sense of how Denisovans and Neanderthals fit into the human family tree.

As it turned out, modern humans share a common ancestor with Denisovans and Neanderthals that lived roughly 600,000 years ago. Later — approximately 400,000 years ago — the Neanderthal and Denisovan lineages split.

Ever since the digging began, Russian researchers have carefully mapped the sedimentary layers in which they found bones and tools. They tried to estimate the ages of the layers, but “the dates were all over the place,” said Dr. Douka.

She and her colleagues at the University of Oxford are experts on determining the age of carbon. Researchers from the University of Wollongong in Australia tried an alternate method called optical dating.

The researchers combined results from the two methods to assemble a single chronology of the cave. The findings are largely in agreement: “It’s definitely a unified story,” said Zenobia Jacobs, an archaeologist at the University of Wollongong.

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Paradoxically, the most recent parts of the cave have yielded some of its biggest mysteries.

Starting around 45,000 years ago, new kinds of artifacts begin showing up in the cave floor. They include pointed pieces of bone, as well as ornaments like stone bracelets and beads. One possibility is that these new tools were made by newly arrived modern humans.

Modern humans evolved in Africa and then expanded out to other continents. They may have made it to what is now Siberia: One human fossil discovered there dates to about 45,000 years ago.

But Michael Shunkov, a co-author of the new studies and the director of Institute of Archaeology and Ethnography at the Russian Academy of Sciences, disagrees with that interpretation.

The sophisticated tools in the Denisova Cave show “no clear indications for outside influences,” he said in an email. Instead, Dr. Shunkov believes that the Denisovans who occupied the cave for perhaps 250,000 years developed this technology on their own.

One way to resolve this question would be to find human fossils from that period.

Dr. Douka and her colleagues have discovered a bone dating back between 45,900 and 50,000 years ago that contains humanlike proteins — but no DNA. It could belong to a modern human, a Neanderthal or a Denisovan.

Researchers are scouring the cave floor for still more fossils. A fossil from around 45,000 years ago could be loaded with surprises.