2 Progress toward Understanding the Origin of the RNA World

Abstract

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 almost 40 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 possibility that RNA catalysts might still be present in contemporary organisms 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 investigators, so it would be futile to attempt a restrictive definition. All RNA World hypotheses include three basic assumptions: (1) At some time in the evolution 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.

There is now strong evidence indicating that an RNA World did indeed exist on the early earth. The smoking gun is seen in the...