In the RNA world, which is a hypothetical idea to explain the origin of life, RNA molecules were considered to have roles in both information storage and as a catalyst. This chapter reviews RNA world studies from its birth to recent advancements. Natural ribozymes and coenzymes containing nucleotide moieties support the hypothesis. For maintenance and evolution of the RNA world, ribozymes that have self-replicating activity had to emerge. Although such a ribozyme has not been discovered in the natural world, in vitro evolution experiments have created ribozymes that have replicative ability, essentially. After the emergence of the replicative ribozyme, ribozymes might have gradually improved its activity by incorporating other biomolecules especially peptides, which could be synthesized spontaneously in the prebiotic world. The RNA-protein (RNP) world may have emerged through the interaction between the RNA world and peptide/protein world. Proteins with higher enzymatic activities could have appeared through Darwinian evolution in the RNP world, and the peptide/protein must have replaced the role of ribozymes as catalysts. Further interactions with other molecular worlds such as the lipid or metabolic worlds accelerated the evolution of the self-replicating system. Finally, DNA, which is chemically more stable than RNA, has taken over the role as the storage of genetic information.
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