DNA of 'Thorin,' one of the last Neanderthals, finally sequenced, revealing inbreeding and 50,000 years of genetic isolation

"Thorin", one of the last Neanderthals to walk the planet, was part of a previously unknown lineage that was isolated for 50,000 years, a new analysis of his DNA finds.

Discovered in 2015 at the entrance to the Grotte Mandrin rock shelter in the Rhône River valley of southern France, Thorin — nicknamed after a dwarf in J. R. R. Tolkien's "The Hobbit" — has sometimes been called the "last Neanderthal" because he may have lived as recently as 42,000 years ago, close to when our closest human relatives disappeared. Although only his teeth and portions of the skull have been recovered so far, Thorin's genome was analyzed to better understand when and how Neanderthals disappeared.

In a study published Wednesday (Sept. 11) in the journal Cell Genomics, a team of researchers led by Ludovic Slimak of the Center for Anthropobiology and Genomics of Toulouse, France, detailed their discovery that Thorin came from a lineage of Neanderthals who were isolated for thousands of years, in spite of the fact that other groups lived nearby.

Slimak, who originally found Thorin's remains, theorized two decades ago that Neanderthals in the Rhône Valley were distinct from those in nearby regions based on differences he noticed in the stone tools at Grotte Mandrin. He proposed that Thorin and his relatives did not adopt the new tool-making style that was seen at other contemporary sites.

"It turns out that what I proposed 20 years ago was predictive," Slimak told Live Science in an email. "The population of Thorin had spent 50 millennia without exchanging a single gene with the classical Neanderthal populations."

Related: 'More Neanderthal than human': How your health may depend on DNA from our long-lost ancestors

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Slimak and colleagues used part of a root of one of Thorin's molars to determine that he was male and to generate a whole-genome sequence. Compared with previously published late European Neanderthal genomes, Thorin was found to have high genetic homozygosity — identical gene variants often indicative of recent inbreeding — and no evidence of interbreeding with modern humans.

"Our results suggest small group sizes and long-term genetic isolation of the Thorin population from other late Neanderthal populations with genetic data available," the researchers wrote in their study.

The isolation of this Neanderthal group raises questions about when and why the species disappeared.

Using a variety of techniques, including radiocarbon dating and assessment of the geological layers in the cave, the research team found that Thorin died between 52,000 and 42,000 years ago. However, according to the study, evidence newly uncovered in 2023 suggests Thorin is much more likely to be 42,000 years old and therefore is among the very last Neanderthals.

"Everything must be rewritten about the greatest extinction in humanity and our understanding of this incredible process that will lead Homo sapiens to remain the only survival of humanity," Slimak said. "How can we imagine populations that lived for 50 millennia in isolation while they are only two weeks' walk from each other? All processes need to be rethought."

Related: 'Simply did not work': Mating between Neanderthals and modern humans may have been a product of failed alliances, says archaeologist Ludovic Slimak

April Nowell, a Paleolithic archaeologist at the University of Victoria in Canada who was not involved in the study, told Live Science in an email that it "supports the hypothesis that Neandertals living in small, socially isolated groups struggled to keep their numbers viable." And, because it's scarce, "any hominin DNA from that time frame is interesting and adds to our knowledge," Omer Gokcumen, an evolutionary genomicist at the University at Buffalo who was not involved in the study, told Live Science in an email. Information from Thorin's genes and final resting place adds to a growing understanding among paleoanthropologists of the Neanderthals' last days on Earth.

While it is commonly assumed that humans were responsible for the extinction of the Neanderthals, the genetic and cultural isolation seen at the Grotte Mandrin rock shelter "raises new lines of questions to explore further their social and ethological organization, which could have played an important role in their later extinction," the researchers concluded.