A new piece to the puzzling tale of the Denisovan hominins has emerged in the form of a jawbone found in a cave on the Tibetan Plateau.
The Denisovans are one of the most head-scratching branches of the prehistoric human family tree. We know them from just a few scraps of bone and large teeth found in a Siberian cave, and from their genetic legacy, which lives on in some modern-day human populations.
Their closest relatives were the Neanderthals, but what they looked like and where they lived remains a mystery. This new find gives us some clues.
The jawbone, encrusted in limestone and still bearing two molar teeth, was first plucked from its resting place in the Baishiya Karst Cave – a one-kilometre-deep cave at the foot of white limestone cliffs in Xiahe county on the eastern edge of the Tibetan Plateau – by a monk in 1980.
Attempts to retrieve ancient DNA from the fossil failed. But researchers were able to isolate fragments of protein from the dentine inside one of the teeth.
Like DNA, proteins comprised a chain of building blocks – specifically, amino acids – and the sequence in the chain can be used to determine which animal made the protein.
Snippets from just eight proteins, all structural variants known as collagens, were isolated from the tooth. It wasn’t much, but it was enough. The proteins were most like those from Denisovans that lived in the Altai Mountains of Siberia.
It’s the first time that a fossil outside of the Denisova Cave has been clearly linked to the group of archaic humans.
The location is also significant. It helps to solve one of the enduring mysteries surrounding the Denisovans’ genetic legacy.
The genomic after-glow of prehistoric pairings between Denisovans and Homo sapiens ancestors is evident in modern-day populations across Asia, New Guinea and Australia.
In the case of present-day Tibetans, a particular variant of the EPAS1 gene, which assists those who have it to live in the low oxygen environments of the Tibetan Plateau, has its origins in the Denisovan genome.
This high-altitude gene variant had perplexed scientists, because the Denisova cave is only 700 metres above sea level.
The Baishiya Karst Cave is 3300 metres above sea level, so it now appears that Denisovan populations adapted to living at this altitude passed a high-altitude gene variant on to humans who later inhabited the region.
“Any mutation that would be favourable to the breathing in an atmosphere impoverished in oxygen would be retained by natural selection,” says led researcher Jean-Jacques Hublin of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany.
“It’s a rather likely scenario to explain how this mutation eventually made its way to present-day Tibetans.”
The location is also significant because the Tibetan Plateau was previously thought to be too extreme an environment to be conquered by any species other than Homo sapiens.
“Nobody ever imagined that archaic humans could be able to dwell in such an environment, and it’s a big surprise because most people thought that challenging environments like the high altitude were colonised only by modern humans like us less than 40,000 years ago,” says Hublin.
The Denisovans, it seems, were there much earlier. A precise age for the fossil hasn’t been determined, but dating of the limestone crust covering the bone puts the individual in the cave at least 160,000 years ago, and potentially even earlier.
“It’s a nice story,” says palaeoanthropologist Tanya Smith from Griffith University in Australia, who wasn’t involved in the study.
Speculation has often swirled around fossil remains found in China about whether or not they could belong to ancient Denisovan populations. But without a skeleton to compare these to, it’s been hard to say.
The Xiahe mandible is the largest Denisovan bone to be uncovered so far, and Hublin and his colleagues suggest that it bears some similarities to other unidentified Chinese remains.
One example is a jawbone found in the Penghu straight off the coast of Taiwan. Like the Xiahe fossil, it has large teeth and lacks a third molar
Absent third molars – or wisdom teeth – are common in modern humans. That wasn’t the case in the Middle Pleistocene, says Hublin.
But the shape of the jawbone and teeth isn’t enough to know for sure.
“It’s sometimes difficult when you’re just comparing one single fossil against another single fossil to be particularly confident in saying these two teeth are the same or they’re different,” explains Smith.
That’s because jawbones and teeth can vary across populations. “People want skulls,” she says.
Until then, ancient proteins could be a way forward, notes Hublin.
By extracting protein fragments from other Chinese specimens, he says, “hopefully it will be possible at some point to make a grouping of, if not all the Denisovans of Asia, at least a subpopulation of so-called Denisovans”.
The study is published in the journal Nature.