Denisovans -- an ancient human species -- adapted to high-altitude low-oxygen environments long before the modern humans arrived, say scientists who have analysed a 160,000-year-old fossil jawbone of the archaic hominins found in the Tibetan Plateau.
The research, published in the journal Nature, shows that Denisovans were more widespread than thought. It also shows that the Denisovans were the first human relatives to roam the 'roof of the world'.
The fossil was originally discovered in Baishiya Karst Cave in Xiahe, China in 1980 by a local monk who donated it to the 6th Gung-Thang Living Buddha who then passed it on to Lanzhou University.
Since 2010, researchers Fahu Chen and Dongju Zhang from Lanzhou University have been studying the area of the discovery and the cave site from where the mandible originated.
In 2016, they initiated a collaboration with the Max Planck Institute for Evolutionary Anthropology (MPI-EVA) in Germany and have since been jointly analysing the fossil.
The Denisovans were first discovered in 2010, when a research team led by Svante Paabo from the MPI-EVA sequenced the genome of a fossil finger bone found at Denisova Cave in Russia and showed that it belonged to a hominin group that was genetically distinct from Neanderthals.
"Traces of Denisovan DNA are found in present-day Asian, Australian and Melanesian populations, suggesting that these ancient hominins may have once been widespread," said Jean-Jacques Hublin, director of the Department of Human Evolution at the MPI-EVA.
"Yet so far the only fossils representing this ancient hominin group were identified at Denisova Cave," Hublin said in a statement.
While the researchers could not find any traces of DNA preserved in this fossil, they managed to extract proteins from one of the molars, which they then analysed applying ancient protein analysis.
"The ancient proteins in the mandible are highly degraded and clearly distinguishable from modern proteins that may contaminate a sample," said Frido Welker of the MPI-EVA and the University of Copenhagen in Denmark.
"Our protein analysis shows that the Xiahe mandible belonged to a hominin population that was closely related to the Denisovans from Denisova Cave," said Welker.
The researchers found the mandible to be well-preserved. Its robust primitive shape and the very large molars still attached to it suggest that this mandible once belonged to a Middle Pleistocene hominin sharing anatomical features with Neanderthals and specimens from the Denisova Cave.
Attached to the mandible was a heavy carbonate crust, and by applying U-series dating to the crust the researchers found that the Xiahe mandible is at least 160,000 years old.
"This minimum age equals that of the oldest specimens from the Denisova Cave," said Chuan-Chou Shen from the National Taiwan University, who conducted the dating.
"The Xiahe mandible likely represents the earliest hominin fossil on the Tibetan Plateau," said Fahu Chen, director of the Institute of Tibetan Research, Chinese Academy of Sciences.
These people had already adapted to living in this high-altitude low-oxygen environment long before Homo sapiens even arrived in the region.
Previous genetic studies found present-day Himalayan populations to carry the EPAS1 allele in their genome, passed on to them by Denisovans, which helps them to adapt to their specific environment.
"Archaic hominins occupied the Tibetan Plateau in the Middle Pleistocene and successfully adapted to high-altitude low-oxygen environments long before the regional arrival of modern Homo sapiens," said Dongju Zhang, of Shandong University in China.
According to Hublin, similarities with other Chinese specimens confirm the presence of Denisovans among the current Asian fossil record.
"Our analyses pave the way towards a better understanding of the evolutionary history of Middle Pleistocene hominins in East Asia," Hublin said.
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