National Academies Press: OpenBook

Effects of Past Global Change on Life (1995)


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Suggested Citation:"ABSTRACT." National Research Council. 1995. Effects of Past Global Change on Life. Washington, DC: The National Academies Press. doi: 10.17226/4762.
Page 233

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CLIMATIC FORCING AND THE ORIGIN OF THE HUMAN GENUS 233 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.

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What can we expect as global change progresses? Will there be thresholds that trigger sudden shifts in environmental conditions—or that cause catastrophic destruction of life?

Effects of Past Global Change on Life explores what earth scientists are learning about the impact of large-scale environmental changes on ancient life—and how these findings may help us resolve today's environmental controversies.

Leading authorities discuss historical climate trends and what can be learned from the mass extinctions and other critical periods about the rise and fall of plant and animal species in response to global change. The volume develops a picture of how environmental change has closed some evolutionary doors while opening others—including profound effects on the early members of the human family.

An expert panel offers specific recommendations on expanding research and improving investigative tools—and targets historical periods and geological and biological patterns with the most promise of shedding light on future developments.

This readable and informative book will be of special interest to professionals in the earth sciences and the environmental community as well as concerned policymakers.

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