Life in the last few million years

The excellent fossil record of the past few million years, combined with th e overwhelming similarity of the biota to extant species, provides an outst anding opportunity for understanding paleoecological and macroevolutionary patterns and processes within a rigorous biological framework. Unfortunatel y, this potential has not been fully exploited because of lack of well-samp led time series and adequate statistical analysis. Nevertheless, four basic patterns appear to be of general significance. First, a major pulse of ext inction occurred 1-2 m.y. ago in many ocean basins, more or less coincident with the intensification of glaciation in the Northern Hemisphere. Rates o f origination also increased greatly but were more variable in magnitude an d timing. The fine-scale correlation of these evolutionary events with chan ges in climate is poorly understood. Similar events probably occurred on la nd but have not been rested adequately. Second, rates of origination and ex tinction in the oceans waned after the pulse of extinction, especially duri ng the past 1 m.y. Thus, most marine species originated long before the Ple istocene under very different environmental circumstances, suggesting that they are "exapted" rather than adapted to their present ecological circumst ances. The same may be true for many terrestrial groups, but not for the ma mmals or fresh-water fishes that have continued to undergo speciation throu ghout the Pleistocene. Third, community membership of late Pleistocene cora l reef communities was more stable than expected by chance. These are the o nly paleoecological data adequate to test hypotheses of community stability , so that we do not know whether community structure involving other taxa o r environments typically reflects more than the collective behavior of indi vidual species distributions. Regardless, the strong evidence for nearly un iversal exaptation of ecological characteristics argues strongly against id eas of coevolution of species in communities. Finally, ecological communiti es were profoundly altered by human activities long before modern ecologica l studies began. Holocene paleontological, archeological, and historical da ta constitute the only ecological baseline for "pristine" ecological commun ities before significant human disturbance. Holocene records should be much more extensively used as a baseline for Recent ecological studies and for conservation and management.