Figure 1 From the primordial soup to the three contemporary kingdoms. Woese (1990) originally used the term "progenote" to describe a common ancestor with an inaccurate translation system, but we make no assumption about the accuracy of translation in the progenote. Accuracy is a matter of degree, and the important point is that translation in this ancestor must have been substantially similar to our own. For clarity, we show DNA as evolving after templated protein synthesis. An interesting alternative, suggested by J. Szostak (Harvard Medical School, personal communication), is that DNA evolved much earlier, perhaps even in the RNA world, as a catalytically inactive storage form of RNA.

self-replicating molecule, early living systems arose with RNA genomes and relatively simple biochemistries, and these evolved into cells with DNA genomes and complex biochemistries. Indeed, the biochemistry of all living organisms is so similar that there is little doubt all life on earth can be traced back to a single common ancestor, which Woese named the "progenote" (Woese, 1990). Moreover, because the translation apparatus is fundamentally similar in all three contemporary kingdoms, this progenote must have been capable of templated protein synthesis substantially like our own.

Here we propose a phylogeny for the origin of tRNA based on the ubiquity and conservation of tRNA-like structures in the replication of contemporary genomes, and we discuss the evidence in contemporary molecules that leads to and supports this phylogeny. The unique aspect of this phylogeny is that it places the origin of tRNA in replication, before the advent of templated protein synthesis. This implies that tRNAs arose before the other components of the translation apparatus, that aminoacyl-tRNA synthetases arose next, and that both tRNA and aminoacyl-tRNA synthetases predated the anticodon and mRNA.



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