Twenty years ago, the remains of six Neanderthals were discovered in a small cave in southern France. Cutmarks and bite marks on these bones suggest that cannibalism was at play – but why?

A new study, published in the Journal of Archaeological Science, has identified a potential reason behind this behavior: climate change.

It turns out that the Neanderthals found in the French cave – two adults, two teens, and two children – lived during the last interglacial period about 120,000 years ago, a time when the planet was heating up. Coping with the change from an icy climate to a much warmer one likely put pressure on the Neanderthals, making it harder for them to obtain food.

The cave at Baume Moula-Guercy in France’s Rhône Valley contains a 40-centimeter-thick (16-inch) layer of rock that arose during the last interglacial period. Trapped in this layer lie various animal remains, which provided the researchers with clues to how the changing climate impacted animal life and, therefore, the food that the Neanderthals relied upon.

“The change of climate from the glacial period to the last interglacial was very abrupt,” study author Emmanuel Desclaux told Cosmos Magazine.

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

The team found that before and after the period of warming, animals such as reindeer and mammoths roamed the Rhône Valley. However, as the climate heated up, these large meaty creatures disappeared, leaving rodents, snakes, and tortoises in their stead. Not a particularly nutritious meal if you’re a hungry Neanderthal.    

It seems this sudden switch in prey drove the Neanderthals to eat each other as a last resort. Marks on teeth found in the cave also suggest that these ancient hominins were going through a period of stress, possibly suffering from disease or malnutrition.

“They weren’t doing anything different to what modern humans would do in the same situation,” Michelle Langley, an archaeologist not involved in the study, said in a statement. The authors note that Neanderthals probably only ate each other in times of desperation as their meat wouldn’t have been as nutritious as that of calorific animals like deer or bison.

Evidence of Neanderthal cannibalism has previously been identified in Spain, Belgium, and Croatia. While the new study provides convincing evidence that this behavior may have appeared due to a climate-induced lack of food, cannibalism may have occurred for cultural reasons too.

“There have been cases of cultural cannibalism, but in this particular case that does not seem to be the case,” said Desclaux.