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.