The evolution of diversity in ancient ecosystems: a review

On a perfect planet, such as might be acceptable to a physicist, one might predict that from its origin the diversity of life would grow exponentially until the carrying capacity, however de¢ned, was reached. The fossil record of the Earth, however, tells a very di¡erent story. One of the most striking aspects of this record is the apparent evolutionary longueur, marked by the Precambrian record of prokaryotes and primitive eukaryotes, although our estimates of microbial diversity may be seriously incomplete. Subsequently there were various dramatic increases in diversity, including the Cambrian èxplosion ' and the radiation of Palaeozoic-style faunas in the Ordovician. The causes of these events are far from resolved. It has also long been appreciated that the history of diversity has been punctuated by important extinctions. The subtleties and nuances of extinction as well as the survival of particular clades have to date, however, received rather too little attention, and there is still a tendency towards blanket assertions rather than a dissection of these extraordinary events. In addition, some but perhaps not all mass extinctions are characterized by long lag-times of recovery, which may re£ect the slowing waning of extrinsic forcing factors or alternatively the incoherence associated with biological reassembly of stable ecosystems. The intervening periods between the identi¢ed mass extinctions may be less stable and benign than popularly thought, and in particular the frequency of extraterrestrial impacts leads t