Primate skulls. X-ray of the skulls of a chimpanzee, Pan troglodytes, and human, Homo sapiens seen from the side. The chimpanzee's cranium (brain-case, upper left) is considerably smaller than that of the human. Both consist of many spongy bones fused at sutures. Chimpanzees have prominent brow ridges (upper left) above their eyes. The large mandible bone (lower jawbone, lower left) has powerful chewing muscles attached to it. Chimpanzees feed largely on fruit by grinding it down with their flat molar teeth. The human jaw (lower right) also houses numerous teeth. Both humans and chimpanzees evolved from a common primate ancestor.