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"Why did life settle on 20 amino acids?" asks Ryan Mehl, Ph.D., previously a researcher at the Scripps Research Institute in La Jolla, Calif., and now on the faculty of Franklin & Marshall College in Lancaster, Pa. "Would more amino acids give you a better organism -- one that could more effectively adapt if placed under selective pressure?"

No, it'll just give you a Cadillac with those stuck-on gold carriage lights.

Simply having another amino acid available doesn't mean that it will be used unless a particular pre-existing gene codes for its use in a protein. The two ways a pre-existing gene could code for its use are 1. a spontaneous point mutation (or several of them, depending on how different the codon is from those for the 20 standard AAs) results in the amino acid substitution in the pre-existing gene, 2. someone deliberately engineers the mutations into a specific gene. The significance of this still has to be seen in the context of standard molecular biological techniques in which amino acid substitutions, deletions, or insertions are cloned into existing genes all the time in order to elucidate the wild type function of the protein in question. The story above would sort of be like someone deciding to include some completely unrelated building materials along with those for a pre-fab house in the hope of seeing whether or how the contractor will incorporate them into pre-existing building plans that don't specify their use.