“The RNA World” means different things to different investigators, so it would be futile to attempt a restrictive definition. 1983) initiated extensive discussion of the role of RNA in the origins of life ( Sharp 1985 Pace and Marsh 1985 Lewin 1986) and led to the coining of the phrase “the RNA World” ( Gilbert 1986). The unanticipated discovery of ribozymes ( Kruger et al. 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 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 over 40 yr ago ( Woese 1967 Crick 1968 Orgel 1968). However, the problem of the origin of the RNA World is far from being solved, and it is fruitful to consider the alternative possibility that RNA was preceded by some other replicating, evolving molecule, just as DNA and proteins were preceded by RNA. Perhaps these concerns will eventually be resolved, and recent experimental findings provide some reason for optimism. A thorough consideration of this “RNA-first” view of the origin of life must reconcile concerns regarding the intractable mixtures that are obtained in experiments designed to simulate the chemistry of the primitive Earth. It might be imagined that all of the components of RNA were available in some prebiotic pool, and that these components assembled into replicating, evolving polynucleotides without the prior existence of any evolved macromolecules. However, arguments regarding whether life on Earth began with RNA are more tenuous. There is now strong evidence indicating that an RNA World did indeed exist before DNA- and protein-based life. The general notion of an “RNA World” is that, in the early development of life on the Earth, genetic continuity was assured by the replication of RNA and genetically encoded proteins were not involved as catalysts.
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