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Life in the Last Few Million Years

Jeremy B. C. Jackson and Kenneth G. Johnson
Paleobiology
Vol. 26, No. 4, Supplement (Autumn, 2000), pp. 221-235
Published by: Paleontological Society
Stable URL: http://www.jstor.org/stable/1571659
Page Count: 15
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Life in the Last Few Million Years
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Abstract

The excellent fossil record of the past few million years, combined with the overwhelming similarity of the biota to extant species, provides an outstanding opportunity for understanding paleoecological and macroevolutionary patterns and processes within a rigorous biological framework. Unfortunately, this potential has not been fully exploited because of lack of well-sampled time series and adequate statistical analysis. Nevertheless, four basic patterns appear to be of general significance. First, a major pulse of extinction occurred 1-2 m.y. ago in many ocean basins, more or less coincident with the intensification of glaciation in the Northern Hemisphere. Rates of origination also increased greatly but were more variable in magnitude and timing. The fine-scale correlation of these evolutionary events with changes in climate is poorly understood. Similar events probably occurred on land but have not been tested adequately. Second, rates of origination and extinction in the oceans waned after the pulse of extinction, especially during the past 1 m.y. Thus, most marine species originated long before the Pleistocene under very different environmental circumstances, suggesting that they are "exapted" rather than adapted to their present ecological circumstances. The same may be true for many terrestrial groups, but not for the mammals or fresh-water fishes that have continued to undergo speciation throughout the Pleistocene. Third, community membership of late Pleistocene coral reef communities was more stable than expected by chance. These are the only paleoecological data adequate to test hypotheses of community stability, so that we do not know whether community structure involving other taxa or environments typically reflects more than the collective behavior of individual species distributions. Regardless, the strong evidence for nearly universal exaptation of ecological characteristics argues strongly against ideas of coevolution of species in communities. Finally, ecological communities were profoundly altered by human activities long before modern ecological studies began. Holocene paleontological, archeological, and historical data constitute the only ecological baseline for "pristine" ecological communities before significant human disturbance. Holocene records should be much more extensively used as a baseline for Recent ecological studies and for conservation and management.

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