A lost lineage
Denisova Cave gives up Siberian clues to distant cousins
During 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 breeding with modern humans. Today, people in places such as 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.
During the past six years, 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 Wednesday. That timeline shows that humans occupied the cave for perhaps as many as 300,000 years. And it raises some intriguing hints that Denisovans might 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 Douka and her colleagues have created “a rigorous and compelling timeline.”
The Denisova Cave is about 30 yards above the Anuy River. The cave has a large main chamber with a high ceiling; from there, passageways lead to smaller chambers. Over the past few hundred thousand years, sediment has slowly built up on the cave floor.
In the 1970s, Russian scientists began digging into that sediment, finding fossils of such animals as 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 who 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,” Douka said.
She and her colleagues at the University of Oxford are experts at 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.
The earliest signs of human life in the cave — simple stone tools — are more than 287,000 years old. The tools alone cannot tell us if those first people were Denisovans or Neanderthals.
But they are not the style known to be made by Neanderthals, suggesting Denisovans might have been the creators.
It’s not until about 200,000 years ago that the oldest Denisovan DNA comes to light. The researchers estimated it to be between 217,000 and 185,000 years old. A Neanderthal DNA sample comes from a layer that formed between 205,000 and 172,000 years ago.
In the millenniums that followed, both Denisovans and Neanderthals left more genetic evidence in the cave. It might have been continually occupied for thousands of years by one group, then abandoned and reoccupied by others.
But Neanderthals and Denisovans must have overlapped at least once during those tens of thousands of years.
In August, researchers reported a bone fragment from a girl whose mother was a Neanderthal and father was a Denisovan. In the new study, researchers estimate that this hybrid child lived between 118,100 and 79,100 years ago.
The researchers found no Neanderthal remains in more recent layers of the cave floor — only Denisovan. A Denisovan tooth dates back to between 84,100 and 55,300 years ago; a Denisovan chip of bone is 76,200 and 51,600 years old.
Paradoxically, the most recent parts of the cave have yielded some of its biggest mysteries.
Starting about 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 such as 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 possibly made it to what is now Siberia: One human fossil discovered there dates to about 45,000 years ago.
But Michael Shunkov, co-author of the new studies and 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, 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.
Douka and her colleagues have discovered a bone dating back between 50,000 and 45,900 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 about 45,000 years ago could be loaded with surprises.
What if the ornaments from that period were made by hybrids of modern humans and Denisovans?
“This dichotomy, that it has to be one or the other, is a little bit old-fashioned,” Douka said.