Jawbone Fossil Reveals More About the Denisovans, a Mysterious Species that Mated With Modern Humans

Jawbone Fossil Reveals More About the Denisovans, a Mysterious Species that Mated With Modern Humans

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A photo of the Xiahe mandible.Image: Jean-Jacques Hublin, MPI-EVA, Leipzig

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In 2010, archaeologists found evidence of a previously unknown hominin, the Denisovans, in a Siberian cave. Researchers are now reporting the discovery of a 160,000-year-old Denisovan jawbone pulled from a cave on the Tibetan Plateau. The fossil is now the first evidence of this mysterious human species outside of Siberia, and the earliest evidence of a hominin presence in this part of the world.

This Denisovan mandible was discovered nearly 40 years ago by a monk who was wandering through Baishiya Karst Cave in Xiahe, China. The cave is located on the Tibetan Plateau, which over 1,240 miles (2,000 kilometers) from Denisova cave in Siberia—the only other place in the world where Denisovan fossils have been unearthed. The details of this discovery were published today in Nature.

We suspected this day would come, and it’s finally happened—the first fossil evidence of this species outside of Denisova cave, which is located in the Siberian Altai Mountains. Archaeologists have only known about the Denisovans—a hominin species closely related to the Neanderthals and not a direct ancestor of modern humans—for the past nine years. Scientists were able to extract DNA fragments from a single Denisovan finger bone found in Denisova cave, allowing them to identify the previously undiscovered species. Today, bits of Denisovan DNA linger on in present day Asian, Australian, and Melanesian populations. This fact alone suggested Denisovans interbred with modern humans (probably around 50,000 to 40,000 years ago), and that they were geographically dispersed. Given all this, the dearth of Denisovan fossils outside of Siberia was somewhat of an enigma.

Like the Neanderthals, the Denisovans eventually went extinct—save for the bits of DNA we inherited from them. Indeed, a remarkable and puzzling aspect of Denisovan DNA is the presence of an allele known as EPAS1. This genetic mutation confers resistance to hypoxia, otherwise known as altitude sickness. Archaeologists couldn’t understand why hominins living in a Siberian cave a mere 2,300 feet (700 meters) above sea level needed a resistance to a high altitude, low-oxygen environment. The discovery of a Denisovan fossil on the Tibetan Plateau, 10,760 feet (3,280 meters) above sea level, seems to solve this mystery. Fascinatingly, the EPAS1 allele lives on in the genome of present-day Himalayans—a trait that likely originated from the Denisovans.

“One of the most spectacular aspects of this new discovery is its location on the Tibetan Plateau,” Jean-Jacques Hublin, the lead author of the new paper and an archaeologist from the Max Planck Institute for Evolutionary Anthropology (MPI-EA), said at a press conference on Monday. “Nobody imagined that archaic humans lived there—we thought only by modern humans like us.”

The discovery, he said, is offering new insights into the Denisovan genome and the special gene variant that protects against hypoxia.

“Now we have an explanation for that,” he said. “The Denisovan population, or populations related to them, lived in high-altitude environments for a very long time, and later passed on this gene to modern populations.”

The Xiahe Denisovans lived during a particularly cold period on the Tibetan Plateau, explained Hublin. Some 160,000 years ago, during the late Middle Pleistocene, the Denisovans had to deal with a “more challenging environment” than the one experienced in the region today—an observation that “blows my mind,” said Hublin.

As noted, this well-preserved mandible, of which only the right half remains, was discovered in 1980 by a monk, but it eventually made its way to Lanzhou University. Since 2010, Lanzhou researchers Fahu Chen and Dongju Zhang, both co-authors of the new study, have been studying the area in which the jawbone was found. The MPI-EA researchers joined the investigation in 2016.

