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CLIMATIC FORCING AND THE ORIGIN OF THE HUMAN GENUS 239 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 predatorsâperhaps 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 NATURE OF EARLY HOMO 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. CLIMATIC FORCING The oldest known fossils representing big-brained Homo are dated at 2.4 Ma (Hill et al., 1992). The oldest known
CLIMATIC FORCING AND THE ORIGIN OF THE HUMAN GENUS 240 manufactured stone tools also date to about this time, and these are customarily attributed to early Homo (Harris, 1983). The tools are simple flakes that represent the so-called Oldowan culture. The youngest gracile australopithecines are not precisely dated, but the famous Taung skull of Australopithecus africanus is now dated at about 2.3 Ma (Delson, 1988). Gracile australopithecines had existed for at least 1.5 m.y. without experiencing appreciable evolutionary change by the time that one of their populations turned into Homo. I have argued above that (1) their postcranial morphology was straitjacketed in an adaptive compromise between terrestrial and arboreal activities, and (2) obligate arboreal activity also prevented them from becoming encephalized appreciably above the level of an ape. It is reasonable to conclude that some kind of environmental change would have been required to end their nearly static evolutionary condition. In particular, what should have been required was a change that caused at least one population to abandon habitual arboreal activity. As it turns out, the onset of the recent ice age at about 2.5 Ma produced exactly the kind of environmental change in Africa that could be expected to have shifted australopithecine behavior in the appropriate direction. Africa became markedly drier, like many other regions of the world at this time (see review by Stanley and Ruddiman, Chapter 7, this volume). As a consequence, forests shrank and grasslands expanded. Fossil pollen reveals that in the Omo Valley region of Ethiopia, climates were warmer and moister than today before about 2.6 to 2.4 Ma, but cooler and drier than today thereafter (Bonnefille, 1983). Similar changes are recorded from Algeria, Chad, and Kenya (Coque, 1962; Conrad, 1968; Bonnefille, 1976; Servant and Servant-Vildary, 1980). Carbon isotopes in soils provide more detailed evidence of this change (Figure 14.6). Samples from a large number of hominid sites reveal no canopied forests after about 2.5 Ma and also document the first occurrence of wooded grasslands at about this time (Cerling, 1992). That the floral changes in Africa had a profound effect on mammals is well established. Close to 2.5 Ma, numerous species of antelopes that had adapted to forest conditions suffered extinction, and during the next few hundred thousand years, there appeared a variety of new savanna-dwelling species, most of which survive as elements of the modern African fauna (Vrba, 1974, 1975, 1985a; Vrba et al., 1989). Micromammals underwent similar changes (Wesselman, 1985). Figure 14.6 Stable carbon isotope composition and inferences about floral composition for paleosol carbonates from East African fossil localities. Isotopic compositions for modern biomes are shown above. Figure after Cerling (1992). It has been suggested that the climatic changes may also in some way have promoted evolutionary turnover within the human family (Vrba, 1975, 1985b; Vrba et al., 1989). The changes of behavior and ontogenetic development that I have attributed to the origin of Homo suggest a particular mode of climatic forcing. Before the shrinkage of forests, troops of australopithecines probably occupied woodlands, which consist of groves or copses of trees separated by small areas of grassland. They could not have climbed well enough to have moved into and through the tall canopies of dense forests. Presumably, they used groves as home bases, sleeping in trees and occasionally feeding in them during the day (Rodman and McHenry, 1980). They may well have spent most of their waking hours on the ground, but only by remaining close enough to the home base to seek arboreal refuge when predators threatened. Modern baboons use trees in this way, even though they feed primarily on grass. Saddled with the low intrinsic rate of natural increase that characterizes species of large primates because of solitary births (as opposed to litters) and lengthy generation times, australopithecine populations could not have sustained themselves in the face of heavy predation without arboreal refugia. Their relatively slow speed and weak natural defenses, in combination with their lack of both controlled fire and manufactured stone weapons, would have created intolerable predation pressure. They would have been easy targets for the multispecies guild of large,