In a huge eureka moment for human evolution, a fossilized jawbone found high on the inhospitable Tibetan Plateau turns out to be Denisovan, an international team of scientists reported Wednesday in Nature.
The jawbone, containing some teeth, dates back about 160,000 years, which throws back the plateau’s settlement by about 120,000 years.
Until now, the Denisovans had been known from exactly one cavern in Siberia, which gave them their name: Denisova Cave, in the Bashelaksky Range of the Altai Mountains.
But certain modern-day Asian populations, including indigenous people in central Asia — notably Tibet, and thousands of kilometers away in low-lying Australia, Papua New Guinea and Melanesia — have a significant component of Denisovan DNA, around 4 to 6 percent. Evidently, their ancestors interbred with the Denisovans.
Thus the theory arose that while Neanderthals occupied mainly Europe in the west, their sister species, the Denisovans, spread east through Asia. But no evidence of the Denisovan range had been found. Until now.
Moreover, the Tibetan Denisovan lived at very high altitude, well beyond the human comfort zone. This supports the discovery that Sherpas and other modern Asian populations gained hypoxia-tolerance genes from the Denisovans.
“One of most spectacular aspects [of the discovery] is of course the location where the specimen was found — the Tibetan Plateau at above 3,300 meters [10,800 feet],” said team head Prof. Jean-Jacques Hublin of the Max Planck Institute for Evolutionary Archaeology. “Until now, nobody imagined that archaic humans could dwell in such an environment. It was a big surprise. Challenging environments like high altitudes were thought to have been only colonized by modern humans like us less than 40,000 years ago,” the paleoanthropologist added.
A mysterious species
Very little is known about this enigmatic species, because so little has been found of them: a fragment of finger; a bit of cranium; some teeth.
The large size of the few teeth found to date in Denisova Cave are also evident in the Tibetan mandible and suggests the Denisovans would have loomed over modern humans. Also, though only two fragments of Denisovan skull have been found (yes, in that same Denisova Cave), they were thicker than typical of Homo sapiens — closer to the Homo erectus type.
It was the world-renowned paleogeneticists Svante Pääbo and David Reich, with others, who managed to extract DNA from the finger-bone fragment found in the cave, and reported in 2010 that it was an unknown species.
Sediment analysis indicates that Denisovans occupied the Siberian cave around 300,000 to about 50,000 years ago. Separate research found that Neanderthals reached the cave about 200,000 years ago.
The evidence is frustratingly spotty, but it has become clear that at least for a time, the sister-species Neanderthals and Denisovans shared the cave. "Sister species" means that the Neanderthals and Denisovans had a common ancestor that remains to be identified, Hublin told Haaretz, adding: "There are several candidates. Maybe something like the Zuttiyeh (Israel) hominin."
n fact, Denisova is where the extraordinary discovery of a first-generation hybrid Neanderthal-Denisovan teenager was found.
The Tibetan jawbone was found 2,400 kilometers from Denisova Cave, and was dated to about 160,000 years ago — in the middle of the range of the Siberian findings.
Later anatomically modern humans lived there too. Some think the various species were interbreeding like rabbits to the bitter end — of the Denisovans and Neanderthals, that is.
The earliest-ever needle known to man, 7 centimeters (2.7 inches) long, made of avian bone and dated to about 50,000 years ago, was also found in that extraordinary cave. Go figure which species made and used it. While various media such as the Siberian Times reported that the manufacturer was likely Denisovan, there is no actual reason to think so. (Nor is there any reason whatsoever to assume that a gorgeous green stone bracelet found in the cave meant Denisovans had advanced aesthetic technique or capacity.)
Home on the mountain range
DNA could not be salvaged from the ancient Tibetan jawbone. It was identified as Denisovan by protein analysis. Finding a single bone does not prove that they thronged throughout Asia, but it definitely shows they spanned at least from Siberia to Tibet.
The discovery reinforces the theory that certain populations of modern humans, not least the Tibetan Sherpas, inherited their tolerance for high altitude from the Denisovans, says the team.
The bone, and Paleolithic stone tools and signs of butchery, were found in Baishiya Karst Cave on the Tibetan Plateau at an altitude of 3,280 meters above sea level. At that height, most of us humans gasp for air and get sick as dogs. Altitude sickness is not a factor of physical fitness, as many a muscled mountaineer has demonstrated.
The genetic quality of hypoxia tolerance that the Sherpas and their neighbors inherited from Denisovans had been a riddle. “That was puzzling because Denisova Cave is not so high in altitude,” said Hublin: it is only about 700 meters above sea level. “But now we have an explanation. Which is, Denisovans or related populations probably lived for a long time in the Tibetan Plateau, and such a mutation could become fixed in this fossil hominin and later passed onto modern invaders.”
Neanderthals and Denisovans are believed to have split from one another well over 400,000 years ago. However, though they merrily interbred, many cavil at calling them separate species, preferring instead to call them “different types.”
So: While the Neanderthal types occupied southern and central Europe, the Denisovan types are believed to have spread through Asia. Hublin notes that a number of hominin fossil specimens found in China are also suspected to be Denisovan. They are not Homo erectus or modern humans, he explained, and are thus good candidates for being Chinese Denisovans. “But this has been impossible to prove to date, simply because in most of these fossils there is no ancient DNA,” Hublin said.
Nor did this mandible, but the team resorted to paleo-proteomics. Protein molecules are not quite as fragile as DNA and may survive longer in the geological record, he explains: They managed to find proteome in the dentine of the teeth, though not the bone itself. “The endogenous proteins are highly degraded and clearly distinguishable from contaminating modern proteins,” the team wrote.
The protein sequences they managed to elucidate are most similar to Denisovans, hence the conclusion that the mandible came from “a hominin population closely related to the Denisovans from Denisova Cave.”
Less cautiously, we will call it Denisovan. The mandible displays archaic morphology “rather common” among Middle Pleistocene hominins and rather distant from Homo erectus.
And what did this Denisovan look like? That’s anybody’s guess. Any artist’s impression of a Denisovan is about as reliable, at this stage, as an artist’s impression of an alien from Alpha Centauri. That said, the latest thinking about Denisovans is that, like us Homo sapiens types, they existed in a range of morphologies. Think about Scandinavians versus Aborigines, for instance.
The Denisovans who bred with the ancestors of today’s Tibetans, or modern southeast Asians possibly as long as 80,000 years ago, may have been profoundly different from the Siberian ones. Genetic analysis indicates that the Denisovans who mixed with humans, resulting in today’s Papua New Guineans, were almost as genetically distinct from their Siberian brethren as they were from Neanderthals.
The analysis of the Papuans indicated two sources of Denisovan DNA from two different Denisovan populations that had been separated for maybe 300,000 years of evolution. Possibly the Denisovans of the north and Denisovans of the south were strikingly dissimilar. Some of the mysterious fossils in China — the most recent at Shushan — share some features with Neanderthals, Hublin shares. But they are not Neanderthals.
Finally, it is possible that the Denisovans went extinct only very recently — just as modern civilizations were arising in the Levant, for instance. The latest genetic analysis of the Denisovan component in Melanesia indicates that interbreeding may have persisted until just 15,000 years ago. There is a village in Israel featuring rudimentary farming that has been dated to 23,000 years ago.
It is possible that the jawbone found on the Tibetan Plateau in a place where we could not survive unaided, at least not for long, isn’t a “pure” Denisovan (whatever that is). Archaic humans got about.