The Trend of Amino-Acid Gain is Consistent with a Doublet Code

The present-day genetic code, while universal, is not the result of a “frozen accident”, but has evolved over time. Analysis of amino-acid frequency has recently identified amino acids that were recruited early and late into the genetic code. We use this observation to lend support to a hypothesis that the current quaternary triplet genetic code comes from an earlier quaternary doublet code, and before that from a binary singlet code.