Transfer RNA and the origins of diversified life.

The evolution of the transfer RNA (tRNA) molecule is controversial but embeds the history of protein biosynthesis, the genetic code, and the origins of diversified life. A new phylogenetic method based on RNA structure that we developed provides new lines of evidence to support the genome tag hypothesis and confirms that the 'top half' of tRNA is more ancient than the 'bottom half'. Timelines of amino acid charging function generated from constraint analyses showed that selenocysteine, tyrosine, serine, and leucine specificities were ancient, while those related to asparagine, methionine, and arginine were more recent. The timelines also uncovered an early role of the second and then first codon bases, identified codons for alanine and proline as the most ancient, and revealed important evolutionary take-overs related to the loss of the long variable arm of tRNA. Furthermore, organismal timelines showed Archaea was the oldest superkingdom, followed by viruses, and superkingdoms Eukarya and Bacteria in that order supporting conclusions from recent phylogenomic studies of protein architecture. Strikingly, results showed that the origin of viruses was not only ancient but was linked to Archaea, supporting the notion that the archaeal lineage is the most ancient on earth and its origin predated diversification of tRNA function and specificity.