In 1990, a 15-year-old boy, Bruno Kowalsczewski, became the first person to enter the Bruniquel Cave in tens of thousands of years. In one chamber, approximately 336 meters into the cave, Kowalsczewski discovered a pile of burnt bones and a number of stalagmites organized into two rings — one between 4 and 7 meters wide and one 2 meters wide. Carbon-dating of a burnt bear bone proved it to be 47,600 years old. This meant that the stalagmite rings were older than any cave painting and that they were constructed by Neanderthals (there was no other hominin in the region at this time). Then, Sophie Verheyden, who works at the Royal Belgian Institute of Natural Sciences, measured the uranium levels of the stalagmites, discovering that they were broken 176,500 years ago. So, what exactly was the purpose of the rings and what does the discovery mean for Neanderthals? The Atlantic explains:

Why did they build the rings and mounds? The structures weren’t foundations for huts; the chamber contains no stone tools, human bones, or any other sign of permanent occupation, and besides, why build shelter inside a cave? “A plausible explanation is that this was a meeting place for some type of ritual social behavior,” says Paola Villa from the University of Colorado Museum. “When you see such a structure so far into the cave, you think of something cultural or religious, but that’s not proven,” adds Verheyden. Indeed, despite some fanciful speculations about cave bear cults, no one really knows.

Nor is it clear how the Neanderthals made the structures. Verheyden says it couldn’t have been one lone artisan, toiling away in the dark. Most likely, there was a team, and a technically skilled one at that. They broke rocks deliberately, and arranged them precisely. They used fire, too. More than 120 fragments have red and black streaks that aren’t found elsewhere in the chamber or the cave beyond. They were the result of deliberately applied heat, at intensities strong enough to occasionally crack the rock. “The Neanderthal group responsible for these constructions had a level of social organization that was more complex than previously thought,” the team writes.

Head over to The Atlantic to read more.

Feature image via Etienne Fabre/SSAC