A Revealing Piece of Ancient Human History, Discovered in a Tibetan Cave

The remarkable, fossilized jawbone has no chin, and the teeth within it are exceptionally large.

The Xiahe mandible
A monk found the Xiahe mandible in a cave high on the Tibetan plateau. (Dongju Zhang, Lanzhou University)

In 2008, scientists working in Denisova Cave—a cold site in Siberia’s Altai Mountains—uncovered a strange pinky bone, broader than a typical human’s. The DNA within that bone revealed that its owner belonged to an entirely new group of ancient hominins, distinct from Homo sapiens or Neanderthals. That group became known as the Denisovans.

Researchers have since decoded the Denisovan genome. But still, no one can say what they looked like. Every known Denisovan fossil would fit in your palm—that pinky, three teeth, and a remarkable bone sliver from a Denisovan-Neanderthal hybrid. And all of these remains came from the same cave.

But now, an international team of scientists has announced the identification of another Denisovan fossil, from a site 1,500 miles away. It’s the right half of a jawbone, found some 10,700 feet above sea level in a cave in China’s Xiahe County, on the eastern edge of the Tibetan plateau. The Xiahe mandible, as it is now known, is not only the first Denisovan fossil to be found outside Denisova Cave, but also the very first Denisovan fossil to be found at all. It just took four decades for anyone to realize that.

The mandible was discovered by a local monk in 1980 and donated to Lanzhou University. There, it lay unstudied until 2010, when a team led by Fahu Chen and Dongju Zhang—a climatologist and an archaeologist, respectively—began examining it in earnest. The world learned about the existence of the Denisovans at around that time, and though fossils had only been recovered from Siberia, it was clear that these hominins likely existed throughout much of East Asia. Smatterings of Denisovan DNA still persist in the genes of living people in this region and beyond, and how else could it have made it into the genomes of modern Tibetans or Melanesians? Still, “I never imagined that [the Xiahe mandible] could be a Denisovan,” Zhang says.

“If it was one, we’d be so lucky,” she adds.

The mandible itself is very thick and sturdy. It has no chin, which rules out modern humans. The teeth within it are exceptionally large, and different in shape and size from those of Neanderthals, Homo erectus, and other known hominins.

The molecules in the specimen were especially telling. The team couldn’t detect any traces of ancient DNA, but it did find the next best thing—fragments of ancient collagen proteins, still lurking in one of the teeth. These fragments closely resemble the proteins of Denisovans, more so than those of Neanderthals, modern humans, or other great apes.

But Katerina Douka of the Max Planck Institute for the Science of Human History notes that methods for analyzing ancient proteins are relatively new, and less well tested than those for studying ancient DNA. Researchers should use both techniques on other specimens to check that they give the same results, Douka says. But for now, based on the data that exist, she agrees that the Xiahe mandible most likely belonged to a Denisovan.

“It confirms that the Denisovans were perhaps widely distributed through East Asia,” Zhang says. For years, scientists had suspected as much. After all, people across East Asia and Melanesia (the region that includes New Guinea and its neighboring islands) have Denisovan DNA in their genes. This pattern—the product of ancient sexual encounters between Denisovans and humans—shouldn’t be possible if the Denisovans were just confined to a small Siberian cave. Instead, it seemed that they were already living in much of East Asia by the time ancient humans also spread through the region.

Indeed, the Xiahe mandible, which is 160,000 years old, is by far the earliest hominin fossil from the Tibetan plateau. Researchers used to think that Homo sapiens was unique in adapting to the Himalayas, but the Denisovans were successfully living on the roof of the world at least 120,000 years earlier. They must also have adapted to extremely thin air—after all, the mandible was found in a cave that’s some 8,000 feet higher above sea level than Denisova itself. “Their presence that high up is truly astonishing,” Douka says.

This helps to explain a remarkable finding from 2014. Back then, Emilia Huerta-Sanchez and her colleagues showed that most Tibetan people carry a mutated version of the EPAS1 gene, which helps them cope with high-altitude air that has 40 percent less oxygen than what most people breathe. And that mutation, the team showed, came from Denisovans. By having sex with these hominins, ancient Tibetans picked up a useful genetic trait that their descendants still benefit from.

That result was surprising, because Denisova Cave is so far from Tibet, and so much lower in altitude. The new mandible resolves that discrepancy. Although it’s unclear whether its owner had the same EPAS1 variant that the other Denisovans did, it at least shows that Denisovans were in the right part of the world. “I was thrilled that they found a Denisovan-like jawbone at high altitude,” Huerta-Sanchez says.

“The new discovery is an important step in understanding the Denisovans, but the big question still remains to be solved,” says Yousuke Kaifu of the National Museum of Nature and Science, Tokyo. And that is: If Denisovans were spread throughout Asia, why do Melanesians have so much more of their DNA than anyone else—5 percent, compared with just 0.2 percent in East Asians, and nothing in other groups?

To answer that question, scientists will need to find more Denisovan bones. Douka and her colleagues have started a project called Finder to do exactly that, by rapidly analyzing small, unidentifiable slivers from various sites in Asia. More intact specimens might also be lying around in museum collections. For example, the Xiahe team notes that its mandible has many similarities to the Penghu 1 mandible, which was fished out of the ocean near Taiwan in 2008. (“I agree that there are some similarities,” says Kaifu, who led the team that analyzed Penghu 1 in 2015.)

China has a long list of similar hominin fossils that have been hard to assign to other species. “Some of those may already be Denisovans,” Zhang says.