The evolution of human bipedality: ecology and functional morphology

Abstract

Contexts that elicit bipedalism in extant apes may provide evidence of the selective pressures that led to hominid bipedalism. Bipedalism was observed most commonly among chimpanzees when they fed on the small fruits of diminutive, open-forest trees. Chimpanzees fed bipedally from such trees either by reaching up to pick fruit while standing on the ground, or from within the tree, in which case bipedalism was frequently stabilized by grasping an overhead branch. The food-gathering function of chimpanzee bipedalism suggests that hominid bipedalism may have evolved in conjunction with arm-hanging as a specialized feeding adaptation that allowed for efficient harvesting of fruits among open-forest or woodland trees. Such evidence is particularly valuable when it is in accord with fossil anatomy. Australopithecus afarensis has features of the hand, shoulder and torso that have been related to arm-hanging in chimpanzees. The australopithecine hip and hind limb clearly indicate bipedalism, but also indicate a less than optimal adaptation to bipedal locomotion compared to modern humans. Locomotor inefficiency supports the hypothesis that bipedalism evolved more as a terrestrial feeding posture than as a walking adaptation. A bipedal postural feeding adaptation may have been a preadaptation for the fully realized locomotor bipedalism apparent in Homo erectus.