In 1980, a Buddhist monk in Tibet entered a sacred cave to pray. On the floor, he found half of a human jawbone, studded with two teeth.
A team of scientists on Wednesday reported that the fossil belonged to a 160,000-year-old Denisovan, a member of a lineage of mysterious, Neanderthal-like humans that disappeared about 50,000 years ago.
The fossil is the first evidence of this species found outside the Denisova Cave in Siberia, buttressing the theory that these relatives of modern humans once lived across much of central and eastern Asia.
“I’m very excited — we have a Denisovan that’s somewhere else than Denisova,” said Bence Viola, a paleoanthropologist at the University of Toronto who was not involved in the new study. “We’d known about Denisovans for 10 years and hadn’t found them anywhere else.”
The new fossil demonstrates that Denisovans were remarkably hardy, able to endure harsh conditions on the Tibetan plateau, at an elevation of 10,700 feet, with only simple stone tools.
The find also suggests that these Denisovans may have evolved genetic adaptations to high altitudes, and that living Tibetans may have inherited those genes thanks to interbreeding between Denisovans and modern humans in prehistoric times.
In the 1970s, Russian researchers began excavating Denisova Cave in Siberia. Over the years, they found a wealth of bones. A few looked like they might have come from humans or an extinct human relative.
Hoping for clues, the archaeologists sent some of the bones to the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, whose experts excel at retrieving DNA from fossils.
Some of the bones contained Neanderthal DNA, it turned out. But in 2010, Max Planck researchers discovered that one finger bone held different genes from an unknown human lineage.
Over the past decade, scientists have discovered more Denisovan teeth and bone fragments, including a chunk of a skull. Denisovans appeared to have lived in the cave, off and on, from 287,000 years ago to about 50,000 years ago.
Judging from their DNA, Denisovans shared a common ancestor with Neanderthals about 400,000 years ago. They interbred with Neanderthals and with our own species. Today, people in East Asia, Australia, the Pacific islands and the Americas all carry some Denisovan DNA.
The spread of Denisovan DNA in living humans strongly suggested that they may have lived throughout East and Southeast Asia. And maybe not just there: Earlier this month, a team of researchers argued that a population of Denisovans reached New Guinea and interbred further with modern humans.
But year after year, no one could find a Denisovan fossil outside the Siberian cave.
In 2010, Dongju Zhang, an archaeologist at Lanzhou University in China, began studying the Tibetan jaw, which had been languishing in storage at her institution.
Right away, she could tell it was humanlike — but not human. “We all have chins, but this doesn’t have one,” Dr. Zhang said in an interview.
Eventually, she located the cave in Tibet where the jaw had been discovered. Monks at a nearby temple told her they regularly found human remains on their visits.
“They said they were half-bone and half-stone,” said Dr. Zhang.
When she and her colleagues made a small excavation in the cave, they found ancient tools, a sign of human occupation.
She emailed photos of the jaw to Jean-Jacques Hublin, a paleoanthropologist at the Max Planck Institute. Intrigued, he traveled to China to examine the fossil, and soon he and Dr. Zhang had begun a collaboration with other experts to learn more.
Chuan-Chou Shen and Tsai-Luen Yu of National Taiwan University handled the task of figuring out how old it was. The jaw still had bits of rock stuck to it, and these contained uranium. By measuring the uranium’s decay into thorium, Dr. Shen and Dr. Yu were able to estimate the bone’s age.
The jaw turned out to be at least 160,000 years old, by far the oldest evidence of humans on the Tibetan plateau. Its antiquity also supported the scientists’ hunch that it did not belong to our own species.
DNA could reveal its true identity. But Qiaomei Fu, a geneticist at the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing, found that no genetic material had survived.
Fortunately, other biological molecules can be found in fossils. At Max Planck, Frido Welker, a molecular anthropologist, discovered ancient proteins in the jawbone’s teeth.
The proteins were not from modern humans; instead, they were a match to Denisovan DNA from Siberia.
With the new discovery and other recent finds, a picture of the Denisovans has grown clearer. Everything about their heads seems to have been big, from their giant molars to their thick jaws to their massive brain cases. Dr. Viola speculated adults may have weighed well over 200 pounds.
“I’d assume they’d be very large and robust individuals,” he said. “These are like a football players.”
The discovery of Denisovans living at high altitude is intriguing for another reason: Tibetans today share a special genetic link to Denisovans.
When most people travel to high elevations, they respond to the low level of oxygen by making extra hemoglobin in their red blood cells.
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“That’s bad, because it makes your blood thick,” said Emilia Huerta-Sanchez, a population geneticist at Brown University who was not involved in the new study.
But many Tibetans don’t make extra hemoglobin, thanks to an unusual version of a gene they carry called EPAS1. In 2014, Dr. Huerta-Sanchez and her colleagues discovered that this unusual gene came from Denisovans.
How did Denisovans end up with a gene that promotes health at high altitudes? And how did it end up becoming so common in Tibetans but so rare in other people?
Until scientists find DNA from Tibetan Denisovans, the history won’t be clear, Dr. Huerta-Sanchez said.
“We don’t know the order of events,” she said. “But Denisovans are such a mysterious group that anything we learn is exciting.”
In recent decades, Chinese paleontologists have found a number of puzzling bones that are almost human and are tens or hundreds of thousands of years old.
Researchers now may compare them to the Tibetan jaw, and search the fossils for ancient proteins.
“Denisovans are already somewhere in a museum drawer,” Dr. Welker predicted. “We just haven’t able to link them together yet.”
https://www.nytimes.com/2019/05/01/science/denisovans-tibet-jawbone-dna.html
2019-05-01 17:02:24Z
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