Ancient piles of broken stalagmites found deep inside a cave in France were made by Neanderthals about 176,000 years ago, way before modern humans were around. This is the first time archaeologists have concluded that our cousins ventured underground and built complex constructions. The structures are proof that Neanderthals were pretty smart and organized, according to the study authors. The meaning of these constructions remains a mystery, but for at least one archaeologist they suggest that Neanderthals may have been religious.

"The date of 176,000 years ago is just mind-blowing."

Since Neanderthals were discovered in the 1800s, archaeologists have debated about just how "evolved" they were. Archaeologists assume that Neanderthals built dwellings, but no well-preserved ruins of such dwellings have ever been found. The only remnants of Neanderthal constructions are disputed and date no later than 50,000 years ago. Today’s study, published in Nature, is the first one to analyze well-preserved structures built by early Neanderthals. The structures are so complex that they resemble those made by modern humans, adding to the theory that Neanderthals were quite intelligent and had some modern human behaviors.

"The date of 176,000 years ago is just mind-blowing," says Brian Hayden, a Neanderthal expert at Simon Fraser University, who did not take part in the study. "There’s no question that they were Neanderthals and that they were using the site."

Neanderthals are among our closest extinct human relatives. They lived between about 400,000 to 40,000 years ago in Europe and parts of Asia, where they were eventually replaced by Homo sapiens. For many years, archaeologists have thought of Neanderthals as pretty backward, but recent discoveries are changing the tide. We now know that Neanderthals used fire, made sophisticated tools, wore clothing, and were skilled hunters. But very few tools and artifacts made by Neanderthals have been found, others aren’t well preserved. That makes it hard for archaeologists to know much about these predecessors of modern humans. Today’s finding suggests a high level of sophistication, Hayden says. "If you’re going that far in[to a cave]," he says, "you need to establish a whole series of things in order to make that possible: you have to have some sort of lighting apparatus, it requires forethought, it requires planning, it requires of sorts of other things, organization so that you can actually do that."

In today’s study, the researchers describe several structures that were found inside Bruniquel cave in southwestern France, about 330 meters from the cave’s entrance. The site was discovered in 1990, but it wasn’t thoroughly analyzed until 2013. The ancient structures are made of more than 400 pieces of stalagmites, similarly sized, piled up, and arranged in two circles. The researchers also found signs of fire on the structures, as well as burned bone fragments. By analyzing the stalagmites as well as the calcite that grew on top of them, the researchers were able to date the site to about 176,500 years ago. At that time, only Neanderthals lived in Europe.

Building such big structures inside a pitch-dark cave involves planning; it means that the Neanderthals used some sort of torch for lighting and that they were organized enough to coordinate a group of people to break the stalagmites and erect the structures. That suggests "that the Neanderthal group responsible for these constructions had a level of social organization that was more complex than previously thought for this hominid species," the study says.

We already knew that Neanderthals made glue as far back as 200,000 years ago

For some, that statement smacks of hype. We already knew that Neanderthals had pretty complex behaviors, says Marie Soressi, faculty of archaeology at Leiden University. Archaeologists found that Neanderthals made glue, for example, as far back as 200,000 years ago. They used a technique called distillation, which requires a pretty good control of fire temperature and underground ovens without oxygen, Soressi says. "That’s a very complex process for me and maybe even more complex than breaking stalagmites and piling them up," she says. "I’m not sure I agree with this statement that they make in the paper. I understand their point, but I also think that we have evidence for Neanderthals having very complex behavior at about the same time."

Dominique Genty, one of the study authors, agrees with Soressi, but says that the study is the first one to prove that early Neanderthals built elaborate constructions in a cave well before Homo sapiens did. "It’s a different kind of proof of social organization," Genty says.

What’s more interesting about the structures, which are some of the earliest ever made, is the mystery surrounding their purpose, Soressi says. The constructions were found inside a dark cave, so they couldn’t be the foundation of a hut, says Paola Villa, a researcher at the University of Colorado Boulder. The researchers also couldn’t find signs, like tools and human bones, that people lived there. For some, like Hayden, that’s pretty clear evidence that the structures have some sort of symbolic or ritual function. "You can’t just argue for anything else," he says. "In one blow, it establishes Neanderthals as having some sense of religion. ... Why would anyone go 300 meters into a deep, dark cave and set up a string of stalagmites in total darkness? Why would you do that? If not for religious or some sort of ecstatic experience."

"In one blow, it establishes Neanderthals as having some sense of religion."

Others aren’t so quick to draw conclusions. Wil Roebroeks, a professor of Paleolithic archaeology at Leiden University and a leading researcher on Neanderthals, says we can’t know for sure what the function of the structure is. "The fact that it was preserved deep in a cave might signal ‘ritual’ to some, but as for Upper Paleolithic art, deep cave settings are prone to preservation of what is destroyed by weathering in the open, and we should not let preservation steer our interpretation," he wrote in an email. "One could even envisage that groups of Neandertal teenagers explored this underground environment deep in the cave (as teenagers tend to do), building fires, breaking off stalagmites and gradually turning them into the structures that 175,000 years later made it into Nature. But that is pure speculation, again, nobody knows…"

The study authors seem to agree. Though the study suggests that the structures could represent some kind of symbolic or ritual behaviors, it makes clear that’s impossible to say for now. "We don’t know what was its use, we really don’t know. No idea." Genty says. "It’s still a mystery."

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