Some adaptive breakthroughs in the history of life have resulted from natural selection for improved adaptation without the influence of environmental change. Other major evolutionary transitions have resulted from selection pressures imposed by a changing habitat. Thus, not only do severe environmental changes alter the biosphere by causing migration and extinction, they also stimulate evolution. In fact, a species can, in a sense, experience both extinction and successful evolution as a result of environmental crisis. A species as it was constituted for 106 to 107 generations may die out and yet have one or more of its populations emerge as a distinctive new species adapted to the new conditions.
The onset of the recent ice age, at about 2.5 Ma, transformed habitats in many regions of the world. A variety of circumstantial evidence suggests that at this time a particular kind of environmental crisis in Africa caused the extinction of one or more species of the human family but also triggered the evolution of the modern human genus, Homo (Stanley, 1992). Homo evolved from a species of the genus Australopithecus. This ancestral genus, having been confined to Africa, could not have escaped the environmental changes that pervaded this continent.
The early evolution of Homo was of unique significance in the history of life. Most importantly, it entailed a marked increase in brain size. The early Homo fossil skull 1590, which belonged to a child about 6 or 7 years old, would have grown to have an adult cranial capacity well in excess of 800 cm3, about twice that of a male belonging to the gracile australopithecine species from which early Homo evolved.
The term ''gracile" means slender, and applies specifically to the jaws, teeth, and skull, which in both of the known gracile species Australopithecus afarensis and A. africanus were less heavily developed than in the "robust" species, which many authors now segregate into the genus Paranthropus. The robust forms were almost certainly evolutionary offshoots of the line of descent that led to Homo. Most experts now regard A. africanus as the gracile species most likely to have given rise to Homo, in part because the known range of this South African form extends to about 2.3 Ma, more than half a million years beyond that of A. afarensis, whose fossil record is confined to northern Africa. In any event, the two gracile species were rather similar in general morphology and presumably therefore in mode of life.
It is important to recognize that the gracile australopithecines existed for more than 1.5 m.y., from about 4 Ma to perhaps 2.3 Ma, without experiencing appreciable evolutionary change. Not only did their cranial capacity remain only slightly above the level of a chimpanzee, but their postcranial anatomy experienced approximate evolutionary stasis as well (McHenry, 1986). They were obviously successful, well-adapted creatures. It has long been understood that australopithecines walked bipedally. Their pelvic configuration is much more human than apelike in form, and their hindlimbs also in many ways resemble those of Homo. Furthermore, fossil bipedal footprints in Tanzania were formed at about 3.7 Ma, long before Homo or robust australopithecines evolved. By default, these tracks are attributed to gracile australopithecines (Leakey and Hay, 1979).
Although there remains no doubt that australopithecines moved bipedally on the ground, the past decade has seen the emergence of abundant evidence that tree climbing was also part of their normal behavioral repertoire (Stern and Susman, 1983; Susman et al., 1984; inter alia). The bones of their forelimbs display numerous traits that evolved as arboreal adaptations in their ancestors. The bones of their hindlimbs were more human in form, but nonetheless retained certain traits that would have enhanced the ability to climb while at the same time restricting performance in walking and running to a level below that of modern humans. The habits of extant primates indicate that australopithecines should have been required to use their appropriate traits in order to avoid predators. The australopithecines' division of activities between terrestrial and arboreal habitats accounts for the failure of their postcranial morphology to evolve appreciably in a human direction for some 1.5 m.y.
In a less direct way, semiarboreal behavior can also account for the gracile australopithecines' failure to evolve appreciably larger brains during the same interval. In fact, it was impossible for the large brain of Homo to evolve until obligate arboreal activity had been abandoned. This restriction related to the developmental mechanism by which the large brain evolved: extension into the postnatal interval of the high in utero rate of brain growth, which in all primates maintains brain weight at about 10% of body weight. In monkeys and apes, this rate gives way to a much lower rate soon after birth. In humans, however, cranial development is delayed, so that the high fetal rate of brain growth is projected through the first year of life after birth. This produces a cranial capacity for a 1-yr-old human infant that is more than double that of an adult chimpanzee. Figure 14.1 illustrates the differing developmental patterns for apes and humans.
The reason that human ancestors could not evolve the large brain of Homo until they totally abandoned arboreal activity is that the developmental delay that yielded the large brain was linked to other aspects of maturation. A general retardation of development produced the large brain by projecting the high fetal rate of brain growth into the postnatal interval. In contrast to infant chimpanzees and