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In 1980, a Buddhist monk in Tibet entered a sacred cave to pray. On the floor he found half of a human jaw bone studded with two teeth.
A team of scientists announced Wednesday that the fossil belonged to Denisovan, 160,000 years old, belonging to a line of mysterious, Neanderthal type humans, who died about 50,000 years ago.
The fossil is the first evidence of this species found outside the cave of Denisova in Siberia, supporting the theory that these modern human parents once lived in much of central and eastern Asia.
"I am very enthusiastic. We have a Denisovan somewhere other than Denisova, "said Bence Viola, a paleoanthropologist at the University of Toronto, who did not participate in the new study. "We knew Denisovans since ten years and had not found them anywhere else. "
The new fossil shows that the Denisovans were remarkably robust, capable of supporting difficult conditions on the Tibetan plateau, at an altitude of 10,700 feet, with only simple stone tools.
The discovery also suggests that these Denisovans may have developed genetic adaptations at high altitudes and that living Tibetans may have inherited these genes through the interbreeding between Denisovans and modern humans in prehistory.
In the 1970s, Russian researchers began searching the cave of Denisova in Siberia. Over the years, they have found a wealth of bones. Some seemed to come from men or from an extinct human parent.
In the hope of finding clues, archaeologists have sent some of the bones to the Max Planck Institute of Evolutionary Anthropology in Leipzig, Germany, whose experts excel at recovering DNA of fossils.
Some bones contained Neanderthal DNA, it turned out. But in 2010, Max Planck researchers discovered that the bone of a finger contained different genes from an unknown human lineage.
During the last decade, scientists have discovered more teeth and Denisovan bone fragments, including a piece of skull. The Denisovans seemed to have lived in the cave from time to time, 287,000 years ago at about 50,000 years ago.
Judging by their DNA, the Denisovans shared a common ancestor with Neanderthals about 400,000 years ago. They crossed with the Neanderthals and with our own species. Today, the peoples of East Asia, Australia, the Pacific Islands and the Americas all possess a Denisovan DNA.
The spread of Denisovan DNA in living humans strongly suggests that they may have lived throughout East and Southeast Asia. And maybe not just there: Earlier this month, a team of researchers claimed that a population of Denisovans arrived in New Guinea and again crossed 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 to study the Tibetan jaw, which was languishing in storage in his institution.
Right away she could say that it was human – but not human. "We all have chins, but that's not the case," said Dr. Zhang in an interview.
Finally, she located the cave in Tibet where the jaw was discovered. Monks from a nearby temple told him that they regularly found human remains during their visits.
"They said they were half bones and half stone," said Dr. Zhang.
When she and her colleagues made a small excavation in the cave, they found old tools, a sign of human occupation.
She sent pictures of the jaw by email to Jean-Jacques Hublin, paleoanthropologist at the Max Planck Institute. Intrigued, he went to China to examine the fossil and, soon, he and Dr. Zhang began collaborating with other experts to learn more.
Chuan-Chou Shen and Tsai-Luen Yu, from the National Taiwan University, were tasked with determining their age. Pieces of rock were still stuck to the jaw and these contained uranium. By measuring the disintegration of uranium into thorium, Dr. Shen and Dr. Yu were able to estimate the age of the bone.
The jaw was at least 160,000 years old, by far the oldest evidence of human presence on the Tibetan plateau. His antiquity also reinforced the intuition of scientists that he did not belong to our own species.
DNA could reveal its true identity. But Qiaomei Fu, a geneticist from the Institute of Paleontology and Paleoanthropology of Vertebrates in Beijing, found that no genetic material had survived.
Fortunately, other biological molecules can be found in fossils. Frido Welker, molecular anthropologist, discovered in Max Planck old proteins in the jaw teeth.
Proteins did not come from modern humans; instead, they corresponded to the Denisovan DNA of Siberia.
With the new discovery and other recent discoveries, the photo of Denisovans has cleared up. Everything in their heads seems to have been big, from their giant molars to their thick jaws and their huge brains. According to Dr. Viola, adults would have weighed well over 200 pounds.
"I suppose they are very big and robust people," he said. "They are like football players."
The discovery of Denisovans living at high altitude intrigues for another reason: Tibetans today share a particular genetic link with the Denisovans.
When most people go to altitude, they respond to low levels of oxygen by producing a hemoglobin supplement in their red blood cells.
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"It's bad, because your blood is thick," said Emilia Huerta-Sanchez, population geneticist at Brown University, who did not participate in the new study.
But many Tibetans do not make extra hemoglobin, thanks to an unusual version of their gene called EPAS1. In 2014, Dr. Huerta-Sanchez and her colleagues discovered that this unusual gene came from Denisovans.
How was Denisovans found with a gene that promotes high altitude health? And how has it become so common among Tibetans and so rare among others?
Until scientists find the DNA of the Tibetan Denisovans, the story will not be clear, said Dr. Huerta-Sanchez.
"We do not know the sequence of events," she said. "But the Denisovans are such a mysterious group that everything we learn is exciting."
In recent decades, Chinese paleontologists have discovered a number of almost human enigmatic bones dating back tens or even hundreds of thousands of years.
Researchers can now compare them to the Tibetan jaw and search fossils for old proteins.
"The Denisovans are already somewhere in a museum drawer," said Dr. Welker. "We have not yet managed to connect them."
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