Since its discovery, the KNM-WT KNM-WT 1700017000 cranium of Australopithecus aethiopicus has been an enigma. Despite its overall appearance, which clearly groups it with the robust australopiths, the magnitude of its prognathismPrognathism and the prognathism-related anatomical features of its calvaria differentiate this specimen from other robust hominins, which are much more orthognathic. Much of the conundrum regarding the positioning of KNM-WT 17000 on the robust clade stems from the traditional view of prognathismPrognathism as a primitive character. However, this assertion is not well founded, given that extreme prognathism is a derived feature in many primates (e.g., baboons, mandrills, geladas, and Celebes crested macaques), as can be deduced from comparisons to outgroups). The acknowledgment of the extreme prognathism in these primates as a derived feature gains support from comparisons between males and females and between adults and subadults of the same species. As is usually the rule among primates (Schultz in Zeitschrift für Morphologie und Anthropologie 52(3):239–255, 1962), females and subadults tend to be more generalized (i.e., less prognathic) than males and adults. Similarly, a comparison between outgroups (such as Cercopithecus aethiops) and the prognathic primate species mentioned earlier supports the interpretation of prognathism as a derived feature. We suggest that the prognathism in KNM-WT KNM-WT 1700017000 be treated the same way in which extreme prognathismPrognathism in other primates is treated, that is, as an autapomorphyAutapomorphy. Our determination that A. aethiopicus is a sister taxon to A. boisei is based on the fact that they share aVisor visor-like zygomatic bone and a unique, heart-shapedHeart-shaped foramen magnumForamen magnum, unlike the round foramen magnum typical of all other primates.