I. AN INVITATION TO tRNA EVOLUTION

".., i l n'est pas de vraie creation sans secret. " Albert Carnus, Le myrhe de Sisyphe Interest in tRNA biochemistry is attested to by the imposing mass of data generated by a veritable army of researchers. The approach to this information used here will be somewhat unique among the large number of existing tRNA reviews in that it will be organized and interpreted within an evolutionary framework. In addition to a discussion and an extension of current hypotheses on tRNA evolution, an evaluation is presented of the basic premise of this review: that tRNA structural and functional data can provide information on early evolutionary events. The importance of tRNA evolution lies in its relation to the origin and development of the genetic code. ' In a sense, the discovery of the universality of the genetic code in eukaryotes and prokaryotes (see Section VII. B. on mitochondria) was a mixed blessing to the field of molecular evolution. * On one hand it clarified the basis of protein sequence comparisons involving different organisms, but on the other it limited the applicability of comparative methods for identifying primitive vs. advanced forms. Thus, genetic code evolutionary models have had to be based on indirect evidence and / or speculation. The primary structure of tRNA, however, does vary, not only as a function of the particular amino acid and codon it relates, but also from species to species across the phylogenetic ~ p e c t r u m. ~ The authors hope to show in this review how statistical and comparative techniques in general can help to shed light on both Darwinian and pre-Darwinian evolution, i.e., biological evolution, speciation, and the biochemical evolution which preceded it. Along with a comparative study of tRNA sequences three other interrelated aspects of Cr iti ca l R ev ie w s