 In this case, a simple Bayesian argument can be used to show that current belief in the universality of the genetic code is well justified. To see this, it must first be noted that the code is arbitrary; there is no necessity in the specific mapping of amino acids to codons in the current standard genetic code. A second premise is that there is an enormously large number of different possible genetic codes. Now, we may ask what the probability is that the exact same genetic code evolved independently in so many different species as have been investigated at the molecular level to date. Surely, this probability is vanishingly small. The probability of coincidence of genetic codes is much, much larger on the assumption that the code did not arise independently in the different species. On this hypothesis, the code arose only once, and the coincidence is due to common descent. However, once a specific genetic code is in place, it is very difficult to change, because any mutational change in the amino acid-codon assignment tends to affect a larger number of different proteins and thus threatens to kill the organism. It is for this reason that Jacques Monod (1974) has described the code as a “frozen accident”. Its mutation rate is just very, very low. These considerations together support the hypothesis of universality in the case of the genetic code.