Human and chimpanzee jaws. Historical artwork comparing the jaws of a modern human (Homo sapiens sapiens, left) and a chimpanzee (Pan troglodytes, right). These are cross-sectional views from the side. From left to right, the lips, teeth, and tongue are shown. The main difference is the protruding human chin, absent in chimpanzees, and the attachment of the tongue muscles. Artwork from 1931, when comparisons such as this were used to decide whether the fossils being discovered at this time were of humans, apes, or an evolutionary link. Artwork from New Discoveries relating to The Antiquity of Man (Sir Arthur Keith, 1931).