Two mysterious stone rings found deep in 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 stalagmites chopped to a similar length and laid out in two oval patterns up to 16 inches high. The notion that Neanderthals could have made them defies long-held assumptions that these hominids were incapable of the complex behavior necessary to work underground.