© 1994 by The Society for Integrative and Comparative Biology
Biodiversity in Geological Time1
Department of Geology and Center for Population Biology, University of California Davis, California 95616
SYNOPSIS. The numbers of animal and plant species extant on Earth have fluctuated dramatically through geological time. Animals and vascular plants were absent from the first three billion years of Earth history, although there is ample evidence of prokaryotic life in rocks as old as 3.5 billion years and fossil eukaryotic organisms in rocks as old as 2.0 billion years. The Cambrian Metazoan Radiation, during a geologically brief interval about 540 million years ago (Ma), saw the appearance of most classes and orders of skeletogenous marine invertebrates. Vascular plants appeared in a subsequent radiation in the mid-Paleozoic (–400 Ma), followed closely by terrestrial vertebrates.
Over the past 400 million years, the trajectories of taxonomic diversity among marine invertebrates, vascular plants, and terrestrial vertebrates were roughly congruent; there were relatively few taxa in each group in the late Paleozoic followed by a striking increase from the late Mesozoic to the levels observed today. The reasons for these increases remain unclear, but both physical and biological processes are likely to have played important roles. Occasional mass extinctions severely reduced taxonomic diversity over geologically brief intervals of time. However, recovery from mass extinctions was invariably a prolonged process. The lesson that diversity, once lost, is regained only slowly over geologic time must not be forgotten as plans are developed to mitigate the coming biodiversity crisis.