A physical analysis of the jawbone and its teeth distinguished the fossil as belonging to a Denisovan individual. Unfortunately, however, no DNA could be extracted from the mandible, but MPI anthropologist and study co-author Frido Welker conducted a cutting-edge protein analysis to further discern the provenance of the fossil. By extracting and analyzing proteins from the molars, Welker was able to identify protein sequences which, like fingerprints, are unique to Denisovans. These coding sequences were then compared to those produced by Neanderthals and modern humans. The “preserved coding sequences were most similar to Denisovans compared to anything else,” Welker said during Monday’s press conference. “We concluded that the mandible belongs to Denisovans.”

A heavy carbonate crust was attached to the mandible, allowing the archaeologists to date the fossil. Erring on the side of caution, the researchers dated the jawbone to 160,000 years ago, but said it could be older.

As for the mandible itself, it contained some ancient features, including very large molars. One of the molars hadn’t yet pierced through the gums, which means it likely belonged to an adolescent Denisovan individual. At the press conference, Hublin said its most distinguishing characteristic was the “robust teeth,” but he admitted “we still know very little of what they looked like.” When pressed to make an educated guess, Hublin said Denisovans “probably looked like an early form of Neanderthals.”

In terms of major takeaways, the new discovery offers at least three. First, we finally have an explanation for why Denisovans had a gene variant to stave off hypoxia. Second, this jawbone is evidence that Denisovans “were connected with other populations of Denisovans,” said Hublin, referencing to the Altai Denisovans in Siberia. And finally, we now know that Denisovans were, as a whole, geographically dispersed.

“Naturally, one wishes that DNA was present in that specimen,” Katerina Douka, an archaeologist at the University of Oxford who’s not affiliated with the new study, wrote in an email to Gizmodo. “Without it, other possibilities cannot be ruled out completely, but I agree that with the currently available data Denisovan is the most likely attribution.”

Chris Stringer, a paleoanthropologist at the Natural History Museum in London—also not involved in the new study—described this “first use” of ancient protein analysis to identify the Denisovan fossil as a “notable landmark.”

“Of course it is early days for this research, and we must remain slightly cautious while both the data from the fossil and from the comparative samples are sparse, but the technique shows great promise for mapping the relationships of fossil hominins where ancient DNA is not preserved,” explained Stringer in an email to Gizmodo.

For Douka, the most exciting aspect of the discovery was not the presence of Denisovans outside of Siberia (which she said she suspected all along), but the location of the new site at over 3,000 meters above sea level. The presence of Denisovans at this altitude during this particular geological period “is truly astonishing,” she said, and a discovery that jibes well with her own research from earlier this year. In that study, Douka’s team dated the oldest Denisovan fossils found in Denisova cave to the same time period indicated by the Tibetan Plateau fossil. This happened during a “weak interglacial period which would have allowed humans,” a group that included Denisovans, Neanderthals, and possibly anatomically modern humans, “to disperse widely, even in uninviting parts of Asia, followed by the penultimate Ice Age, which saw a return to colder conditions and the rather surprising presence of Denisovans in Siberia and, as we know now, the Tibetan plateau,” said Douka.

Stringer said the Xiahe fossil is significant for a number of reasons, including the identification of primitive physical features as compared to Neanderthals and modern humans.

“Not only in the primitive-looking teeth resembling those of more ancient humans, already known from Denisova Cave, but also a short and very robust mandible,” Stringer told Gizmodo. “That a strongly built jaw is not just an idiosyncratic feature of the Xiahe Denisovans is indicated by its resemblance to the Penghu mandible from the Taiwan region of China, which many of us had previously considered to be Homo erectus-like.”

Indeed, as both Stringer and the researchers behind the new study pointed out, archaeologists should now re-evaluate ambiguous hominin fossils found in Asia to determine if they’re actually of Denisovan origin. At the same time, archaeologists should head back into Baishiya Karst Cave to search for more fossils and possibly artifacts associated with the Denisovans.

That, however, could prove difficult given the cave’s holy status among the local population. As Hublin explained to Gizmodo in an email: “Permission needs to be obtained from the local government—not that easy.”

https://gizmodo.com/jawbone-fossil-reveals-more-about-the-denisovans-a-mys-1834444888


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