176,500 years ago, long before modern humans left Africa for the Eurasian continent, a band of Neanderthals conducted an elaborate ritual deep inside Bruniquel Cave in a region we know today as southern France. The Neanderthal group wrested hundreds of stalagmites from the floor of the cave to build elaborate circular structures, their work illuminated only by firelight. Discovered by archaeologists in the 1990s, the cave system is so large that many of its great treasures are hidden far from its entrance, which suggests it was thoroughly explored and probably inhabited for some period of time. This new part of the cave, analyzed only recently, adds to our understanding of Neanderthal social life.

The Neanderthal structure was mostly undisturbed for tens of thousands of years with the exception of a few hibernating bears. Recounting their discovery in Nature, a group of archaeologists say there is no question that the structures were created deliberately by humans, especially because there is evidence that the stalagmites were wrenched from the cave floor and stacked in circular patterns. Burn marks on the roughly 400 stones show that fires were built inside the structure, and one area contains burned bones. The bones could mean that this was a feasting place, but its difficult-to-reach location and the nature of the design suggest a more symbolic use. Based on the burn patterns, it seems that the structures themselves were designed to light on fire, creating what would have appeared to be circles of flaming stone in the otherwise pitch-black cave.

There is little evidence of human activity in the space other than the unusual structures, which don't resemble any of the art or funeral rites associated with more recent Neanderthal dwellings we've discovered. Most Neanderthal sites are from the past 50 thousand years, and these contain paintings, ochre for body decoration, and graves full of flowers . Some contain complex tools made after contact with modern humans from Africa, and it's often difficult to say whether they were made by Neanderthals, Homo sapiens, or some combination of the two. After all, there was a roughly 10,000 year period when Neanderthals and Homo sapiens co-existed in Europe, and we know they formed families and had children together.

What makes this site so extraordinary isn't just its great age, but the certainty that it was made by Neanderthals whose culture had not yet been changed by modern humans. This structure represents pure Neanderthal expression. It supports the theory that human culture and symbolism emerged in different populations all over the world throughout the past 200 thousand years. Though modern humans from Africa came to dominate the world after they crossed into Eurasia about 80 thousand years ago, evidence from Bruniquel Cave suggests that people outside Africa were developing their own symbolic systems, too.

As for what those long-ago Neanderthals were doing with their circles of flaming stone structures, there are no easy answers. "We could assume that they represent some kind of symbolic or ritual behavior, but could they rather have served for an unknown domestic use or simply as a refuge? Future research will try to answer these questions," write the researchers. What we know for certain is that this discovery is "unprecedented." It shows that early humans were able to live—or at least gather—deep inside a cave by providing their own lighting. Plus the structure itself is so elaborate that the archaeologists conclude, "the Neanderthal group responsible for these constructions had a level of social organization that was more complex than previously thought for this hominid species."

Nature, 2016. DOI: 10.1038/nature18291