The ancient Denisovans are cloaked in mystery but this fossil find is already changing that

The Tibetan plateau is home to Baishiya Karst Cave, a place long-revered by Tibetans who go there to pray or recuperate from illness. And the cave is home to some impressive fossil finds. That’s what a Tibetan monk discovered in 1980 when he stumbled across a jawbone with two very large teeth. The jawbone was human, but it was definitely not from a modern human.

The Xiahe jawbone, named for the county in China where it was discovered. Featured image by Dong Ju Zhang, Lanzhou University via National Geographic

Now, that jaw bone — thanks to a study published Wednesday in the journal Nature, is shining a brighter light into the extinct world of the Denisovan people, National Geographic reports. Detailed analysis of the fossil’s physical features and proteins that were extracted show that its’ owner died approximately 160,000 years ago. Remains of these enigmatic people, a sister group of the Neanderthals had only been found in a single cave in Siberia’s Altai Mountains.

“I just couldn’t believe that at the moment my colleagues told me,” said the study’s co-author Dong Ju Zhang, of China’s Lanzhou University. “I was really excited.”

Her excitement is understandable. The jaw, now known as the Xiahe mandible after the county in China where it was found, widens our knowledge of these ancient humans. Previously Denisovan fossils had only been found in Siberia’s Denisova cave, but people all across Asia and Australasia have Denisovan DNA in their genomes. So the discovery of the Xiahe jaw bone some 1400 miles from the cave in Siberia shows that these people were wanderers indeed.

Anthropologists believe that ancestral Denisovans split from their Neanderthal relatives about 400,000 years ago and headed east into Asia, while Neanderthals spread across Europe and western Asia. Modern humans left Africa about 200,000 years ago. Their migration began slowly but over time waves of modern humans made their way out. Those who headed to the Middle East bred with Neanderthals while those who headed east into Asia bred with local Denisovans. And while some of us carry genetic fingerprints from Neanderthals, others of us carry Denisovan DNA.

Neanderthal man. Photo by Allan Henderson, license CC Attribution 2.0 via Flickr

It’s thanks to the Denisovans that modern-day Tibetans are so well adapted to the thin air in which they live. Scientists had long wondered if their ancient forebears were adapted for higher altitudes because Denisova cave is just 2300 feet above sea level. But Baishiya Karst Cave is 10,760 feet above sea level so this amazing fossil provides evidence that these people did indeed live at higher altitudes. And the mandible is four times the age of the earliest known human activity in the Tibetan Plateau’s demanding climate.

“Thanks to this study we are ‘cornering’ Denisovans,” says Maria Martinón-Torres, director of Spain’s National Research Center on Human Evolution, in an email to National Geographic. “Their portrait is progressively less blurred.”

The race to find out more about these ancient humans began when a finger bone of a young girl who lived more than 50,000 years ago was unearthed in Denisova cave, Science reports. Paleogeneticists extracted DNA from the bone and realized she was a previously undiscovered human species. Researchers from the Max Planck Society in Leipzig, Germany have also sequenced DNA from several other individuals found in the cave. But because the fossils were very fragmented, scientists weren’t really able to paint a portrait of what Denisovans looked like.

With the discovery of the Xiahe mandible, the portrait can be fleshed out. Zhang traced the jawbone’s origins by interviewing people living in Xiahe and many said they remembered seeing human bones in the huge cave which is situated right next to a Buddhist shrine. The cave has long been considered a holy place and is a popular tourist attraction. The monk who discovered the fossil recognized its uniqueness and gave it to his superiors. They consulted scholars and then gave the fossil to Lanzhou University.

The jaw is much more heavily built than a modern human’s and researchers had no clue how to classify it. So it languished on shelves for years, Zhang said.

Then she and geologist Fahu Chen, also from Lanzhou University and the Institute of Tibetan Plateau Research in Beijing had an idea. They took the fossil to paleoanthropologist Jean Jacques Hublin, also of the Max Planck Institute. And when he saw the huge molars, Hublin immediately suspected it was Denisovan. But the paleogeneticists couldn’ extract DNA from the jaw. So Frido Welker, one of Hublin’s graduate students did something else entirely different.

In his doctoral work, Welker studied key differences in the amino acid proteins among Neanderthals, modern humans, and Denisovans. He was able to extract collagen from the fossil jaw and found that its amino acid sequence most closely matched that of Denisovans.

And because the jaw had a carbonate crust, researchers were able to measure the radioactive decay of uranium in the carbonate, thus allowing them to determine the age of the fossil. That’s why they can confidently say this fossil is at least 160,000 years old.

Modern-day Tibetan man. Photo by Rod Waddington, license CC SA 2.0 via Flickr

The age of the fossil also makes it apparent that Denisovans had tens of thousands of years to adapt to Tibet’s higher altitudes. The fact that Tibetan people living today have Denisovan genes that help them deal with thinner air is likely because these ancient humans interbred with modern humans.

“It seems likely that ancestral Tibetans interacted with Denisovans as they began to move upslope,” said archaeologist David Madsen of The University of Texas at Austin in an email.

The jaw’s features may make it easier for researchers to spot other Denisovan fossils.

“It’s distinct large molars and premolar roots differ from those of Neanderthals,” Hublin wrote, adding that the jawbone “is very primitive and robust.”

He can also see a pointed resemblance to another jawbone found off the coast of Taiwan that’s known as the “Penghu mandible.”

And for scientists, protein analysis can build bridges where there weren’t any before.

“The protein analyses allow us to see landscapes where DNA cannot reach” — that can be anything from warmer climates to much older sites where fragile DNA won’t hold up, Martinón-Torres says. Researchers have a wellspring of fossils they are anxious to test for proteins or compare with the Xiahe jaw.

The implications are huge.

“Forget the textbooks,” says UK archaeologist Robin Dennell of the University of Sheffield. “Human evolution in Asia is far more complex than we currently understand, and probably does involve multiple lineages, some of which probably engaged with our species.”

Ever so slowly, the puzzle of human evolution is being filled in by missing pieces.

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