A researcher takes measurements in the Bruniquel Cave. (Etienne Fabre/SSAC)

This deep inside the cave, sunlight was just a memory. Mineral-laden water dripped from the ceiling, accumulating on spiny stalagmites below. Every footstep echoed and long shadows cast by the fires danced on the damp, rugged walls.

Hundreds of centuries ago, someone trudged into that foreboding darkness. Methodically, they broke apart hundreds of stalagmites and arranged the pieces in cryptic piles on the cave floor: two stone rings, one vast and one small, and several piles containing charred rock. They may have been holding a religious ceremony. Or perhaps they simply needed a place to cook away from the cave bears and the cold. The purpose of these structures remains an enigma.

But their creators are not. In a study in the journal Nature, scientists report that these strange stalagmite piles found in the south of France were formed roughly 176,000 years ago by Neanderthals.

That analysis makes the cave structures the oldest hominid constructions ever found, and marks Bruniquel Cave as the first known to be used by early humans. It is among the most sophisticated relics we have from our Neanderthal cousins — and provides the latest addition to a growing pile of evidence that these early humans were much smarter than we once believed.

"This is proof of evolution" beyond what some scientists have imagined, said Dominique Genty, a paleoclimatologist at the Laboratory of Climate Sciences and the Environment, in France.

The structures were first discovered in the early 1990s by a teenage boy with a sense of curiosity. According to the Atlantic magazine, 15-year-old Bruno Kowalsczewski spent three years hauling away rocks and rubble from the hillside until he uncovered a passageway so narrow that only the thinnest members of the local spelunking club could squeeze through. Gingerly, they made their way through the rough-hewn tunnel, past puddles and piles of animal bones and spindly mineral formations, until they arrived at a wide chamber 300 meters deep and found the extraordinary constructions inside.

Suspecting they'd stumbled upon something more than a simple bear den, the cavers called in an expert: the late archaeologist François Rouzaud. Using radiocarbon dating, which measures the age of material based on how many of the carbon atoms in them have undergone radioactive decay, he estimated that a bit of burned bear bone found on one of the structures was about 47,600 years old — making it almost as old as modern humans' presence in Europe.

But Rouzaud died before the survey could be completed, and his research was never reported in a widely read journal. The findings that did get published — a drawing of the structures alongside Rouzaud's estimates — were not enough to convince other scientists to take a closer look. The cavers who owned Bruniquel sealed it off to protect its contents, and the strange formations were mostly forgotten.


A 3D reconstruction of the structures in the Bruniquel Cave. (Xavier Muth - Get in Situ, Archéotransfert, Archéovision -SHS-3D, base photographique Pascal Mora)

More than a decade later, Genty got a call. A Belgian paleoclimatologist named Sophie Verheyden was putting together a team to reinvestigate Bruniquel. She knew that the dates uncovered by Rouzaud were somewhat suspect — radiocarbon dating is imprecise at the best of times and stops being reliable entirely at about 50,000 years. But the relatively new technology of uranium series dating, which can be applied to non-organic matter, would allow her to pinpoint when the stalagmites themselves were broken.

Genty swiftly agreed to join; he still remembered the intriguing illustration published with Rouzaud's study years ago. But nothing could have prepared him for entering the cave itself.

"It's wonderful," he said. "When you see it, the first thing you think is: 'Okay, this is not natural. This was made by people.'"

Their findings indicate that the stalagmites were broken sometime in the range of 174,400 to 178,600 years ago, long before modern humans had reached Europe. Since the researchers didn't find evidence of cave bears in the spot — which, for what it's worth, still wouldn't explain who'd built the fires — the structures could have been created only by Neanderthals.

That comes as a surprise, because scientists have long believed that such sophisticated structures were beyond the capacity of humans' now-extinct cousins. The effort required to move some 4,400 pounds of material and arrange it in such a specific pattern is enormous. Several people must have been involved, which means that they were capable of working together to execute a plan. And they did it all in the dark.

"Imagine going deep inside the cave, far from the light," Genty said. "It's a very different world. You have fear of the unknown."

That the structures' creators did so anyway shows they were driven to pursue something often considered uniquely human: "They explored."

Paola Villa, an archaeologist at the University of Colorado at Boulder who wasn't involved in the study, told the Associated Press that the site "provides strong evidence of the great antiquity of those elaborate structures and is an important contribution to a new understanding of the greater level of social complexities of Neanderthal societies."

The Bruniquel find comes on the heels of several other studies attempting to overhaul Neanderthals' primitive reputation. The past few decades have seen scientists uncover evidence that these ancient cousins used tools, organized hunts, created art, participated in rituals and were capable of complex communication. Villa is one of the biggest proponents of this view.

But other researchers have their doubts. Harold Dibble, an archaeologist at the University of Pennsylvania, told Nature that he'd like to see more conclusive evidence that the structures were built by humans. And even if the Bruniquel team can provide it, he isn't convinced that it challenges what Neanderthals were capable of.

“To me, constructing some sort of structure — things a lot of animals do, including chimps — and equating that with modern cultural behavior is quite a leap,” Dibble said.

But Genty says that's not quite what he's arguing. Building something as complex as the Bruniquel circles was unusual for any hominid at the time. Outside the cave, the next oldest unambiguously man-made structures are just 20,000 years old, according to the Atlantic. And nothing from the prehistoric past has been found in a spot so deep and dark.

"It shows that the progress they made, was more or less the same as our ancestors who were in Africa at the time," he said. (Anatomically modern humans arose in East Africa roughly 200,000 years ago.) Neanderthals may be primitive by today's standards, or even by the standards of the modern humans who first encountered them in Europe about 50,000 years ago.

But by the standards of their time, Genty said, the Neanderthals could have been innovators.

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