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A tooth unearthed from a remote cave in Laos is helping to sketch an unknown chapter in the human story.
Researchers believe the tooth belonged to a young female who lived at least 130,000 years ago and was likely a Denisovan – an enigmatic group of early humans first identified in 2010.
The lower molar is the first fossil evidence placing Denisovans in Southeast Asia and may help untangle a puzzle that had long vexed experts in human evolution.
The only definitive Denisovan fossils have been found in North Asia – in the eponymous Denisova cave in Siberia’s Altai Mountains in Russia. Genetic evidence, however, has tied the archaic humans most closely to places much further south – in what’s now the Philippines, Papua New Guinea and Australia.
“This demonstrates that the Denisovans were likely present also in southern Asia. And it supports the results of geneticists who say that modern humans and the Denisovans might have met in Southeast Asia,” said study author Clément Zanolli, a researcher in paleoanthropology at CNRS, the French National Center for Scientific Research and the University of Bordeaux.
Archaeologists uncovered the tooth in a place known as Cobra Cave, 160 miles (260 kilometers) north of Laos’ capital, Vientiane, where excavations began in 2018. The study, which published in the journal Nature Communications on Tuesday, estimated the molar was between 131,000 and 164,000 years old, based on analysis of cave sediment, the dating of three animal bones found in the same layer, and the age of rock overlying the fossil.
“Teeth are like the black box of an individual. They preserve a lot of information on their life and biology. They have been always used by paleoanthropologists, you know, to describe species or to distinguish between species. So for us paleoanthropologists (teeth) are very useful fossils,” Zanolli said.
Comparison with archaic human teeth
The researchers compared the ridges and dips on the tooth with other fossilized teeth belonging to archaic humans and found it didn’t resemble teeth belonging to Homo sapiens or Homo erectus – an archaic human that was the first to walk with an upright gait whose remains have been found across Asia. The cave find most closely resembled a tooth found in a Denisovan jawbone found on the Tibetan plateau in Xiahe county, in Gansu province, China. The authors said it was possible, though less likely, it could belong to a Neanderthal.
“Think about about it (the tooth) as if you are traveling into (a) valley between mountains. And the organization of these mountains and valleys is very typical of a species,” Zanolli explained.
Analysis of some protein in enamel from the tooth suggested that it belonged to a female.
Denisovan DNA lives on in some humans today because, once our Homo sapiens ancestors encountered the Denisovans, they had sex with them and gave birth to babies – something geneticists call admixture. This means we can look back into human history by analyzing current-day genetic data.
The “admixing” happened was thought to have happened more than 50,000 years ago, as modern humans moved out of Africa and likely crossed paths with both Neanderthals and Denisovans. But pinning down exactly where it happened has proven difficult – particularly in the case of Denisovans.
Any addition to the meager hominin fossil record of Asia is exciting news, said Katerina Douka, an assistant professor of archaeological science at the department of evolutionary anthropology at the University of Vienna. She wasn’t involved in the research.
She said she would have liked to see “more and extensive evidence” that the tooth was definitively Denisovan.
“There is a chain of assumptions the authors accept in order to confirm that this is a Denisovan fossil,” she said.
“The reality is that we cannot know whether this single and badly preserved molar belonged indeed to a Denisovan, a hybrid or even an unknown hominin group. It might well be a Denisovan, and I would love it to be a Denisovan, because how cool would that be? But more confident evidence is needed,” she said.
In deeming the Laos tooth Denisovan, the researchers in this study relied heavily on a comparison with the Xiahe jawbone, Douka said. However, the jawbone, while thought by many to be Denisovan, was not an open-and-shut case. No DNA had been retrieved from the fossilized jawbone, only “thin” protein evidence, she added.
“Anyone working on this hominin group, where many major questions still remain, wants to add new dots on the map. The difficulty is in reliably identifying any fossils as that of a Denisovan,” she said. “This lack of robust biomolecular data, however, reduces significantly the impact of this new find and it is a reminder of how difficult it is to work in the tropics.”
The study authors said they planned to try and extract ancient DNA from the tooth, which, if possible, would provide a more definitive answer but the warm climate means that could be a long shot. The research team also plans to continue excavating the site after a pandemic-induced hiatus in the hope of more discoveries of ancient humans that lived in area.
“In this kind of environment, DNA doesn’t preserve well at all but we’ll do our best,” said study coauthor Fabrice Demeter, an assistant professor at the Lundbeck Foundation GeoGenetics Centre in Denmark.