[This question](https://biology.stackexchange.com/questions/651/minimal-number-of-amino-acids-required) got me thinking about amino acids and the ambiguity in the genetic code. With 4 nucleotides in RNA and 3 per codon, there are 64 codons. However, these 64 codons only code for 20 amino acids (or 22 if you include [selenocysteine](http://en.wikipedia.org/wiki/Selenocysteine) and [pyrrolysine](http://en.wikipedia.org/wiki/Pyrrolysine)), so many of the amino acids are coded by multiple codons.

Is there any hypothesis as to why there are only 22 amino acids and not 64? Is it possible that there were 64 (or at least more than 22) at an earlier time?