The coevolution theory (CET) of the genetic code postulates that the genetic code and codon assignment to amino acid coevolved with amino acid biosynthesis. In particular, the coevolution theory proposes that primordial proteins consisted only of those amino acids readily obtainable from the prebiotic environment, representing about half the twenty encoded amino acids of today, and the missing amino acids entered the system as the code expanded along with pathways of amino acid biosynthesis. Namely, as primordial organisms succeeded to synthetize new amino acids, codon were re-assigned and genetic was reshaped accordingly. The isolation of genetic code mutants, and the antiquity of pretran synthesis revealed by the comparative genomics of tRNAs and aminoacyl-tRNA synthetases, have combined to provide a rigorous proof of the fundamental tenets of the theory.
Recently, Fournier and co-workers casted new doubts on CET investigating Aminoacyl-tRNA synthetases (aaRS) which are universally distributed across cellular life. In particular, sequence reconstructions of the paralog ancestors of isoleucyl- and valyl- RS provide strong empirical evidence that at least for this divergence, the genetic code did not co-evolve with the aaRSs; rather, both amino acids were already part of the genetic code before their cognate aaRSs diverged from their common ancestor.
A co-evolution theory of the genetic code.
Proc Natl Acad Sci USA 1975, 72:1909-1912.
Gregory P. Fournier, Cheryl P. Andam, Eric J. Alm and J. Peter Gogarten
Molecular Evolution of Aminoacyl tRNA Synthetase Proteins in the Early History of Life
Origins of Life and Evolution of Biospheres. Volume 41, Number 6 (2011), 621-632