Astrobiology Workshop, Macquarie University July 12-13 2001

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The RNA world and the origins of prokaryotes and eukaryotes.

Anthony Poole (Institute of Molecular Biosciences, Massey University, New Zealand)

The RNA world is the name given to a period in the early evolution of life prior to the advent of genetically-encoded protein synthesis and DNA. Reconstructing this period is possible through the identification of RNA relics found in extant organisms. Importantly, because this period predates the emergence of prokaryotes (archaea and bacteria) and eukaryotes, the distribution of relic RNAs can shed light on the evolution of these groups. The vast majority of relics from the RNA world are found in the eukaryotes, and have been replaced by protein equivalents in archaea and bacteria. Preliminary data on pathways which utilise thermally unstable metabolites are giving a picture that is consistent with the RNA dataset, suggesting that both archaea and bacteria underwent reductive evolution in adaptation to high temperatures. Both datasets suggest that the LUCA was not a thermophile, and that many features of prokaryote biochemistry are in fact derived, with many ancestral pathways having been retained in the eukaryote lineage. This picture challenges the standard assumption that prokaryotes in extreme environments are indicative of the most ancient forms of life on Earth, and has serious implications for the suggestion that life on Earth may have evolved elsewhere.