contemporary genomes. This phylogeny is unique in suggesting that the function of tRNA in replication dates back to the very beginnings of life on earth, before the advent of templated protein synthesis. The origin we propose for tRNA has distinct implications for the order in which other components of the modern translational apparatus evolved. We further suggest that the "top half" of modern tRNA—a coaxial stack of the acceptor stem on the TΨC arm—is the ancient structural and functional domain and that the "bottom half" of tRNA—a coaxial stack of the dihydrouracil arm on the anticodon arm—arose later to provide additional specificity.


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