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 nucleotide combinations. Therefore some amino acid are coded by several different nucleotide combinations. While Leucine is coded by 6 codons, Tryptophan 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 nucleic acid that does not modify the coded amino-acid (third-position wobble).
Therefore, I would expect that the genetic code cannot not entirely be explained by "it happened to occur this way the first time (at the origin of life or in the last universal common ancestor [LUCA]) and it never changed".
So, my questions are:
Why some amino acids are coded by a big set of codons while others are coded only by one codon?
And more specifically, why methionine is coded by only one codon (start codon = AUG) where all other amino-acids (except Tryptophan, Selenocysteine and Pyrrolysine) are coded for by more than one codon?
Or more broadly: Why (by which mechanisms, which selective pressure if any, ...) has the genetic code evolved the redundancies?