National Academies Press: OpenBook

Effects of Past Global Change on Life (1995)

Chapter: REFERENCES

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Suggested Citation:"REFERENCES." National Research Council. 1995. Effects of Past Global Change on Life. Washington, DC: The National Academies Press. doi: 10.17226/4762.
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Page 241
Suggested Citation:"REFERENCES." National Research Council. 1995. Effects of Past Global Change on Life. Washington, DC: The National Academies Press. doi: 10.17226/4762.
×
Page 242
Suggested Citation:"REFERENCES." National Research Council. 1995. Effects of Past Global Change on Life. Washington, DC: The National Academies Press. doi: 10.17226/4762.
×
Page 243
Suggested Citation:"REFERENCES." National Research Council. 1995. Effects of Past Global Change on Life. Washington, DC: The National Academies Press. doi: 10.17226/4762.
×
Page 244

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CLIMATIC FORCING AND THE ORIGIN OF THE HUMAN GENUS 241 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. REFERENCES Aiello, L., and C. Dean (1990). An Introduction to Human Evolutionary Anatomy, Academic Press, London,596 pp.

CLIMATIC FORCING AND THE ORIGIN OF THE HUMAN GENUS 242 Bonnefille, R. (1976). Implications of pollen assemblage from the Koabi Fora Formation, East Rudolf, Kenya, Nature 264, 487-491. Bonnefille, R. (1983). Evidence for a cooler and drier climate in the Ethiopian uplands towards 2.5 Myr ago, Nature 303, 487-491. Brain, C. K. (1981). The Hunters or the Hunted? An Introduction to African Cave Taphonomy, University of Chicago Press, Chicago, 365 pp. Cartmill, M. (1974). Pads and claws in arboreal locomotion, in Primate Locomotion, F. A. Jenkins, ed., Academic Press, New York, pp. 45-83. Cerling, T. (1992). Development of grasslands and savannahs in East Africa during the Neogene, Palaeogeography, Palaeoclimatology, Palaeoecology 97, 241-247. Conrad, G. (1968). Evolution Continental Post-Hercynienne du Sahara Algerien, CNRS, Paris. Coque, R. (1962). La Tunisie Présaharienne, Amrand Colin, Paris, 476 pp. Count, E. W. (1947). Brain and body weight in man: Their antecedents in growth and evolution, Annals of the New York Academy of Sciences 46, 993-1122. Deacon, T. W. (1990). Problems of ontogeny and phylogeny in brain-size evolution, Internat. J. Primatol. 11, 237-282. Delson, E. (1988). Chronology of South African australopith site units, in Evolutionary History of the "Robust" Australopithecines, F. E. Grine, ed., Aldine de Gruyter, New York, pp. 317-324. Devore, I., and S. L. Washburn (1963). Baboon ecology and human evolution, in African Ecology and Human Evolution, F. C. Howell and F. Bourliere, eds., Aldine, Chicago, pp. 335-367. Harris, J. W. K. (1983). Cultural beginnings: Plio-Pleistocene archaeological occurrences from the Afar, Ethiopia, African Archaeol. Rev. 1, 3-31. Hill, A. S., A. Deino, G. Curtiss, and R. Drake (1992). Earliest Homo, Nature 355, 719-722. Holt, A. B., D. B. Cheek, E. D. Mellits, and D. E. Hill (1975). Brain size and the relation of the primate to the nonprimate, in Fetal and Postnatal Cellular Growth, D. B. Cheek, ed., John Wiley & Sons, New York, pp. 23-44. Jungers, W. L., and J. T. Stern (1983). Body proportions, skeletal allometry and locomotion in the Hadar hominids: A reply to Wolpoff, J. Human Evol. 12, 673-684. Kay, R. F. (1985). Dental evidence for the diet of Australopithecus, Ann. Rev. Phys. Anthrop. 14, 315-341. Kennedy, G. E. (1983). A morphometric and taxonomic assessment of a hominine femur from the lower member, Koobi Fora, Lake Turkana, Amer. J. Phys. Anthrop. 61, 429-436. Krogman, W. G. (1972). Child Growth, University of Michigan Press, Ann Arbor, 231 pp. Kruuk, H. (1972). The Spotted Hyena, University of Chicago Press, Chicago, 335 pp. Latimer, B., and C. O. Lovejoy (1990). Hallucal tarsometatarsal joint in Australopithecus afarensis, Amer. J. Phys. Anthrop. 