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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,

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