  A primordial genetic code is proposed, having only four codons assigned, GGC
meaning glycine, GAC meaning aspartate/glutamate, GCC meaning alanine-like and
GUC meaning valine-like. Pathways of ambiguity reduction enlarged the codon
repertoire with CUC meaning leucine, AUC meaning isoleucine, ACC meaning
threonine-like and GAG meaning glutamate. Introduction of UNN anticodons, in a
next episode of code evolution in which nonsense elimination was the leading
theme, introduced a family box structure superposed on the original mirror
structure. Finally, growth rate was the leading theme during the remaining
repertoire expansion, explaining the ordered phylogenetic pattern of
aminoacyl-tRNA synthetases. The special role of natural aptamers in the process
is high-lighted, and the error robustness characteristics of the code are shown
to have evolved by way of a stepwise, restricted enlargement of the tRNA
repertoire, instead of by an exhaustive selection process testing myriads of
codes.

Populations and Evolution