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When archaeologist Ludovic Slimak unearthed five teeth in a rock shelter in the Rhône Valley in France in 2015, it was immediately clear that they belonged to a Neanderthal. They were the first intact remains of this prehistoric species to be discovered in this country since 1979.
But the unique find, named Thorin after a character from The Hobbit, remained a closely guarded secret for nearly a decade while Slimak and his colleagues deciphered the find’s significance – a delicate undertaking that pitted ancient DNA experts against archaeologists.
“We faced a big problem,” said Slimak, a researcher at France’s National Center for Scientific Research and Paul Sabatier University in Toulouse. “Genetics suggested that the Neanderthal we called Thorin was 105,000 years old. But from the archaeological context (of the specimen), we knew it was somewhere between 40,000 and 50,000 years old.”
“What the DNA suggested did not match what we saw,” he added.
It took the team nearly ten years to piece together the story of the enigmatic Neanderthal, adding a new chapter to the long-standing mystery of why these people disappeared about 40,000 years ago.
The study, published Wednesday in the journal Cell Genomics, found that Thorin belonged to a lineage or group of Neanderthals that was isolated from other groups for about 50,000 years. This genetic isolation was the reason Thorin’s DNA appeared to come from an earlier time than was actually the case.
Until now, geneticists assumed that there was a genetically homogeneous Neanderthal population at the time of the extinction. However, the new study shows that there were at least two populations in Western Europe at that time – and they lived surprisingly close to each other.
“The Thorin population has not exchanged genes with other Neanderthal populations for 50,000 years,” Slimak said in a press release.
“So we have 50 millennia in which two Neanderthal populations, living about ten days’ march apart, coexisted and completely ignored each other.”
Slimak said the discovery suggests that Neanderthal communities were small and isolated – factors that could be crucial to understanding their extinction, since isolation is generally considered an evolutionary disadvantage.
Lower genetic diversity could make adaptation to climate change or disease more difficult, while lower social interaction between groups could make the exchange of knowledge and technology more difficult.
“They were happy in their valley and didn’t have to move, whereas Homo sapiens are constantly exploring and wanting to see what’s beyond that river, beyond that mountain. (We have) this need to move and this need to build a social network,” Slimak said.
This pattern of small, culturally and genetically isolated populations was probably a major factor in the extinction of the Neanderthals, which occurred roughly at the same time as the arrival of Homo sapiens in Europe, he said.
DNA from Homo sapiens fossils from this period shows that these early arrivals interbred with Neanderthals – traces of these encounters have been preserved in modern-day human populations. However, no corresponding genetic evidence of this interbreeding has been found in Neanderthal fossils from this period, including Thorin’s remains, the study says.
Whatever behavior led to this lack of genetic mixing, along with small and isolated Neanderthal populations like the one Slimak and his colleagues identified, probably contributed to the Neanderthals’ disappearance, says Chris Stringer, head of research in human evolution at the Natural History Museum in London, who was not involved in the study.
“Whatever the reasons for this imbalance (social, biological?), it contributed to the demise of the last Neanderthals, as their already small populations lost reproductive-age individuals to the other species without any replenishment in return,” Stringer said by email.
“Coupled with the economic competition of newcomers for resources, this could have led to a demographic collapse.”
It is not clear whether Thorin’s complete skeleton is buried in the Grotte Mandrin, as the rock shelter in the Rhone Valley near Malataverne in France is called. The remains were found near the surface in soft, unstable ground, and excavation is proceeding slowly, Slimak said, with archaeologists pulling out “one grain at a time.” It is also unclear whether or not the specimen, which is believed to be a male, was intentionally buried.
Archaeologists have unearthed more of Thorin’s remains: 31 teeth, part of his jaw and five finger bones. The shape of his teeth is typical of a Neanderthal, but he had two extra lower molars – a feature that sometimes indicates an inbreeding population, the study said.
Genetic mystery solved
Initial genetic analysis suggested that Thorin was much older, as his genome was different from that of other later Neanderthals and similar to the genome of earlier humans who lived more than 100,000 years ago..
To understand Thorin’s origins and determine the age of his remains, the team analyzed chemical isotopes in his bones and teeth to draw conclusions about the climate he lived in, based on the water he may have drunk and other factors. A Neanderthal in Europe 105,000 years ago would have enjoyed a much warmer climate than a Neanderthal living 45,000 years ago during the Ice Age.
“We worked for seven years to figure out who was wrong – the archaeologists or the genomicists,” Slimak said in the press release.
Slimak has been involved in the excavation of the Mandrin Cave for more than three decades and has made a number of exciting discoveries in the rock shelter. It is the only known site where Homo sapiens and Neanderthals lived alternately, and also the first evidence of the use of bows and arrows outside Africa.
“Grotte Mandrin always provides surprises,” said Stringer.
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