AN ancient lineage of humans' closest relatives has been unearthed in France - and scientists say the discovery is a world-first.
A fossilized Neanderthal was discovered in a cave system in the Rhône Valley in 2015. He was nicknamed Thorin, after the Lord of the Rings character.
DNA analysis revealed Thorin lived between 42,000 and 50,000 years ago, in a small community separated from the rest of the world.
He is just one member of a previously unknown lineage that split from other Neanderthals around 100,000 years ago and remained genetically isolated for over 50,000 years.
"Until now, the story has been that at the time of the extinction there was just one Neanderthal population that was genetically homogeneous, but now we know that there were at least two populations present at that time," geneticist Tharsika Vimala said.
The discovery, published today in Cell Genomics, could elucidate why the species went extinct.
And that isn't all, as scientists uncovered another shocking detail.
Thorin's fossilized remains were discovered in Grotte Mandrin, a cave system previously known to scientists for housing humans.
Despite being found seven years ago, they are still being excavated with painstaking care.
New evidence reveals that Thorin lived within walking distance of another Neanderthal population.
"The Thorin population spent 50,000 years without exchanging genes with other Neanderthal populations," co-first author Ludovic Slimak explained.
"We thus have 50 millennia during which two Neanderthal populations, living about ten days' walk from each other, coexisted while completely ignoring each other."
Slimak - who is responsible for finding Thorin's remains - believes this is proof Neanderthals "biologically concieved our world very differently from us."
Coexisting in such a way would be "unimaginable" for ancient humans, Slimak explained.
While they are often confused as our ancestors, Neanderthals are an entirely different species than Homo sapiens.
There is overlap between our genetic records - and evidence of crossbreeding - but Neanderthals had larger brains, stockier bodies, and more prominent brows.
Senior author Martin Sikora believes Thorin's genome is a remnant of some of the earliest Neanderthal populations in Europe.
"The lineage leading to Thorin would have separated from the lineage leading to the other late Neanderthals around 105,000 years ago," Sikora explained.
Based on the position of Thorin's remains within the cave's sediment, archeologists suspect he lived between 40,000 and 45,000 years ago, making him a "late Neanderthal."
To determine his age, the team extracted DNA from his teeth and jaw and compared his genetic information to previously seequenced Neanderthal genomes.
The intitial analysis suggested Thorin was actually older than the archeological age estimate.
In fact, his genome bore close resemblance to those of Neanderthals who lived more than 100,000 years ago.
The researchers worked for seven years to clear up the mystery, analyzing isotopes from his bones and teeth to date Thorin's remains.
Isotopes are atoms of the same element that have an equal number of protons and an unequal number of neutrons, causing their weights to differ.
Radioactive isotopes decay over time, while stable isotopes do not - making both important markers in archaeology.
In Thorin's case, isotopic analysis indicated he lived in a frigid climate.
As late Neanderthals lived during the Ice Age and early Neanderthals enjoyed a much warmer climate, this finding confirmed suspcions that he fell into the former category.
Thorin's genome most closely resembled an individual excavated in Gibraltar, leading researchers to suspect his population migrated to France.
"This means there was an unknown Mediterranean population of Neanderthals whose population spanned from the most western tip of Europe all the way to the Rhône Valley in France," Slimak said.
Knowing that Neanderthal communities were small and insular is key to understanding why they died out.
Isolation is generally considered a disadvantage. Over time, as members of the population breed, they become increasingly genetically similar.
This means they are succeptible to the same diseases and are less able to adapt to a changing climate.
Vimala pointed out that isolation also "limits you socially because you're not sharing knowledge or evolving as a population."
To put Thorin's lineage in context, the reserachers must sequence other genomes from the same time period.
The next step is looking for genes from other parts of the world, where the team expects to find other "deeply structured populations."