Primate skeletons. Historical artwork of the skeletons of five primates. From right to left are the skeletons of a gibbon (family Hylobatidae), an orang-utan (Pongo pygmaeus), a chimpanzee (Pan troglodytes), a gorilla (Gorilla gorilla) and a human (Homo sapiens). The artwork is taken from Der Kampf um den Entwickelungsgedanken, by the German biologist Ernst Haeckel (1834-1919). Haeckel wrongly believed that the gibbon, like the other primates shown here, was a close relative of humans, because it is the only ape which always moves upright on two legs.