Tam Ngu Hao 2.(Laos): tooth discovered may be first Denisovan fossil from South-East Asia
ABC Science
Belinda Smith
This tooth lay buried in a cave for around 150,000 years until it was brought to light in 2019.(Supplied: Fabrice Demeter (University of Copenhagen/CNRS Paris))
A single ancient tooth found in a Laos cave may be the first physical evidence of an extinct, enigmatic group of humans called Denisovans in South-East Asia.
The molar is unveiled in a study published in Nature Communications today by an international team, which concluded it once belonged to a young girl who lived as far back as 164,000 years ago.
And if added to the Denisovan catalogue, this would be just the third site in the world for such fossils — and one that's thousands of kilometres south of the others.
"When it was found, I wasn't surprised, but I was definitely excited," said study co-author Kira Westaway, a geochronologist at Macquarie University.
Traces of dalliances with Denisovans have been found in the genome of present-day populations, particularly in people indigenous to Australia, Papua New Guinea and some Pacific Islands, and, to a lesser extent, mainland South-East Asia.
Yet despite this, scarcely any Denisovan fossils have been identified — not just in South-East Asia, but anywhere full stop.
So far, there's a pinky finger bone and a few teeth from Denisova Cave in Russia, plus a jaw bone and a few teeth from Baishiya Cave on the Tibetan Plateau.
A bone from a child with a Neanderthal mother and Denisovan father was recently reported too.
The Laos tooth — found in the tropics — showed how adaptable these ancient humans were, Dr Westaway said.
"If you look at the time range [of the Laos tooth], Denisovans were freezing their arses off in Russia, they were adapting to high altitudes in Tibet, and they were living in balmy tropical caves of Laos all at the same time, but doing it 100,000 years earlier than modern humans were doing it."
The tale a tooth can tell
So who did the molar — the first or second tooth from the back of a lower left jaw — belong to?
Archaeologists were clued in on the age of the tooth's owner by its pristineness. It was an adult or permanent tooth, but had no signs of wear and tear, suggesting it hadn't yet grown through the gum and into the mouth when its owner died.
Top, bottom and side views of the tooth discovered in Tam Ngu Hao 2.(Supplied: Fabrice Demeter (University of Copenhagen/CNRS Paris))
In the absence of usable DNA — the heat and humidity of the tropics degrades genetic material quickly — they analysed more robust proteins in the tooth enamel to ascertain if it belonged to a boy or girl.
Males produce a particular form of a protein that's involved in enamel production. The molar did not contain that protein type.
This all pointed to a girl, who was between 3.5 and 8.5 years old when she died.
Now — what type of human was she?
Our species Homo sapiens might be the only type of human knocking around the planet today, but we know quite a few branches of our family tree have, at some point (or points), called South-East Asia home.
The team compared lumps and bumps on the tooth's surface and under the enamel with other teeth of a range of different humans — including Neanderthals, Homo erectus, Denisovans and Homo sapiens — to rule them out (or in).
For instance, the chewing surface of the tooth was exceptionally crinkled, so much so it couldn't be H. sapiens, which has a smoother surface.
Teeth previously discovered in Denisova Cave helped in the identification process, said study co-author Renaud Joannes-Boyau, a geochemist at Southern Cross University.
But overall, the tooth looked remarkably similar to its counterpart attached to what many believe is a Denisovan jawbone found in a cave in Tibet.
There were also some similarities to Neanderthal teeth, Dr Joannes-Boyau said, but that was expected.
"We know Denisovans were very, very close to Neanderthals, and we actually believe that Denisovans were once probably a geographically [isolated population] of Neanderthals, but it's not really clear yet."
Narrowing down her place in human history
The researchers also needed to get a handle on the tooth's age. Directly dating the tooth could be possible, but that technique would involve cutting into it, so the team decided to leave it whole and date everything around the precious fossil instead.
The tooth was found buried in hard sediments in Tam Ngu Hao 2, or Cobra Cave, up in the Annamite mountains of Laos.
The area is a warren of limestone caves, some of which are famous for ancient human fossilised remains. For instance, the nearby Tam Pa Ling, or Cave of the Monkeys, yielded evidence of modern human habitation some 60,000 years ago.
Flinders University geoarchaeologist and study co-author Mike Morley was working at Cave of the Monkeys when the entrance to Cobra Cave, some 30 metres up a rocky hillside, was found.
He climbed up to have a look.
Tam Ngu Hao 2 was not a spacious cavern, and probably not a place humans would have lived, he said.
"It has a rounded part of the top, but then it's narrower at the bottom ... like a keyhole.
But Tam Ngu Hao 2 was part of a larger network of chambers and tunnels. Bones, teeth and other bits and bobs were washed from the surface into the cave and covered in silt and sand.
It was in this jumbled mix that the tooth was later found.
Dr Morley analysed sections of the sediment to make sure it hadn't been "reworked", which can mix newer fossils and sediments with older stuff.
Dr Joannes-Boyau directly dated a few herbivore teeth from the same thick sediment layer the tooth was found in, while Dr Westaway calculated how long ago the sediment itself was buried.
Yet more team members dated the cave's flowstone — a hard, mineralised sheet atop the fossil layer that built up as mineral-rich water dribbled into the cave and deposited crystals as it evaporated.
Together, they suggest the girl lived and died towards the end of the Middle Pleistocene, some 164–131,000 years ago.
Fossils are few and far between
Andy Herries, an archaeologist at La Trobe University who was not involved with the study, said the researchers had "done everything they possibly could to get the best information out of [the tooth] in terms of its age and its morphology".
One sticking point, he added, may be attributing the tooth to a Denisovan based heavily on its similarity to the Tibetan plateau jawbone.
There's still some controversy over whether it is Denisovan or not.
But, Professor Herries said, it makes sense that the Laos tooth is indeed Denisovan, based on present-day genetic studies of people living in the region.
What will help cement the tooth's Denisovan status is finding more fossils like it.
"We've got massive amounts of data for Homo erectus, and massive amounts of data for Neanderthals, and massive amounts of data for Homo sapiens.
"The sample size for Denisovan teeth is the ones from the Tibetan mandible and the ones from Denisova Cave. That's it.
"The amount of data that we have for Denisovans is tiny."
Griffith University archaeologist Adam Brumm, who was not involved in the study either, agreed that the tooth was likely Denisovan.
"Denisovans were clearly established in parts of eastern Asia and genetic analysis also suggests they might have lived in South-East Asia too; in fact, some evidence places them as far east of Laos as New Guinea."
He cautioned that it's "always dangerous to draw big conclusions from a solitary find" like a tooth, but "hominin fossils are so rare that every new specimen is important", especially in South-East Asia.
And the discovery of Homo floresiensis, also known as The Hobbit, also started with a single tooth. It was only years later that more of its skeleton came to light.
"This [study] is more of a 'watch this space' find than a Eureka moment," Professor Brumm said.
"One thing is for sure: the South-East Asian fossil record has yielded all sorts of amazing surprises in recent decades, so who knows what else this team might unearth at the site?"