The article, The Finely Tuned Genetic Code by Jonathan M. explains why it is unlikely that life arose through the unguided action of natural law. The article explains that the genetic code is finely tuned for efficiency (it is not random) and it is unlikely this efficiency could have arisen through evolution. It is impossible to explain how the genetic code could have evolved because a change in the code would affect every protein in the cell. For example, if you changed every letter "n" to the letter "p" in an entire book, it would cause many words to be spelled incorrectly. In a living cell, such a change in the genetic code would be catistrophic. This is strong evidence that the genetic code and therefore life itself did not arise through the unguided action of natural laws but is the result of intelligent design. (Read the the linked article for the full argument.)
How Is the Genetic Code Finely Tuned?
As previously stated, the genetic code is degenerate. This means that multiple codons will often signify the same amino acid. This degeneracy is largely caused by variation in the third position, which is recognized by the nucleotide at the 5' end of the anticodon (the so-called "wobble" position). The wobble hypothesis states that nucleotides that are present in this position can make interactions that aren't permitted in the other positions (though it still leaves some interactions that aren't allowed).
But this arrangement is far from arbitrary. Indeed, the genetic code found in nature is exquisitely tuned to protect the cell from the detrimental effects of substitution mutations. The system is so brilliantly set up that codons differing by only a single base either specify the same amino acid, or an amino acid that is a member of a related chemical group. In other words, the structure of the genetic code is set up to mitigate the effects of errors that might be incorporated during translation (which can occur when a codon is translated by an almost-complementary anti-codon).
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The Non-Evolvability of the Genetic Code
Changes in codon assignments would be catastrophic to the cell because such a mutation would ultimately lead to changes to the amino acid sequence in every protein produced by the cell. This means that one cannot have a significantly evolving genetic code, though -- it may be granted -- there are one or two minor variations on the standard genetic code. Some have tried to argue around this by positing that the lesser-used codons can be redesignated to a different but related amino acid, thus allowing the genetic code to become optimized. There are, however, significant difficulties with this proposal. For one thing, it seems highly unlikely that by virtue of replacing some of the lesser-used amino acid assignments with a related amino acid that one could attain the level of optimization which we find in the conventional code.
Furthermore, the question is naturally raised as to what selective-utility would be exhibited by the new amino acids. Indeed, they would have no utility until incorporated into proteins. But that won't happen until they are incorporated into the genetic code. And thus they must be synthesized by enzymes that lack them. And let us not forget the necessity for the dedicated tRNAs and activating enzymes which are needed for including them in the code.
One related difficulty with standard evolutionary explanations is that a pool of biotic amino acids substantially less than 20 is liable to substantially reduce the variability of proteins synthesized by the ribosomes. And prebiotic selection is unlikely to sift the variational grist for this trait of amino-acid-optimality prior to the origin of self-replicative life (in many respects, "prebiotic selection" is somewhat oxymoronic).
There is also the added problem of the potential for codon mapping ambiguity. If, say, 80% of the time a particular codon specifies one amino acid and 20% of the time specifies another, this mapping ambiguity would lead to cellular chaos.
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