Effects of Past Global Change on Life (1995)
Chapter: ABSTRACT
14
Climatic Forcing and the Origin of the Human Genus
STEVEN M. STANLEY
The Johns Hopkins University
ABSTRACT
With the onset of the modern ice age, climatic changes in Africa caused grasslands to expand and forests to contract. This environmental shift appears to account for the evolution of Homo about 2.4 million years ago (Ma). Evolution could not have established the mode of development that produces the large brain of Homo until human ancestors were fully terrestrial. This mode of development produces the long interval of physical helplessness that distinguishes humans from all other mammals; it yields infants that cannot cling to mothers. The more mature infants of apes can cling to their mothers, allowing the mothers to climb trees. Gracile australopithecines, from which the genus Homo evolved, retained an apelike pattern of development, arboreal adaptations, from their ancestors. Thus, even females with infants could have been adept tree climbers, and the need to avoid predators must have required that gracile australopithecines be habitual climbers. In contrast, the small pelvic dimensions and large brain of early Homo point to delayed development, helpless infants, and a totally terrestrial mode of life. The origin of Homo, about 2.4 Ma, appears to have resulted from the onset of the recent ice age, when climatic changes in Africa caused savannas to expand at the expense of woodlands. These changes must have had a severe impact on australopithecines, as they did on other groups of mammals. In particular, they should have compelled many australopithecine populations to abandon the arboreal activity that had maintained evolutionary stability for more than 1.5 million years (m.y.). The resulting crisis conditions presumably caused the extinction of many populations, but the evolution of a huge brain through delayed development was now possible for the first time. Selection pressure for superior intelligence fostered the development of advanced social behavior and tool manufacture, which offset the problems created by helpless infants and the loss of arboreal refugia and food resources. African antelopes experienced parallel changes, with forest-adapted species suffering heavy extinction and a variety of new species coming into existence.
