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CLIMATIC FORCING AND THE ORIGIN OF THE HUMAN GENUS 238 upright climbing, but not extensive acrobatic activity of the sort engaged in by chimpanzees and orangutans. Once in a tree, they probably walked bipedally on limbs, gripping branches with their forelimbs for support. The Arboreal Imperative It is clear that the traits described in the preceding section evolved as arboreal adaptations in australopithecine ancestors, although unfortunately the fossil record of African primates for the Late Miocene and very early Pliocene is too poor to reveal the identity of these more heavily arboreal predecessors. The upright climbing posture of australopithecine ancestors was retained during the evolutionary transition to activity on the ground; there is no evidence that knuckle walking of the sort employed by modern apes had any place in human ancestry. We would not predict that a transition from a strictly arboreal mode of life to an essentially terrestrial one would occur instantaneously on a geological scale of time. Whether the change took place gradually or in steps, there should have been intermediate stages represented by taxa that divided their time between activities in trees and activities on the ground. The morphology of the australopithecines suggests that they were such taxa. As Figure 14.1 shows, the australopithecines' pattern of development was apelike, so that the ability of infants to cling should have permitted a mother to climb. One might nonetheless hypothesize that the australopithecines had converted to a totally terrestrial life but, through some kind of evolutionary inertia, retained inherited arboreal traits. There are major difficulties with this idea, however. Traits that had evolved as arboreal adaptations persisted for a total of about 3 m.y. in the australopithecine complex of species. The robust australopithecines inherited from the graciles most of the "arboreal" traits that the latter themselves had inherited. In general, the australopithecines remained intermediate between apes and humans in a variety of locomotory features. Their forelimbs possessed many apelike traits. Their hindlimbs, having been occupied extensively with bipedal locomotion on the ground, were more human in form but seemingly remained compromised by residual arboreal activities: It is highly unlikely that evolution would have failed to improve terrestrial locomotion by eliminating these deleterious traits had they not been employed in essential arboreal activities. Short legs were one of the traits that natural selection for improved locomotion on the ground might be expected to have eliminated if stabilizing selection were not maintaining them because of their value in climbing. The short legs of australopithecines relative to body weight must have reduced endurance in bipedal locomotion by increasing the number of strides per unit of distance traversed (Jungers and Stern, 1983). Similarly, the relatively long toes of australopithecines, though useful for climbing, would have reduced speed and endurance in running. Toes of some minimum length, apparently approximated in modern humans, are necessary for gripping the substratum and providing balance. Longer toes, however, lengthen the moment arm about which the body's center of gravity must rotate upward and forward during the so-called toeoff stage of running (Stanley, 1992; Figure 14.5). It is not difficult to understand why australopithecines should have retained compulsory arboreal activities. Their dental morphology has been taken to indicate that fruits and seed pods formed a large part of their diet (Kay, 1985), and climbing would have expanded the range of available food items of this type. Even more important, however, should have been the need to elude large African predators. Figure 14.5 Illustration of the reason that lengthening of human toes would impair running. Arrow shows the force that leg muscles apply to the long tendon of the big toe in elevating the body during the toe-off stage of running. The tendon attaches to the distal bone of the toe. A longer proximal phalanx of the toe (P) would increase the lever arm of the forces opposing elevation of the body.
