BERLIN (AP) — Two mysterious stone rings found deep inside a French cave were probably built by Neanderthals about 176,500 years ago, proving that the ancient cousins of humans were capable of more complex behavior than previously thought, scientists say. 
 
 The structures were made from hundreds of pillar-shaped mineral deposits, called stalagmites, which were chopped to a similar length and laid out in two oval patterns up to 40 centimeters (16 inches) inches high. They were discovered by chance in 1990, after remaining untouched for tens of thousands of years because a rockslide had closed the mouth of the cave at Bruniquel in southwest France. 
 
 While previous research had suggested the structures pre-dated the arrival of modern humans in Europe around 45,000 years ago, the notion that Neanderthals could have made them didn't fit long-held assumptions that these early humans were incapable of the kind of complex behavior necessary to work underground. 
 
 Using sophisticated dating techniques, a team led by archaeologist Jacques Jaubert of the University of Bordeaux, France, found that the stalagmites must have been broken off the ground around 176,500 years ago "making these edifices among the oldest known well-dated constructions made by humans." 
 
 "Their presence at 336 meters (368 yards) from the entrance of the cave indicates that humans from this period had already mastered the underground environment, which can be considered a major step in human modernity," the researchers concluded in a study published online Wednesday by the journal Nature. 
 
 Jaubert ruled out that the carefully constructed rings, which show traces of fire, could have come about by chance or been assembled by animals such as the bears and wolves whose bones were found near the entrance of the cave. 
 
 "The origin of the structures is undeniably human. It really cannot be otherwise," he told The Associated Press. 
 
 The Neanderthals who built them must have had a "project" to go so deep into a cave where there was no natural light, said Jaubert. They probably explored underground as a group and cooperated to build the rings, using fire to illuminate the cave, he said. "These are exceptional tours, certainly for extraordinary reasons we do not yet know." 
 
 Paola Villa, an archaeologist at the University of Colorado at Boulder who wasn't involved in the study, said 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 authors said the purpose of the oval structures — measuring 16 square meters (172 sq. feet) and 2.3 square meters (25 sq. feet) — is still a matter of speculation, though they may have served some symbolic or ritual purpose. 
 
 "A plausible explanation is that this was a common meeting place for some type of ritual social behavior," Villa suggested. 
 
 Wil Roebroeks, a Neanderthal expert at the University of Leiden, Netherlands, noted that the structures in Bruniquel may represent only the tip of the iceberg of Neanderthal culture, but most relics would have been made of organic material and decayed long ago. 
 
 "Bruniquel cave (shows) that circular structures were a part of Neanderthals' material culture," said Roebroeks, who called the rings "an intriguing find, which underlines that a lot of Neanderthal material culture, including their 'architecture,' simply did not survive in the open." 
 
 Roebroeks, who also wasn't involved in the study, said the fact that similar rings haven't been found anywhere else makes it hard to test any theory about how they came to be. 
 
 "One could even envisage that groups of Neanderthal 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 (the journal) Nature," he said. 
 
 ___ 
 
 Follow Frank Jordans on Twitter at http://www.twitter.com/wirereporter ||||| These weren’t natural formations, and they weren’t the work of bears. They were built by people. 
 
 Recognizing the site’s value, the caver brought in archaeologist Francois Rouzaud. Using carbon-dating, Rouzaud estimated that a burnt bear bone found within the chamber was 47,600 years old, which meant that the stalagmite rings were older than any known cave painting. It also meant that they couldn’t have been the work of Homo sapiens. Their builders must have been the only early humans in the south of France at the time: Neanderthals. 
 
 The discovery suggested that Neanderthals were more sophisticated than anyone had given them credit for. They wielded fire, ventured deep underground, and shaped the subterranean rock into complex constructions. Perhaps they even carried out rituals; after all, there was no evidence that anyone actually lived in the cave, so what else were the rings and mounds for? 
 
 Rouzaud would never know. In April 1999, while guiding colleagues through a different cave, he suffered a fatal heart attack. With his death, work on the Bruniquel Cave ceased, and its incredible contents were neglected. They’ve only now re-entered the limelight because Sophie Verheyden went on holiday. 
 
 A life-long caver, Verheyden works at the Royal Belgian Institute of Natural Sciences, where she specializes in stalagmites. She treats them as time capsules, using the chemicals within them to reconstruct the climate of past millennia. So when she learned about Bruniquel Cave, while visiting the region on holiday and seeing a display at a nearby castle, she had only one thought: Why hadn’t anyone dated the broken stalagmites themselves?” 
 
 She knew that Rouzaud’s date of 47,600 years was impressive but suspect. Carbon-dating is only accurate for samples younger than 50,000 years, so the Bruniquel material was hitting the technique’s limits. They could well have been much older. To get a better estimate, Verheyden assembled a team including archaeologist Jacques Jaubert and fellow stalagmite expert Dominique Genty. In 2013, they got permission to study the site and crawled into it themselves. “I’m not very big, and I had to put one arm before me and one behind to get through,” says Verheyden. “It’s kind of magical, even without the structures.” 
 
 After drilling into the stalagmites and pulling out cylinders of rock, the team could see an obvious transition between two layers. On one side were old minerals that were part of the original stalagmites; on the other were newer layers that had been laid down after the fragments were broken off by the cave’s former users. By measuring uranium levels on either side of the divide, the team could accurately tell when each stalagmite had been snapped off for construction. 
 
 Their date? 176,500 years ago, give or take a few millennia. 
 
 “When I announced the age to Jacques, he asked me to repeat it because it was so incredible,” says Verheyden. Outside Bruniquel Cave, the earliest, unambiguous human constructions are just 20,000 years old. Most of these are ruins—collapsed collections of mammoth bones and deer antlers. By comparison, the Bruniquel stalagmite rings are well-preserved and far more ancient.
– For tens of thousands of years, the secrets of France's Bruniquel Cave went unknown, its mouth closed off by a rock slide. That began to change in the late '80s, the Atlantic reports, when a boy named Bruno Kowalsczewski started clearing away what obstructed the opening; three years later, in 1990, the then-15-year-old's 100-foot path allowed local cavers entry. One caver made it more than 1,000 feet in and found something wild: two stone rings up to 16 inches high intentionally made from pieces of chopped-up stalagmites (pillar-shaped mineral deposits that stick up from the ground), probably by Neanderthals some 176,500 years ago, scientists now say. A press release notes "until now the oldest formally proven cave use dated back only 38,000 years." While previous research had suggested the structures predated the arrival of modern humans in Europe around 45,000 years ago, the AP reports the notion that Neanderthals could have made them didn't fit long-held assumptions that these early humans were incapable of the kind of complex behavior necessary to work underground. "Their presence at [1,102 feet] from the entrance of the cave indicates that humans from this period had already mastered the underground environment, which can be considered a major step in human modernity," the researchers concluded in a study published online Wednesday by the journal Nature. The authors said the purpose of the oval structures—measuring 172 square feet and 25 square feet—is still a matter of speculation, though they may have served some symbolic or ritual purpose.