This Tibetan Genetic Adaptation Baffled Scientists, But An Ancient Jawbone May Hold The Answer

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It’s 1980, and a Buddhist monk is trekking through the mountainous terrain of the Tibetan Plateau. Eventually, at an altitude of more than 10,000 feet, he comes to the opening of a curious cave. Here, he finds what appears to be the ancient remains of a jaw. At this point in time, this individual can’t possibly understand how important his find will one day prove to be. But in reality, it’ll ultimately shed light on a special genetic trait exhibited by the Tibetan people of today.

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It was only in 2010, however, that this jawbone was subjected to the level of scientific scrutiny that it deserved. Indeed, that was the year in which Dongju Zhang turned her attention to the object. Having just completed her Ph.D. around that time, it should come as no surprise that she was so eager to get stuck into the work.

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Zhang wasn’t acting alone, however. In fact, she’d initially taken an interest in the bone thanks to a push from her academic mentor Fahu Chen. It was actually Chen himself who headed up the research, which involved several experts in addition to Zhang. All these people’s work ultimately culminated in a study which was published in May 2019.

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Essentially, it’s taken almost four decades since the bone’s discovery for its true significance to start to be recognized. But with this piece of research from 2019, the scientific community now has a new compelling idea over which to pore. Basically, it revolves around a potential connection between this ancient jawbone and an evolutionary adaptation seen today in the people of Tibet.

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The region of Tibet is situated in Asia. It encompasses most of the Tibetan Plateau, a significant expanse of highland which spreads across roughly 970,000 square miles. For a sense of scale, we can say that the plateau is about five times larger than France. It reaches into parts of both China and India.

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Tibet is considered to be the loftiest place in the world. On average, it tends to stretch to about 16,000 feet into the sky. Having said that, the region’s most significant point is much higher. This is Mount Everest, the tallest mountain in the world, which reaches a height of more than 29,000 feet.

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Over the years, archaeologists have attempted to piece together the earlier stages of Tibet’s history. Certain pieces of evidence have indicated that early species of human were present in the region roughly 500,000 years ago. But modern humans, specifically, seem to have shown up there much later, about 21,000 years ago.

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Nowadays, the number of Tibetan people isn’t known precisely, but a figure of 6.5 million has been posited. The population is no longer focused specifically in Tibet, of course, with some being located in other parts of China. Tibetans also inhabit India, Bhutan and Nepal, as well as other, more faraway parts of Earth.

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The Tibetan people communicate in a variety of different tongues. Some of these languages can actually be so distinct that they’re essentially incomprehensible to each other. A majority of Tibetans are Buddhists, though a native religion known as Bon has also survived amongst some. Islam is also practiced by a minority.

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Genetically speaking, Tibetans today share many of the same characteristics and traits as other peoples of eastern and central Asia. They’re also unique, however, in that they can live at significant altitudes, whereas other people cannot. This is a fascinating quality of the Tibetan people, and one which Fahu Chen’s 2019 study might shed light on.

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The jawbone – or mandible, as lower jaws are also known – at the center of this study was found at the Baishiya Karst Cave. This is a sacred Buddhist site which attracts pilgrims and monks. It was here, in 1980, that an unnamed monk came across the ancient jaw. This is now referred to as the Xiahe mandible: it acquired this moniker as a reference to the county in which the jawbone was found.

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This monk later brought the Xiahe mandible to a significant individual within the Buddhist faith. After that, the find eventually ended up at China’s Lanzhou University, where it lay in relative obscurity for quite some time. In 2010, however, Dongju Zhang – with encouragement from supervisor Fahu Chen – took an interest in it.

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Attached to the mandible are a pair of big teeth, quite unlike what you’d expect to see in modern humans. Moreover, it appears to be without a chin, a feature which is apparently specific to our species. As such, all of this was rather telling for researchers. The jawbone, it seemed, had once belonged to another species of human.

