Early evolution: prokaryotes, the new kids on the block

Prokaryotes are generally assumed to be the oldest existing form of life on earth. This assumption, however, makes it difficult to understand certain aspects of the transition from earlier stages in the origin of life to more complex ones, and it does not account for many apparently ancient features in the eukaryotes. From a model of the RNA world, based on relic RNA speci es in modern organisms, one can infer that there was an absolute requiremen t for a high-accuracy RNA replicase even before proteins evolved. In additi on, we argue here that the ribosome (together with the RNAs involved in its assembly) is so large that it must have had a prior function before protei n synthesis. A model that connects and equates these two requirements thigh -accuracy RNA replicase and prior function of the ribosome) can explain man y steps in the origin of life while accounting for the observation that euk aryotes have retained more vestiges of the RNA world. The later derivation of prokaryote RNA metabolism and genome structure can be accounted for by t he two complementary mechanisms of r-selection and thermoreduction. BioEssa ys 21:880-889, 1999. (C) 1999 John Wiley & Sons, Inc.