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A tooth found in a southeast Asian cave could expand our knowledge of one of our most enigmatic relatives.
The Laos molar is potentially the most recently discovered fossil of the Denisovans, an extinct hominin species of which very little is known.
A young girl living hundreds of thousands of years ago has provided another piece of evidence of how our relatives evolved.
The molar found in Laos is believed to belong to a young Denisovan, a group of hominins of which only a handful of fragments, including teeth and a finger bone, have been discovered. If confirmed as belonging to this group, it would begin to explain the presence of Denisovan DNA in the genetics of people living in southeast Asia today.
The team of scientists behind the discovery, published in Nature Communications, claim the tooth is the 'smoking gun' for this hypothesis. However, Prof Chris Stringer, a Museum human evolution expert, has said further work is needed to confirm the origin of the tooth.
'Any new human fossil from an under-represented area like Laos is important, especially if it's non-sapiens as this clearly seems to be,' says Chris, who was not involved in the study. 'The authors have done an excellent job in describing and dating the find, but at the moment I'd prefer to describe it as a putative Denisovan.
'One of the problems is as of yet, there are no comparable lower molars from Denisova Cave itself, so the tooth is indirectly linked to the Denisovan fossils through a series of morphological inferences using the Xiahe jawbone, another putative Denisovan.
'If it is a Denisovan fossil, it would considerably extend the known physical and ecological ranges of these humans into southeast Asia.'
Studying human evolution is complex, with many species having been named over the years. Different species have varying levels of support, depending on the amount of genetic and fossil evidence available.
The Denisovans are a relatively recent addition to our family tree, and were first described in 2010. Their name originates from the location in which they were found, Denisova cave, and the first identified fossils consisted of just a single finger bone and a tooth.
Despite the lack of fossil evidence, the location of the cave in Siberia and its cool climate has meant that DNA within these remains was well-preserved enough to get a complete genomic sequence.
The Denisovan genetics suggest they were more closely related to Neanderthals than to us. It is believed that their common ancestor split off from the ancestor of Homo sapiens hundreds of thousands of years ago.
It is known is that the Denisovans were present at Denisova cave at least 195,000 years ago, while the most recent fossil has been dated to around 50,000 years ago.
During this time, the Denisovans are known to have interbred with other human species, including Neanderthals and Homo sapiens.
Genetic analysis of humans alive today suggests that the Ayta Magbukon people of the Philippines share almost 5% of their DNA with the Denisovans, with Australian Aboriginals, Torres Strait Islanders and Papuans from New Guinea sharing around 4%.
This genetic evidence suggests that Denisovans lived in southeast Asia, and may have migrated into Australasia, but the fossil evidence for this is lacking. While one fossil jaw bone found in Xiahe, China, has been linked to Denisovans through similarities in its protein composition, genetic material has not been extracted from this specimen and so a direct link has not been made.
This latest tooth uncovered in Laos feeds into the suggestion that these ancient hominins were living in southeast Asia hundreds of thousands of years ago.
The molar was found in the Tam Ngu Hao Two cave, also known as the Cobra Cave. The surrounding area has previously revealed early Homo sapiens fossils, with ongoing searches leading to the discovery of the new fossil.
The cave is partly infilled by sediment that is believed to have washed into the cave, bringing a selection of fossils with it. In addition to the hominid tooth, this has included the remains of Stegodon, an ancient mammal related to mammoths and elephants, as well as tapirs, deers and cows.
Both the fossils and the sediment were dated, indicating that the hominid tooth was deposited in the cave between 164,000 and 131,000 years ago.
Co-author Prof Renaud Joannes-Boyau says, 'The good agreement of the different dating techniques on both the sediment and fossils attest to the quality of the chronology for the species in the region.
'This has a lot of implications for population mobility in the landscape.'
The hominin tooth was then analysed, with its structure and protein signature suggesting it belongs to a girl of between three and nine years old. While its protein composition was compared to other hominins, it didn't share any significant similarities.
The researchers did not extract DNA from the sample as the hot and humid conditions of Laos means that DNA rapidly breaks down. Porcupines are also known to gnaw teeth remains in southeast Asia, which also affected the preservation of the fossil.
Instead, the scientists compared the structural features of the tooth to other samples, and found it shared many similarities with the Xiahe mandible. From this they concluded it was likely Denisovan, but could not rule out that the fossil may belong to a Neanderthal, a discovery which would also significantly expand the known range of this species.
In the meantime, researchers continue to hunt for better preserved specimens and DNA sequences of Denisovans in southeast Asia and elsewhere, in the hope of discovering more about these mysterious hominins.
'We will be more confident about finds such as the Tam Ngu Hao Two molar and the Xiahe mandible being Denisovans when we have more complete Denisovan fossils or good quality ancient DNA and proteomes from existing fossils,' adds Chris.
'Unfortunately, there is very little chance of ancient DNA from the molar itself, and the enamel proteomic data could only identify it as from a probable female Homo individual. However, there is obviously enormous potential for further exciting discoveries in this region of Laos.'