Wong J T, Bronskill P M
J Mol Evol. 1979 Jul 18;13(2):115-25. doi: 10.1007/BF01732867.
The production of some nonproteinous, and lack of production of other proteinous, amino acids in model prebiotic synthesis, along with the instability of glutamine and asparagine, suggest that not all of the 20 present day proteinous amino acids gained entry into proteins directly from the primordial soup. Instead, a process of active co-evolution of the genetic code and its constituent amino acids would have to precede the final selection of these proteinous amono acids.