82, 125-133. Latimer, B., J. C. Ohman, and C. O. Lovejoy (1987). Talocrural joint in African hominoids: Implications for Australopithecus afarensis, Amer. J. Phys. Anthrop. 74, 155-175. Leakey, M. D., and R. L. Hay (1979). Pliocene footprints in the Laetolil beds at Laetoli, northern Tanzania, Nature 202, 7-9. Leakey, R. E. F., A. Walker, C. V. Ward, and H. M. Grausa (1989). A partial skeleton of a gracile hominid from the upper Burgi member of the Koobi Fora Formation, east Lake Turkana, Kenya, in Hominidae, G. Giacobini, ed., Jaca, Milan, pp. 209-215. Marean, C. W. (1989). Saber-tooth cats and their relevance for early hominid diet and evolution, J. Human Evol. 18, 559-582. McHenry, H. M. (1986). The first bipeds: A comparison of the A. afarensis and A. africanus postcranium and implications for the evolution of bipedalism, J. Human Evol. 15, 177-191. McHenry, H. M. (1991). Sexual dimorphism in Australopithecus afarensis, J. Human Evol. 20, 21-32. Rodman, P. S., and H. M. McHenry (1980). Bioenergetics and the origin of hominoid bipedalism, Amer. J. Phys. Anthropol. 63, 371-378. Schaller, G. B. (1972). The Serengeti Lion, University of Chicago Press, Chicago, 480 pp. Servant, M., and S. Servant-Vildary (1980). L'environment quaternaire du bassin du Tchad, in The Sahara and the Nile, M. A. J. Williams and H. Faure, eds., Balkema, Rotterdam, pp. 133-162. Skeat, W. W., and C. O. Blagden (1906). Pagan Races of the Malay Penninsula, MacMillan, New York, 724 pp. Stanley, S. M. (1992). An ecological theory for the origin of Homo, Paleobiology 18, 237-257. Stern, J. T., and R. L. Susman (1983). The locomotory anatomy of Australopithecus afarensis, Amer. J. Phys. Anthrop. 60, 279-317. Susman, R. L., J. T. Stern, and W. L. Jungers (1984). Arboreality and bipedality in the Hadar hominids, Folia Primatol. 43, 113-156. Tague, R. G., and C. O. Lovejoy (1986). The obstetric pelvis of A. L. 288-1 (Lucy), J. Human Evol. 15, 237-255. Vrba, E. S. (1974). Chronological and ecological implications of the fossil Bovidae at the Sterkfontein australopithecine site, Nature 250, 19-23. Vrba, E. S. (1975). Some evidence of chronology and palaeoecology of Sterkfontein, Swartkrans and Kromdraai from the fossil Bovidae, Nature 254, 301-304. Vrba, E. S. (1979). A new study of the scapula of Australopithecus africanus from Sterkfontein; Amer. J. Phys. Anthrop. 51, 117-130. Vrba, E. S. (1980). The significance of bovid remains as indicators of environment and predation patterns, in Taphonomy and Paleoecology, A. K. Behrensmeyer and A. P. Hill, eds., University of Chicago Press, Chicago, pp. 247-271. Vrba, E. S. (1985a). African Bovidae: Evolutionary events since the Miocene, S. African J. Sci. 81, 263-266. Vrba, E. S. (1985b). Ecological and adaptive changes associated with early hominid evolution, in Ancestors: The Hard Evidence, E. Delson, ed., Alan R. Liss, New York, pp. 63-71.

CLIMATIC FORCING AND THE ORIGIN OF THE HUMAN GENUS 243 Vrba, E. S. (1988). Late Pliocene climatic events and hominid evolution, in Evolutionary History of the "Robust" Australopithecines, F. E. Grine, ed., Aldine de Gruyter, New York, pp. 405-426. Vrba, E. S., G. H. Denton, and M. L. Prentice (1989). Climatic influences on early hominid behavior, Ossa 14, 127-156. Wesselman, H. B. (1985). Fossil micromammals as indicators of climatic change about 2.4 Myr ago in the Omo Valley, Ethiopia, S. African J. Sci. 81, 260-261. Wood-Jones, F. (1900). Arboreal Man, E. Arnold, London,230 PP.

CLIMATIC FORCING AND THE ORIGIN OF THE HUMAN GENUS 244

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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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