Ape years ago (Mya) the clades diverged into newer clades. The lesser apes are the gibbon family, Hylobatidae, of sixteen species; all are native to Asia. Their major differentiating characteristic is their long arms, which they use to brachiate through trees. Their wrists are ball and socket joints as an evolutionary adaptation to their arboreal lifestyle. Generally smaller than the African apes, the largest gibbon, the siamang, weighs up to ; in comparison, the smallest "great ape", the bonobo, is . Formerly, all the great apes except humans were classified as the family Pongidae, which conveniently provided for separating the human
Killer ape theory often associated with the theory, because of similarities and because Robert Ardrey has developed both. The expression "killer ape" does not mean an outstanding aggressive kind of ape; in fact the term refers to anthropological analysis of human aggression. Accordingly, the killer ape is a notably belligerent species on which our instincts might be rooted, because this very ancestor could establish itself due to its special aggression. As founder of this thesis, Raymond A. Dart (1893–1988) dealt with this issue in his professional article "The predatory transition from ape to man", 1953. Dart refers to the Australian anatomist Grafton Elliot