group-hunting terrestrial predators that, in the Pliocene as today, would have sought out as preferred prey animals that were easiest to catch. Even a grove of trees that initially served well as a refuge could not have sufficed indefinitely. In time, a troop would have exhausted food resources within close range of any home base. It would then have been required to move across grassland at the risk of suffering predation.
Before 2.5 Ma, woodlands were widespread and numerous groves of trees were separated by narrow zones of grassland. When forests shrank and fragmented with the onset of the ice age, however, many populations of australopithecines must have suffered a devastating intensification of predation pressure. Shrinking groves of trees offered smaller stores of food, which necessitated more frequent migration, and expanding grasslands increased the risk of predation by lengthening dangerous journeys. Presumably, many populations suffered extinction. Others may have survived for a time in areas that continued to support woodlands of moderate extent.
Widespread replacement of woodland habitats by grasslands is also exactly the kind of environmental forcing factor that could be expected to have obliged some populations to abandon habitual arboreal activity. Such a restriction of behavior automatically opened the way for encephalization through evolutionary extension of Phase I growth into the postnatal interval: physically helpless infants, though ecologically problematical, were now tolerable because mothers no longer climbed trees. Overriding the problems of raising highly dependent offspring, coping with predators, and losing arboreal food resources were the profound advantages of brain expansion—especially the ability to offset relatively weak physical attributes with innate cunning, advanced cooperative behavior, and sophisticated weaponry. These advantages of encephalization applied not only to avoidance of predators but also to development of hunting prowess that expanded trophic resources on the ground.
An important aspect of this scenario is that the first step was a simple change in behavior—one that amounted to a reduction of the preexisting behavioral repertoire. The result was that powerful natural selection pressures were brought to bear, so that major morphological changes ensued. Furthermore, the evolutionary retardation of development that produced encephalization was a relatively simply change, in that it represented only a modification of timing, not the origin of an entirely new pattern of development. This is not to say that the brain changed only by expanding. There was also a reorganization of brain anatomy, which we are only beginning to understand (Deacon, 1990).
The general evolutionary scenario outlined here entailed a shift to a new adaptive zone, not by an entire populous species but by a relatively small population of such a species that survived an environmental crisis. Other populations may have survived for a time with little change, in areas where environmental deterioration was less extensive. At least one fossil individual dated at about 1.6 Ma had a relatively small brain and more apelike proportions than individuals assigned unequivocally to early Homo (Leakey et al., 1989). In addition, two robust australopithecine species persisted well into Pleistocene time. The enormous molars and powerful jaw muscles of these forms endowed them with the ability to process a wider variety of plant foods than gracile forms, however, and this may have increased their chances for survival by reducing the need to migrate to new food supplies. Even these forms died out at about 1 Ma. This was approximately the time when glacial maxima and minima became more extreme (Stanley and Ruddiman, Chapter 7, this volume) and when carbon isotopes show that true savannas appeared (Cerling, 1992). Perhaps the increased severity of droughts during glacial maxima caused the extinction of the robust australopithecines.
There is evidence that Australopithecus africanus persisted to about 2.3 Ma (Delson, 1988), but we do not now know for sure that it survived beyond the origin of Homo at about 2.4 Ma. Thus, we cannot know for sure whether Homo emerged from the entire surviving population of the decimated ancestral australopithecine species or whether the ancestral species gave rise to Homo by the evolutionary divergence of just one of its populations and then survived for a time alongside it, though possibly in other geographic regions. Discovery of a temporal overlap within the poorly documented interval between 2.5 and 2.0 Ma would settle the issue in favor of evolutionary branching, as opposed to the bottlenecking of an entire species.
The mechanism of climatic forcing that I have described is compatible with either possibility, in that environmental deterioration must have been a complex process in time and space, and different populations were undoubtedly subjected to different patterns of environmental change. In any event, by mid-Pleistocene time, only the fully terrestrial genus Homo remained.
We tend to think of the environmental changes associated with the onset of the Plio-Pleistocene ice age as constituting a deterioration of habitats. Thus, it might seem a great irony that the origin of our genus, which we inevitably view as a positive event, was wrought by what, from a different perspective, has been widely viewed as an environmental crisis.
Aiello, L., and C. Dean (1990). An Introduction to Human Evolutionary Anatomy, Academic Press, London,596 pp.