Our ancient ancestors had a prominent ridge of bone above their eyes, but scientists are not sure of its purpose—or why it disappeared. Some have suggested the bone provided filler between the brain and eyes or reduced strain on the face during eating. But by changing the size of the brow ridge in a digital model of a Homo heidelbergensis skull, researchers have found that neither of the theories holds up, Popular Science reports. Instead, the disappearance of the brow ridge could have given humans an evolutionary advantage by allowing us to communicate subtly using our eyebrows, the authors speculate in Nature Ecology & Evolution. However, other scientists doubt the conclusions, pointing out that chimpanzees can convey subtle emotional information even without flat foreheads.