Lack of genetic variation in Neanderthals may explain their disappearance

The extinction of Neanderthals remains a mystery, but the study of a specimen from the French region of Rhône, whose genetic lineage spent 50,000 years without mixing with other groups, opens up a new hypothesis: their genetic isolation.

Neanderthals inhabited Eurasia until 40,000 years ago, coexisting with our ancestor, Homo sapiens, before disappearing.

“It was the last moment in which there were several humanities on Earth, a strategic and deeply enigmatic moment, since we do not understand how an entire humanity, which existed from Spain to Siberia, could suddenly become extinct,” says Ludovic Slimak, a researcher at Paul Sabatier University in Toulouse and co-author of the study published this Wednesday (11) in the journal Cell Genomics.

The specimen, named “Thorin” in reference to a character by the writer JRR Tolkien, was found in 2015 in the Mandrin Cave (Drôme), inhabited alternately by Neanderthals and Homo sapiens.

This discovery is rare, as this is the first Neanderthal discovered in France since 1978. Only about 40 have been found in all of Eurasia.

“As soon as the body was discovered, I sent a small bone fragment, a piece of a molar, to Copenhagen so that the teams we collaborated with could do the genetic analysis. We had been trying to obtain DNA from Mandrin for ten years, whether from animals or humans, but we had never been able to do it, because DNA degrades very quickly when the remains leave the ground,” says Slimak.

When the results came back, there was confusion. According to archaeological analyses, “this body was between 40,000 and 45,000 years old, but for geneticists, it was 105,000 years old. One of the two parties must have made a mistake,” he explains.

– Consanguinity –

It took seven years of research to solve the puzzle. Isotope analysis showed that Thorin lived in an extremely harsh climate, corresponding to the Ice Age, which only late Neanderthals experienced.

However, its genome is very old. “It is a vestige of the earliest Neanderthal populations in Europe,” said geneticist and lead author of the study Martin Sikora of the University of Copenhagen in a statement.

According to him, the lineage leading to Thorin would have separated from that of other late Neanderthals around 105,000 years ago.

This lineage then spent 50,000 years “without any genetic exchange with classical European Neanderthals,” even with populations that lived just two weeks’ walk away, Slimak explains.

An isolation unthinkable for their cousin Homo sapiens, especially since the Rhône Valley was then one of the main migratory corridors between northern Europe and the Mediterranean.

“Archaeology has long told us that Neanderthal populations lived in very small territories, a few dozen kilometers around a site,” the archaeologist recalls. It was known that they lived in small groups, with problems of inbreeding.

In the case of Homo sapiens, territories are “infinitely larger, with tens of thousands of square kilometers,” Slimak said, adding that “the diffusion of objects, shells, sociability and the construction of structured social networks are universal” across the species.

These two populations “did not understand or organize the world in the same way”, which would be “a fundamental key to understanding” the extinction of the Neanderthals.

“When you’re isolated for so long, you limit the genetic variation available, which means you’re less able to adapt to climate change and pathogens. You also limit yourself socially, as you don’t share or evolve as a population,” says Tharsika Vimala, a geneticist at the University of Copenhagen and co-author of the study.