  The genetic code is nearly universal, and the arrangement of the codons in
the standard codon table is highly non-random. The three main concepts on
origin and evolution of the code are the stereochemical theory; the coevolution
theory; and the error minimization theory. These theories are not mutually
exclusive and are also compatible with the frozen accident hypothesis.
Mathematical analysis of the structure and possible evolutionary trajectories
of the code shows that it is highly robust to translational error but there is
a huge number of more robust codes, so that the standard code potentially could
evolve from a random code via a short sequence of codon series reassignments.
Thus, much of the evolution that led to the standard code can be interpreted as
a combination of frozen accident with selection for translational error
minimization although contributions from coevolution of the code with metabolic
pathways and/or weak affinities between amino acids and nucleotide triplets
cannot be ruled out. However, such scenarios for the code evolution are based
on formal schemes whose relevance to the actual primordial evolution is
uncertain, so much caution in interpretation is necessary. A real understanding
of the code's origin and evolution is likely to be attainable only in
conjunction with a credible scenario for the evolution of the coding principle
itself and the translation system.
