One of the biggest debates in archaeology is whether Neandertals were capable of the kind of abstract and symbolic expression that prehistoric modern humans demonstrated in abundance—for example, by painting animal images on the walls of caves like Chauvet and Lascaux in France and Altamira in Spain. Possible evidence for Neandertal art was reported a couple of years ago in the Spanish cave of El Castillo, but researchers are not sure whether Neandertals or modern humans painted a red disk on its wall 41,000 years ago—right around the time that modern humans entered Europe. Now, archaeologists working at Gorham’s Cave, a former Neandertal haunt on the coast of Gibraltar, report online today in the Proceedings of the National Academy of Sciences that they have found this crosshatched pattern etched into the hard rock floor of the cave (see photo above). The pattern was deeply incised using some sort of stone tool and was found under archaeological layers dating back at least 39,000 years—but containing stone tools that only Neandertals made. The image is somewhat reminiscent of a 75,000-year-old geometric pattern found at Blombos Cave in Africa, and indeed the Gorham’s team argues that it is proof positive that Neandertals were just as capable of abstract thought as modern humans. The claim is likely to attract some skepticism, however, from archaeologists who have argued that such simple patterns are poor evidence for complex symbolic expression.