A Mediterranean seaside cave in Gibraltar holds what researchers believe is some of Europe's oldest art. The X-shaped markings were etched into the rock walls at least 39,000 years ago, according to a new study published in Proceedings of the National Academy of Sciences. The source of the artwork is subject to some debate, however. The study authors report that Neanderthals should receive the credit, while others researchers think that Homo sapiens are the more likely creators.

Researchers have been investigating the Neanderthal cave, called Gorham's Cave, since the 1980s, Nature News reports. There, they've uncovered insights into Neanderthal life. (The Neanderthals who lived there dined on fish, shellfish and birds, for instance.) But it wasn't until 2012, when researchers discovered a tiny hidden passage, that the art came to light, Nature continues. The "frisbee-sized" drawings are etched onto a flat, table-like rock surface and are several millimeters deep.

Sediment that covered the art work dates back to the time of the Neanderthals, Nature writes, so the researchers concluded that the engravings must also date back to that time or earlier—perhaps even up to 45,000 years ago. The scientists have no idea what the shapes mean; they could be abstract art, or they could be some sort of map. What they are confident of, however, is that the markings were deliberately created, Nature continues.

Paul Tacon, an expert in rock art at Griffith University, who was not involved in the study, praised the significance of the finding. "We will never know the meaning the design held for the maker or the Neanderthals who inhabited the cave but the fact that they were marking their territory in this way before modern humans arrived in the region has huge implications for debates about what it is to be human and the origin of art," he told the Associated Press.

But other outside researchers, however, question whether or not Neanderthals should actually receive credit for the etchings. The sediment could have moved around, for example, making it seem like that art is older than it actually is. And either way, the art itself is not that impressive, Harold Dibble, an anthropologist at the University of Pennsylvania, told Nature. "It takes more than a few scratches - deliberate or not - to identify symbolic behavior on the part of Neanderthals," he said.