All previously known fossils from the mysterious hominids come from a Siberian cave
Dongju Zhang/Lanzhou University
Denisovans reached what’s now called “the roof of the world” at least 160,000 years ago.
Found in a Tibetan Plateau cave, a partial lower jawbone represents a Denisovan who is the oldest known hominid to reach the region’s cloud-scraping heights, researchers report online May 1 in Nature.
The fossil suggests that these perplexing, extinct members of the human lineage weathered the plateau’s frigid, thin air long before humans did. Many researchers have assumed that, as far as hominids go, only Homo sapiens settled in that high-altitude, low-oxygen environment, probably no earlier than 40,000 years ago (SN: 12/22/18, p. 6).
“It blows my mind that Denisovans lived on the Tibetan Plateau,” paleoanthropologist and study coauthor Jean-Jacques Hublin of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, said at an April 29 news conference.
Until now, Denisovans were known only from a handful of fossils unearthed in Siberia’s Denisova Cave, and from ancient DNA extracted from one of those bones. Researchers regard Denisovans, who inhabited Denisova Cave from around 300,000 to 50,000 years ago (SN: 3/2/19, p. 11), as close relatives of Neandertals and possibly a distinct Homo species.
The jaw’s microscopic protein structure and anatomy peg it as a Denisovan, geoarchaeologist Fahu Chen of the Chinese Academy of Sciences in Beijing and his colleagues report. However, the team wasn’t able to extract any Denisovan DNA from the fossil. Rocky material attached to the bottom of the jaw enabled the calculation of its minimum age.
Found in 1980 by a Buddhist monk as he explored a cliff-side cave on the plateau in Xiahe, China, the jaw adds to evidence that Homo evolution in Asia was a complicated affair, Hublin said. Denisovans and Stone Age H. sapiens occasionally interbred, leaving traces of Denisovan DNA in present-day Asians, Melanesians and Australians. Recent research also showed that two different Denisovan populations left a genetic mark on Papua New Guineans, one perhaps as recently as 15,000 years ago (SN: 4/27/19, p. 15).
Scientists also knew that modern Tibetans had inherited a genetic variant from Denisovans that aids survival at high altitudes (SN: 8/9/14, p. 8). That discovery fueled suspicions that Denisovans had ascended into mountainous parts of Asia far from Denisova Cave, which sits only 700 meters above sea level. Baishiya Karst Cave, where the new fossil was discovered, is 3,280 meters above sea level. Denisovans probably evolved a genetic tweak to deal with the Tibetan Plateau’s thin air long before passing that gene to H. sapiens via interbreeding, Hublin suggested.
Like teeth previously found in Denisova Cave, two molars preserved in the Xiahe jaw are larger than those of Neandertals and resemble teeth of older Homo species. A molar at the back of the Xiahe jaw had not fully erupted, indicating that this individual was an adolescent at the time of death. The jaw itself is strongly built and shorter than those of Neandertals.
Two other Stone Age Homo jaws, one dredged from ocean waters near Taiwan and the other unearthed in northern China (SN Online: 1/16/19), resemble the Xiahe find. Those fossils might also come from Denisovans, says paleoanthropologist Chris Stringer of the Natural History Museum in London.
SEEING DOUBLE Researchers digitally reconstructed a Denisovan jaw by using the right half of a jaw fossil, found on the Tibetan Plateau, to create a mirror image stand-in for the fossil’s left side, shown in gray.
In addition to analyses of the Xiahe jaw and teeth, fragments of six proteins extracted from the fossil displayed chemical sequences that matched corresponding protein sequences of Denisovan fossils from Denisova Cave more closely than those of Neandertals, H. sapiens and modern apes. Proteins, which preserve in teeth and bones better than DNA does, contain amino acid sequences that distinguish between living and fossil species of various animals, including hominids, says bioarchaeologist and study coauthor Frido Welker of the Max Planck Institute for Evolutionary Anthropology and the University of Copenhagen.
Analyses of ancient proteins show great promise for identifying evolutionary relationships among fossil hominids, Stringer says. Preserved proteins may help determine, for example, whether the Taiwanese and northern Chinese jaws come from Denisovans, especially if those specimens don’t yield any DNA.
The Xiahe fossil fits a scenario in which Denisovans inhabited much of East Asia, says paleoanthropologist Antonio Rosas González of the National Museum of Natural Sciences in Madrid. But he sees no answer at this point to the mystery of whether Denisovans belonged to a genetically diverse Neandertal species or represented a distinct Homo species with close Neandertal ties.
F. Chen et al. A late Middle Pleistocene Denisovan mandible from the Tibetan Plateau. Nature. Published online May 1, 2019. doi:10.1038/s41586-019-1139-x.
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