Their bones were discovered in a cave in Southeastern France in the 1990s. All up, six Neanderthals: two adults, two adolescents, and two children.

Across Europe, there are over 200 sites that feature the ancient remains of Neanderthals such as these, but few of them tell the same grisly story this cave contains, according to a new archaeological analysis.

Whoever they were, the way these ancient humans left the world some 120,000 to 130,000 years ago could well have been marked by desperation, hunger, and savage brutality.

Why? Because these long-gone Neanderthals lived during the last interglacial period – a time when the world was swiftly transitioning from an ice age into a much hotter climate.

"The change of climate from the glacial period to the last interglacial was very abrupt," palaeontologist Emmanuel Desclaux from the French National Centre for Scientific Research (CNRS) told Cosmos.

"We're not [talking] in terms of geological scale, but more a human scale. Maybe within a few generations, the landscape totally changed."

As the world got hotter, the seas rose against the shorelines. Plants transformed. Animals changed. Hunter-gatherer communities, successfully adapted for tens of thousands of years to extreme cold, were facing the unknown.

We know little about how they fared in this trying period, but archaeological layers preserved in about 220 European sites offer some clues, based on rare vestiges of isolated human remains, including teeth, fragments of skulls, and other bones.

But in most of these places, the remains don't look like they do in the cave Baume Moula-Guercy (BMG) in Southeastern France.

This site features a perhaps unparalleled level of bone and charcoal preservation. This has allowed archaeologists to reconstruct the natural environment and landscapes the Neanderthals experienced during the Eemian period, the researchers note in their paper.

But it also reveals something else: evidence of Neanderthal cannibalism that the authors propose came about in response to the "profound upheaval" brought about by climate change.

In the BMG cave, 120 bones from the six individuals were found. They were mixed in with animal bones, and analysis reveals something severe had happened to these ancient human remains.

"Cut marks are spread over 50 percent of the human remains and distributed over the entire skeleton from the cranium and mandible to the metapodials and phalanges," the team writes.

"Percussion marks are visible on all the skulls, all the long bones and other bones of adults and children."

Based on the evidence, which also reveals potential signs of crushing and chewing, the researchers suggest the layout and condition of the bones are consistent with a single episode of cannibalism. It was likely forced upon the eaters, who may have been starved of their usual animal prey in the shifting climate.

"Skeletal elements from all regions of the body suggests that they were intact prior to the human butchery event," the paper states.

"Further, none of the remains is in anatomical relationship to one another, indicating the [bodies] were completely dismembered."

Of course, the cannibalism proposal remains hypothetical, but based on the evidence the researchers have analysed, it aligns with other things we know about Neanderthals. They usually buried their dead, and were likely forced to abandon parts of Europe during the period, due to instances of famine brought upon by a changing world, and a lack of protein-rich prey.

For those who stayed behind, conditions were tough. Some may have killed one another to survive. Others became food.

But as horrible as this scenario is, the researchers are eager to emphasise it doesn't make these ancient Neanderthals monsters. Instead, they were people who had no other option, if they wanted to keep on living.

"The cannibalism highlighted at Baume Moula-Guercy is not a mark of bestiality or sub-humanity," the researchers explain.

"The synthesis of the data makes it possible to interpret this occurrence as a short and single episode of survival endo-cannibalism in response to nutritional stress induced by rapid and radical environmental changes."

The findings are reported in Journal of Archaeological Science.