  The genetic code is profoundly shaped by an origin in ancient RNA-mediated
interactions, needing an extended development to reach the Standard Genetic
Code (SGC). That development can serially use RNA specificities, a
ribonucleopeptide transition (RNPT), finally code escape and diaspora. An index
of evolutionary plausibility based on least selection takes simultaneous
account of speed and accuracy of evolution, identifying favored evolutions.
Combining RNA world specificities allowed convergence of early coding to SGC
assignments. Secondly, this was sufficient to launch a post-RNA-world RNPT. The
RNPT allowed biosynthesis of complex amino acids, depending heavily on late
code fusions between coexisting independent codes. Thirdly, escape from
fluctuating, but highly-evolved codes of the RNPT applied a near-ideal
selection for fastest-evolving and most accurate/useful genetic codes.
Concurrently, a code and its microbial carrier suited to a free-living
existence necessarily evolved. The established unity of life on Earth likely
traces to SGC ascendancy during escape from the RNPT, and code diaspora.
