It has been suggested that the present-day triplet genetic code evolved from a doublet code when there were fewer amino acids available for primitive protein synthesis.

As determined by comparisons of ancient and recently evolved proteins, cysteine, tyrosine, and phenylalanine appear to be late-arriving amino acids. In addition, they are considered to have been absent in the abiotic Earth. All three of these amino acids have only two codons each, while many others, earlier in origin, have more. Is this mere coincidence, or might there be some underlying explanation?