  We investigated the error-minimization properties of putative primordial
codes that consisted of 16 supercodons, with the third base being completely
redundant, using a previously derived cost function and the error minimization
percentage as the measure of a code's robustness to mistranslation. It is shown
that, when the 16-supercodon table is populated with 10 putative primordial
amino acids, inferred from the results of abiotic synthesis experiments and
other evidence independent of the code evolution, and with minimal assumptions
used to assign the remaining supercodons, the resulting 2-letter codes are
nearly optimal in terms of the error minimization level. The results of the
computational experiments with putative primordial genetic codes that contained
only two meaningful letters in all codons and encoded 10 to 16 amino acids
indicate that such codes are likely to have been nearly optimal with respect to
the minimization of translation errors. This near-optimality could be the
outcome of extensive early selection during the co-evolution of the code with
the primordial, error-prone translation system, or a result of a unique,
accidental event. Under this hypothesis, the subsequent expansion of the code
resulted in a decrease of the error minimization level that became sustainable
owing to the evolution of a high-fidelity translation system.