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In order to clarify things, scientists tried to retrieve some DNA from the mandible. When this proved unsuccessful, however, they attempted to study the proteins which lay within it. With that, they noted that the jawbone exhibited characteristics suggestive of a recently classified species of human known as the Denisovan.

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In 2008 a finger was found in a Siberian cavern known as Denisova Cave. Over the next couple of years, this specimen was subjected to testing. And while it exhibited similarities to the Neanderthal, it appeared to be different in other ways. As such, the bone was eventually proclaimed to have belonged to a new species of human. This was to be referred to as the Denisovan.

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According to a paleoanthropologist named Bence Viola, the Xiahe mandible is what one would expect from a Denisovan specimen. “To me it hits the expected morphology really well,” the specialist from the University of Toronto told National Geographic magazine in 2019. “It really looks like what one would have hoped for.”

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Ultimately, the Denisova Cave fossil was the first Denisovan bone to be officially classified as such. But it was the Xiahe mandible which had been the first to actually be discovered, which it had been decades before. It just so happens that nobody realized the body part’s significance for quite some time.

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When they eventually started working on it, though, the researchers from Lanzhou University sought to establish how old the Xiahe mandible actually was. To do so, they analyzed the rock within which the bone had been found. From that, they concluded that it dated back about 160,000 years – an age comparable to the Denisova Cave discovery.

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If this date is correct, it implies that Denisovans were found in Tibet well before modern humans ever showed up. To be more precise, it seems that there was a period of about 120,000 years before our species joined the Denisovans there. And that’s not all that the Xiahe mandible can tell us.

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Before the Xiahe mandible had been classified, the only other evidence of the Denisovans came from the Siberian Denisova Cave. Baishiya Karst Cave, however, is over 1,400 miles away. As such, this tells us that Denisovans were living in other parts of Asia than had initially been comprehended by experts.

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Moreover, the discovery implies that Denisovans were capable of living at notably high altitudes. After all, the Baishiya Karst Cave in which the Xiahe mandible was discovered is more than 10,700 feet above sea level. It’s also said that weather conditions in the time of the Denisovans could have become extremely cold, perhaps something like -22°F at times.

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The fact that there’s evidence indicating Denisovans’ ability to survive in extreme conditions could have significant implications. You see, most modern humans are unable to live at great heights. That’s because up high – where there’s less oxygen – the human body produces a higher quantity of a protein known as hemoglobin.

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Hemoglobin is useful in that it allows oxygen to be transported around the body more effectively. However, if our bodies are exposed to too much, it can interfere with the workings of the heart. As such, this might cause the onset of altitude sickness or even bring on a heart attack – which, obviously, could prove deadly.

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This is the case for most people on Earth. The Tibetan people, however, don’t necessarily produce too much hemoglobin in these conditions. The reason for this, apparently, is down to EPAS1, a special gene which has been nicknamed the “super-athlete gene.” That’s because it ensures the body uses its oxygen more adeptly.

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But how exactly is it that the Tibetan people have managed to come by this handy gene? Well, the answer, it seems, could be indicated by the Xiahe mandible. Basically, it all has to do with the fact that early species of human were known to interbreed with one another. In doing so, they would have exchanged certain genetic traits.

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Modern humans are thought to have derived from Africa. However, about 200,000 years ago, they started migrating to other parts of the world. At some point, they made it to the Middle East, where they came across and mated with Neanderthals. Elsewhere, there were others who traveled to Asia and bred with Denisovans.

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The resulting offspring from these encounters would’ve been born with genetic traits taken from each of these species. And evidence of this can be seen in the Tibetan people today. The EPAS1 gene, for instance, is thought to trace back to the Denisovans. At one time, it appeared that this gene came into being for reasons which did not concern altitude. But the Xiahe mandible has since suggested otherwise.

