The fossil record reveals that the group of large mammalian predators that inhabited the African continent during Early Pliocene time was much like the one that exists today. In fact, the Early Pliocene fauna was slightly more diverse. In addition to the single living species of lion, leopard, and cheetah, and the three living species of hyenas, there were at least five species that are now extinct: two additional hyenas and three sabertooth cats. It is inconceivable that australopithecines could have withstood the predation pressure of this formidable array of carnivores had they not habitually climbed trees. Significantly, large terrestrial herbivores in modern Africa closely resemble their predators in running speed. The difficulty that modern carnivores encounter in capturing these speedy prey is indicated by two facts: first, they focus heavily on young, old, and sick animals; and second, the availability of food generally limits their population sizes (Kruuk, 1972; Schaller, 1972). Australopithecines, having been even slower than modern humans, would have been no match for predators. Lacking fire and stone weapons, they would also have had little ability to ward off attacks on the ground.
Australopithecines in trees would have faced few effective predators—perhaps only leopards and the false sabertooth, Dinofelis. (The true sabertooth cats, having had long, fragile canine teeth, are thought to have specialized on pachyderms; summary by Marean, 1989). Furthermore, leopards are solitary, territorial predators rather than group hunters, so that their density and hunting prowess are both relatively low. A treed australopithecine with a sharp stick might have warded off a leopard whose forelimbs were occupied in clinging to a branch, but australopithecines confined to the ground would frequently have found themselves within the ranges of several species of social predators with excellent night vision and a preference for nocturnal hunting.
Modern primates that resemble australopithecines in body size offer a test of the idea that australopithecines would have needed to climb trees in order to reduce predation pressure. Chimpanzees and baboons both habitually employ trees as arboreal refuges in two ways. They sleep in trees at night, and they flee into them during waking hours when threatened by predators. Male baboons, with their formidable canine teeth, have been seen to face down leopards, yet when lions are in the vicinity, trees are as important a limiting resource for baboons as are food and water (Devore and Washburn, 1963).
In addition, accumulations of gracile australopithecine bones in South African cave deposits appear to be the products of predation (Vrba, 1980; Brain, 1981). These primates, together with baboons, greatly outnumber ungulates and are represented primarily by cranial remains. The implication is that the australopithecines were under heavy predation pressure. In summary, for more than 1.5 m.y., australopithecines retained traits that made them much better climbers than modern humans. The fact that evolution failed to rid them of these traits despite the fact that some of the traits were deleterious to terrestrial locomotion suggests that stabilizing selection was maintaining the traits. The source of this stabilizing selection is readily found in the need to climb trees frequently to feed and especially to avoid numerous species of fast, powerful, group-hunting predators.
The taxonomy of early representatives of the genus Homo is controversial, in part because of a patchy fossil record. "Early Homo" is a convenient label for fossil representatives of the genus older than Homo erectus, which ranges back to about 1.6 Ma. Traditionally, early Homo specimens have been assigned to the single species Homo habilis, which many workers now judge should be divided into two or more species. Details aside, it is clear that by about 2 Ma there existed some members of the genus whose brain capacities were at least twice the average for a gracile australopithecine. In addition, some members of early Homo had a pelvis that was not compressed from front to back, like that of a gracile australopithecine, but was instead remarkably like that of a modern human, except in having a smaller inlet, which required that babies be born at a smaller size than ours. Two known femora (thigh bones) of early Homo that are dated at about 1.9 Ma are also well within the length range for modern human females (Kennedy, 1983). In contrast, Lucy's femur is considerably shorter even than that of a female pygmy (Figure 14.3). All of the early Homo fossils mentioned above are quite similar to the equivalent skeletal parts of Homo erectus and modern humans, indicating a high level of adaptation to terrestrial locomotion.
The pelvic dimensions of early Homo indicate a small birth size; yet the remarkably large early Homo skull KNM-ER 1590, representing a small child, would have expanded into the Homo erectus range in adulthood (perhaps exceeding 900 cm3 in cranial capacity). The enormous amount of brain growth between birth and adulthood in early Homo would have required a considerable extension of the Phase I interval of growth into the postnatal interval (Stanley, 1992). Thus, early Homo could not have been an obligate tree climber: its infants could not have clung to their mothers.
The oldest known fossils representing big-brained Homo are dated at 2.4 Ma (Hill et al., 1992). The oldest known