This article proposes that the genetic code was not fully formed before the divergence of life into three kingdoms. Rather, at least arginine and tryptophan evolved after the diversification of archaea, bacteria and eukaryotes, and were spread by horizontal gene transfer. Evidence for this hypothesis is based on data suggesting that enzymes for biosynthesis of arginine and tryptophan, and for arginine tRNA ligase, have shorter divergence times than the underlying lineages. Also, many of these genes display 'star' phylogenies. This proposal is an extension of the idea that the genetic code was unified because of the evolutionary pressure from horizontal gene transfer. These considerations further undermine the need to postulate the existence of a 'last common ancestor'; a simpler model would be that multiple lineages gave rise to life today.
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