doned and the new one to which it was directed (which raises the question as to why natural selection would keep pushing the intermediate forms toward the new, more specialized adaptive type). This is what happened in Simpson’s favorite evolutionary example, that of horses passing from browsers to grazers, as well as in the postural changes in our ancestors: “The new feature, for which the specialization was adaptive, was the ability to graze, to eat harshly abrasive food. Nevertheless the ability to eat less abrasive food, to browse, was not thereby lost. The development of upright posture in man and utilization of the hands for manipulation, only, and not locomotion, perhaps provide a better example of specialization that broadened rather than restricted the general adaptive type” (Simpson, 1950).
Nevertheless, other attendees of the symposium did use the expression quantum evolution, and in a manner very close to its original meaning, to explain the origin of human bipedal posture. W. W. Howells (1950) argued: “It is true that bipedal walking was the more radical line of change. As Washburn says, this was undoubtedly a case of quantum evolution, a conceptual contribution of Simpson (1944).” According to S. L. Washburn (1950) “The derivation of this type from an ape is best regarded as a case of rapid or quantum evolution (Simpson, 1944).” For Washburn, the key was in the iliac blade and the gluteus maximus muscle: “The argument runs as follows: among apes who were living at the edge of the forests and coming to the ground, were some who had shorter ilia. These ilia had to be more bent back for obstetrical reasons and in some this carried gluteus maximus far enough so that it became effective in finishing extension. This started a new selection which favored bigger gluteus muscles and ilia still further bent.”
In the australopithecines, the pelvis had already undergone the necessary changes for obligate bipedalism, but other parts of the skeleton reflected this new posture. The unavoidable question is whether the passing from a nonbipedal adaptive type (that of the common ancestor of humans and chimpanzees) to an obligate bipedal one (like the australopithecines) occurred directly and only once or whether a transitional form had previously existed with a generally primitive skeleton but with some particular key feature (perhaps in the iliac blade as suggested by Washburn) that made an early form of facultative bipedal locomotion, still compatible with some degree of life in the trees, possible.
Currently, there are three known genera that predate Australopithecus and that, according to their respective discoverers, are our ancestors and were facultative bipeds: Sahelanthropus (Brunet et al., 2002; Zollikofer et al., 2005), Orrorin (Senut et al., 2001; Pickford et al., 2002), and Ardipithecus (Lovejoy et al., 2009; White et al., 2009). To date, only a single preaustralopithecine pelvis has been recovered, belonging to the Ardipithecus ramidus