1 Prospects for Understanding the Origin of the RNA World

The general idea that, in the development of life on the earth, evolution based on RNA replication preceded the appearance of protein synthesis was first proposed about 30 years ago (Woese 1967; Crick 1968; Orgel 1968). It was suggested that catalysts made entirely of RNA are likely to have been important at this early stage in the evolution of life, but the pos-sibility that RNA catalysts might still be present in contemporary organ-isms was overlooked. The unanticipated discovery of ribozymes (Kruger et al. 1982; Guerrier-Takada et al. 1983) initiated extensive discussion of the role of RNA in the origins of life (Pace and Marsh 1985; Sharp 1985; Lewin 1986) and led to the coining of the phrase “the RNA World” (Gilbert 1986). The RNA World means different things to different authors, so it would be futile to attempt a restrictive definition. All RNA World hypotheses include three basic assumptions: (1) At some time in the evo-lution of life, genetic continuity was assured by the replication of RNA; (2) Watson-Crick base-pairing was the key to replication; (3) genetically encoded proteins were not involved as catalysts. RNA World hypotheses differ in what they assume about life that may have preceded the RNA World, about the metabolic complexity of the RNA World, and about the role of low-molecular-weight cofactors, possibly including peptides, in the chemistry of the RNA World. It should be emphasized that the existence of an RNAWorld as a pre-cursor of our DNA/protein world is a hypothesis. We find it an attractive hypothesis and believe that it derives some support from the results of experiments that it has inspired. The demonstration that the peptide-bond-