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Now we turn to a mystery found in a French cave. It appears that a group of Neanderthals walked into this cave 176,000 years ago and started building something. Neanderthals were our closest relatives, but they were not exactly known as builders or cave explorers. And scientists can't really figure out what they were up to. Here's NPR's Christopher Joyce.
CHRISTOPHER JOYCE, BYLINE: A French archaeologist first ventured deep into this Bruniquel Cave in southwestern France in 1990. He discovered something strange. Someone had broken stalagmites from the floor and arranged them in two large circles. But the archaeologist died before he could fully explore the site. Twenty-three years later, in 2013, a crew of scientists managed to get back there. Geologist Dominique Genty with France's National Center for Scientific Research was there.
DOMINIQUE GENTY: It's very strange. It's kind of obvious that it is not natural.
JOYCE: They found the circles of stone spikes, almost like 2-foot-high fences. More stone fragments lay in piles nearby. It was like a huge Lego set, a Neanderthal Lego set because all of this calcite stone dated back 176,000 years, long before modern humans arrived in Europe. Writing in the journal Nature, the team says it's the most complex Neanderthal structure ever found.
Why did they build it, though? There are only clues. For example, the stalagmite pieces all showed signs of being burned. Was it a ritual or perhaps, maybe, a sort of fireplace to warm them or maybe repel cave bears? No one knows. And then there's the location, over 1,000 feet from the cave entrance. Archaeologist Marie Soressi with Leiden University in the Netherlands is a Neanderthal expert. And she says that's astonishing.
MARIE SORESSI: What is most surprising for me is that this discovery is showing that Neanderthals ventured underground and far away from any source of natural light.
JOYCE: The scientists found pieces of burned animal bone at the site, which they think could have been used as torches because the fatty interior of bone burns. Soressi points out that recent discoveries keep stoking the argument that Neanderthals were, in fact, not dumb throwbacks compared to modern humans.
SORESSI: I think we have, by now, many different lines of evidence to show that Neanderthals and even Neanderthals 200,000 years ago had cognitive capabilities not so different from our direct ancestors.
JOYCE: But still quite capable of doing things that puzzle us even now. Christopher Joyce, NPR News.
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