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CLIMATIC FORCING AND THE ORIGIN OF THE HUMAN GENUS 236 and slow maturation of human infants compared to the offspring of apes. The evolution of highly immature infants in human evolution represented a profound ecological sacrifice. Offspring that must be carried, fed, and protected for years occupy parents' time that could otherwise be spent in important activities such as acquisition of food. Such infants also complicate the avoidance of predators. For natural selection to have produced this deleterious developmental pattern, there had to be some overriding benefit. Encephalization was clearly the change that provided this benefit: not only did it have great adaptive value, but it was as profound a change as the delay in development. In degree of brain expansion immediately after birth, modern humans rank first among mammals (Count, 1947), just as they rank first in the length of their postnatal interval of physical helplessness (Krogman, 1972). No such developmental delay characterized gracile australopithecines. Fossils reveal that these animals closely resembled apes rather than humans in pattern of brain growth. It is most meaningful here to consider the pattern for males, because their mean birth size is larger than that of females, so that its maximum value can be estimated from the size of the female pelvis. Pelvic inlet breadth is the dimension that limits cranial size and therefore body size for neonates. This dimension for the pelvis of the famous "Lucy" skeleton, which represents Australopithecus afarensis, indicates that male birth size for this species approximated that for a chimpanzee or orangutan (Tague and Lovejoy, 1986). Other fossils show that adult male body size averaged at least 45 kg, not far from the mean for chimpanzees (McHenry, 1991), and that adult male brain size (averaging about 480 cm3) was only slightly above the chimpanzee mean (Figure 14.1). Apes do not give birth to neonates as large as their pelvic dimensions would allow. Thus, they do not develop brains as large as they might, even without postnatal extension of the Phase I portion of the brain-body growth curve. The probable reason is that apes' forelimbs are so heavily occupied in locomotionâarboreal climbing and terrestrial knuckle walkingâthat extensive tool use is impossible. Encephalization beyond the present level is therefore unwarranted, given the costs that accompany brain expansion: the demographic sacrifice that a lengthened gestation time would entail, for example, and the high energy expenditure required for growth of brain tissue. As a result, there is no reason to believe that the slight postnatal extension of the Phase I slope in gracile australopithecines resulted in helpless infants that could not cling to climbing mothers. One might ask why natural selection should not simply have expanded the australopithecine brain without being required to retard development in general, with all the attendant problems. The primary answer is that an organ such as the brain does not develop in isolation, but is morphogenetically linked to other anatomical systems. As a result, a general delay in development was by far the simplest mechanism for brain expansion. All that was needed was prolongation into the postnatal interval of a pattern of development that was already in place. The complexity of other potential evolutionary mechanisms was so great that the probability of their occurrence was very low. A likely second problem with other mechanisms would have been their inability to expand the brain dramatically soon after birth. Delayed development offered the key advantage of producing a large brain during the first year, thus permitting the immense human learning process to proceed rapidly at a very young age. THE LIFE OF GRACILE AUSTRALOPITHECINES Even if clinging neonates permitted australopithecines to engage in arboreal activity, habitual climbing would have been possible only if the adults possessed appropriate adaptations. In fact, numerous of their morphological traits indicate that although these animals were adapted for bipedal locomotion on the ground, they were nonetheless much more adept climbers than modern humans. Furthermore, as I will explain below, one can make a strong case that australopithecines were compelled to put their climbing abilities to use in their everyday life. Arboreal Traits Modern humans can climb trees better than many members of advanced civilizations recognize, not only by shinnying but also by what amounts to walking up tree trunks. Members of certain Malaysian tribes are excellent barefoot climbers, gripping a tree trunk with a hand on each side, taking small steps upward by applying their splayed feet to the trunk, and then regripping with the hands at a higher level (Wood-Jones, 1900; Skeat and Blagden, 1906). Sometimes they climb so rapidly in this manner as to be described as "running" up trees. A variety of attributes would have made australopithecines more adept at these activities than modern humans are (Figure 14.2). A trait of australopithecines that would have given them a substantial advantage over modern humans in "walking" up trees was their larger ratio of arm length to leg length. This is reflected in the humerofemoral index, which is the ratio of upper arm length to upper leg length (Figure 14.3). The australopithecines' relatively long arms permitted the upper torso to tilt backward, so that gravity contributed more to the strength of the hands' grip (Cartmill, 1974; Jungers and Stern, 1983). This principle is the one employed by a repairman who climbs a telephone pole by looping a strap around the pole and his waist.
CLIMATIC FORCING AND THE ORIGIN OF THE HUMAN GENUS 237 Figure 14.2 A gracile australopithecine "walking" up a tree. Key adaptations rendering australopithecines superior to modern humans in this activity are long arms relative to leg length; powerful wrists and hands with long, curved toes; ankles capable of great dorsiflexion; and small body size. Figure 14.3 Relative lengths of the humerus (upper forelimb bone: left-hand member of each pair) and femur (upper hindlimb bone) for a pygmy chimpanzee, human pygmy, and Lucy (female Australopithecus afarensis). The length ratio for Lucy is intermediate. (See Stanley, 1992, for sources.) The australopithecine forelimb exhibits additional traits that would have enhanced climbing ability. The glenoid cavity, the socket of the shoulder blade that receives the head of the humerus, is more upward directed than in modern humans (Vrba, 1979; Stern and Susman, 1983; Stanley, 1992). This is advantageous for suspensory activity. Various skeletal features indicate that the australopithecines' wrists and hands were more powerful relative to body size than those of modern humans. In addition, their finger bones were long and curved, resembling those of chimpanzees (Figure 14.4). In fact, their hands were more apelike than human in form (Stern and Susman, 1983; Aiello and Dean, 1990). All of these traits would have served australopithecines well in climbing. Figure 14.4 Strong curvature of a proximal phalanx (finger bone) from the hand of an individual of Australopithecus afarensis (AL 333-W4). This bone resembles the equivalent bones from the fingers of Pan (chimpanzees). The hindlimbs of australopithecines also bore toes that were comparatively long and curved (Stern and Susman, 1982; Latimer and Lovejoy, 1990). Although these animals could not have grasped tree trunks or branches with their hind feet in the manner of apes, which have highly prehensile big toes, they were nonetheless better equipped than modern humans to grip arboreal substrata with curled toes. In addition, their ankles allowed for much greater upward flexure of the foot than ours (Latimer et al., 1987), which would have been very useful for vertical climbing (Figure 14.2). Finally, the relatively small body sizes of gracile australopithecines would have facilitated climbing by offering a higher ratio of strength to weight than characterizes modern humans. New estimates suggest that females averaged about 30 kg and males about 45 kg (McHenry, 1991). In summary, australopithecines were certainly better climbers than modern humans. There is no question, however, that they were also less proficient at arboreal activity than modern apes. Their morphology suggests ability for