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Experts once believed that EPAS1 had emerged in Denisovans in order to help them to handle intensive exercise. Then, it was thought, the gene was later utilized for other functions. But the Xiahe mandible suggests that Denisovans lived in areas defined by great heights. As such, it seems that the gene emerged to cope with these significant altitudes.

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As the head of the Spanish National Research Center on Human Evolution, María Martinón-Torres is undoubtedly an authority on matters of this nature. And in an email to National Geographic, she proclaimed that the Xiahe mandible has been influential in detailing the Denisovans. As she put it, “Thanks to this study we are ‘cornering’ Denisovan. Their portrait is progressively less blurred.”

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The notion that the Tibetan people of today can trace their ability to live in high altitudes to the Denisovans isn’t exactly new. In fact, a piece of research published back in 2014 already argued such a thing. Now, though, the Xiahe mandible appears to back up the claims of this other work.

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Yet things aren’t quite so straightforward. There are, in fact, those who are cautious to subscribe to these new theories. One example is Emilia Huerta-Sanchez, a genetics expert from Brown University. “I agree with the authors [of the 2019 study] that it could be that this hominin group [the Denisovan] was high-altitude adapted,” the Rhode Island-based academic stated. “But I don’t think we know for certain.”

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As we saw earlier, the researchers working on the Xiahe mandible failed to extract DNA from the specimen. As such, we can’t be certain that the individual to which it once belonged actually did possess the genetic trait to help it survive at high altitudes. Yet while she’s expressed doubt that this is the case, it does appear that Huerta-Sanchez wishes it to be so.

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But there are other reasons to be skeptical of such claims, too. You see, it’s thought that the Denisovans were an incredibly diverse species. In fact, another study from 2019 has posited that what we call Denisovans can actually be split into three different categories, which themselves are incredibly divergent from one another. This calls into question the very notion of the Denisovan as a specific species.

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Yet even though doubts exist, the new way to study fossilized proteins and reveal new information is in and of itself exciting to experts. As Martinón-Torres elaborated to National Geographic, “I love the way cutting-edge techniques have been put together to make the dead speak. Paleo-genetics was a revolution in the paleoanthropological field and now proteomics constitutes another frontier [of] research, opening a door to unforeseen dimensions of knowledge.”

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Of course, there are questions which remain unanswered with regard to the Xiahe mandible. Yet the jawbone could prove useful to scientists down the line. For instance, new fossils discovered throughout Asia may be compared to it. With that, they might subsequently be identified as having belonged to a Denisovan.

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And on top of everything else, the Xiahe mandible has shown that there are potential anthropological treasures to be discovered up high. As paleoanthropologist Viola pointed out to National Geographic, “The high mountains of Asia are really, really unknown. People usually just assumed nobody lived there.”

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Viola is himself conducting research at a lofty site thought to have once housed ancient species of human. This is the Kyrgyzstan’s Sel’ungur Cave, which is situated about 6,200 feet above sea level. Viola and his colleagues initially thought that this place had accommodated Neanderthals, but the Xiahe mandible research has suggested an alternative. As he put it, “Maybe they were Denisovans.”

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The Xiahe mandible may completely upend previous notions related to early human species. Indeed, these are the thoughts of one of the experts involved in putting together the 2019 paper. As Jean-Jacques Hublin proclaimed to newspaper The Guardian, “Until today, nobody imagined that archaic humans could be able to dwell in such an environment.”

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The Baishiya Karst Cave itself might yet inform scientists even further. You see, in addition to the Denisovan jawbone, other skeletal remains have been found inside here. These seemingly belonged to different animals, and they were discovered alongside what appear to be tools. It remains to be seen if these things can be linked to the Denisovans, but further investigations will hopefully tell us more.

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Whatever the case may prove to be down the line, the research centered on the Xiahe mandible has already been celebrated as groundbreaking. The very fact that it utilized protein testing to identify an ancient human species is itself significant. Perhaps using similar methods in future will help to classify all sorts of fossils littered around China today.

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