After looking at this question, some other questions poped in my mind.

The DNA code is redundant, there are 20 amino acids for 64 possible nucleases combinations. Therefore some amino acid are coded by several different nucleases combinations (=codons). While the Leucine is coded by 6 codons, the Trp is coded only by 1 codon. It is worth saying the set of codons that code for one given amino acid tend to look alike each other more than random. Usually it is only the last nucleobasis for which changement does not modify the coded amino-acid.

Therefore, I would expect that the genetic code cannot not entirely be explained by "it happened to occur this way the first time and it never changed".

So, my questions are: How (by which mechanisms) does the genetic code evolved? Why some amino acid are coded by a big set of codons while others are coded only by one codon? And more specifically, why the start codon is coded by only one codon where all amino-acid (except the Trp but including the stop codon) are coded by more than one codon?

I should specify that my question is neither "why AUG and not AGG?", nor "why methionine rather than something else"