BIODIVERSITY IN GEOLOGICAL TIME

The numbers of animal and plant species extant on Earth have fluctuate d dramatically through geological time. Animals and vascular plants we re absent from the first three billion years of Earth history, althoug h there is ample evidence of prokaryotic life in rocks as old as 3.5 b illion years and fossil eukaryotic organisms in rocks as old as 2.0 bi llion years. The Cambrian Metazoan Radiation, during a geologically br ief interval about 540 million years ago (Ma), saw the appearance of m ost classes and orders of skeletogenous marine invertebrates. Vascular plants appeared in a subsequent radiation in the mid-Paleozoic (simil ar to 400 Ma), followed closely by terrestrial vertebrates. Over the p ast 400 million years, the trajectories of taxonomic diversity among m arine invertebrates, vascular plants, and terrestrial vertebrates were roughly congruent; there were relatively few taxa in each group in th e late Paleozoic followed by a striking increase from the late Mesozoi c to the levels observed today. The reasons for these increases remain unclear, but both physical and biological processes are likely to hav e played important roles. Occasional mass extinctions severely reduced taxonomic diversity over geologically brief intervals of time. Howeve r, recovery from mass extinctions was invariably a prolonged process. The lesson that diversity, once lost, is regained only slowly over geo logic time must not be forgotten as plans are developed to mitigate th e coming biodiversity crisis.