Cue the Laos Cobra cave tooth, first clue in the Denisovan hunt down under

Denisovans, the mysterious archaic humans first identified in a cave in Siberia, also lived down under. We know that because indigenous Papuans and Australians carry their DNA – around 4% was bequeathed by a Denisovan ancestor.

Since that discovery in 2010, the race has been on to find the actual remains of a Denisovan in this part of the world. Now an international team may have found them in the form of a 160,000–130,000-year-old tooth unearthed from Cobra cave in the remote Annamite mountains of northern Laos.

“We have essentially found the ’smoking gun’ – this Denisovan tooth shows they were once present this far south,” says geoarchaeologist Mike Morley, a team member from Flinders University, in Adelaide.

“It’s incredibly exciting; we’ve never seen them in a warm climate before,” says geochronologist Kira Westaway, a team member from Macquarie University, Sydney.

Outside the research team, some remain circumspect about the tooth’s identity. “The authors have done a great job in describing and dating it, but I’d prefer to say it’s a putative Denisovan fossil,” says seasoned anthropologist Chris Stringer, the research leader in human origins at the British Natural History Museum.

Denisovan tooth laos
Denison tooth. Credit: Demeter et al, 2022 / University of Copenhagen

Denisovans are the only archaic humans ever to have been identified by DNA alone. The first clue came from a finger bone fragment, one of the middle bones of a pinky, found in a jumble of bone fragments in Denisova Cave near the border of Siberia and China. The pinky bone looked like it came from a human. But its DNA showed that, while human-like, it was not from a modern human or a Neanderthal but something quite distinct.

So distinctive was the DNA, that geneticists could pick out traces of it in modern human populations. The DNA of nearby Eurasians carried only 0.1% but on the other side of the world, the DNA of indigenous Papuans and Australians carried 4%.

The DNA story said the Denisovans most likely interbred with modern people somewhere in South East Asia. So where were their physical remains?

The only other Denisovan remains to date were identified in 2019, when an oversized human-like jaw with huge teeth, originally found by a meditating monk in a Tibetan cave, turned out to be Denisovan. The identification was made after researchers extracted collagen protein from the very large teeth and found it was a match to that predicted by the Denisovan DNA sequence. Denisova Cave later also yield three large Denisovan teeth.

Read also: Denisova DNA yields more secrets

To find Denisovan fossils down under, most pundits placed their bets on the islands that serve as stepping stones between the Asian mainland and Australia/Papua New Guinea. The Annamite mountains were not on the radar of Denisovan hunters. But they were for Fabrice Demeter, a paleoanthropologist at the University of Copenhagen and lead author of the new study. He had gone there to answer a different question: how did modern humans travel from Africa through Asia? Laos turned out to be an important part of the route: in 2012 the ‘cave of monkeys’ aka Tam Pa Ling Cave, delivered the oldest remains of modern humans on mainland southeast Asia, with one jaw dating to around 70,000 years ago.

But adventurous climbers had also found another promising cave nearby, the Cobra cave, or Tam Ngu Hao 2.

Unlike the cave of monkeys it was not habitable. But it served, as limestone caves do, as a trap for fossils swept from surrounding hillsides during floods. Inside, a cemented gravelly deposit turned out to be a treasure trove of fossilised bones, mostly teeth of giant herbivores such as ancient bison, elephants and rhinos. Amidst them was a prize gem: one archaic-looking human molar.

How old was it? Archaeology is plagued by arguments about dates so the researchers deployed independent approaches. Kira Westaway from Macquarie dated the cemented gravel deposit using infra-red stimulated luminescence dating. Jian-Xin Zhao from University of Queensland dated the flowstones above the gravel using a uranium series. Renaud Joannes-Boyau from Southern Cross University, in Lismore, dated the co-cemented animal teeth using a combined uranium series and electron spin resonance technique (the hominin tooth was too precious to drill into).

Denison tooth laos
Mike Morley at Flinders Microarcheology Lab. Credit: Flinders University

Finally Mike Morley from Flinders examined hair-thin slices of the cemented deposits under the microscope to check the integrity of the structure. Were the baubles in this cement cake all baked in at the same time, or had some fallen in later, perhaps when a tree root pierced the layer?

The forensic analyses all agreed: the layer was intact and had been deposited between 164,000 and 131,000 years ago. The minimum age for the tooth then, was 131,000 years old. It did not belong to a modern human.

Had the cave been in the northern hemisphere, the key suspect would be a Neanderthal but their kind had never been found this far south. Could it be, at last, a remnant of the long-sought southern Denisovan?

The tooth did not yield any DNA for an ID; no surprise since the tropics are unkind to DNA preservation. So Clément Zanolli, an expert at identifying hominin teeth at the University of Bordeaux, in France, was brought in to adjudicate. His method is akin to the time-honoured approach of identifying species from the molar cusp pattern, but with a high-tech advance. Instead of scanning the outer cusps, which may be worn or broken, he uses a miniature CT scanner to look at the pristine pattern just underneath the enamel layer, the so-called enamel-dentine junction, or EDJ.

Zanolli has used his technique to look at the EDJ of diverse members of the South East Asian archaic human family – including Indonesian Homo erectus and the diminutive Homo luzoneniensis (found on the island of Luzon, in the Philippines) and Homo floresiensis (found on the Indonesian island of Flores). His analysis placed the tooth closest to that of the Denisovan teeth found in the Tibetan cave and Denisova Cave.

Protein traces from the tooth enamel also suggested the owner was a young woman, because no male-specific version of the amelogenin protein was detected.

But to close the case on whether or not she really was a tropical Denisovan, more of her kind will need to be found. Watch this space. There’s surely more to come!

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