The genetic code evolved around transfer RNA (tRNA).
Generally, the complexity of the genetic code in mRNA (messenger RNA) has been considered to be 4x4x4=64 codons. 
The complexity of the genetic code in tRNA, however, is 4x4x2=32 anticodons.
The problem of evolution of the genetic code, therefore, has been cut in half by Dr. Burton.
The Burton lab has described how the genetic code evolved and why the complexity of the standard genetic code is limited to 20 encoded amino acids + stops.
The genetic code was built via tRNA charging errors and evolved to closure and universality through fidelity mechanisms.