The evolution of the genetic code took place in an anaerobic environment.

We have compared orthologous proteins from an aerobic organism, Cytophaga hutchinsonii, and from an obligate anaerobe, Bacteroides thetaiotaomicron. This comparison allows us to define the oxyphobic ranks of amino acids, i.e. a scale of the relative sensitivity to oxygen of the amino acid residues. The oxyphobic index (OI), which can be simply obtained from the amino acids' oxyphobic ranks, can be associated to any protein and therefore to the genetic code, if the number of synonymous codons attributed to the amino acids in the code is assumed to be the frequency with which the amino acids appeared in ancestral proteins. Sampling of the OI variable from the proteins of obligate anaerobes and aerobes has established that the OI value of the genetic code is not significantly different from the mean OI value of anaerobe proteins, while it is different from that of aerobe proteins. This observation would seem to suggest that the terminal phases of the evolution of genetic code organization took place in an anaerobic environment. This result is discussed in the framework of hypotheses suggested to explain the timing of the evolutionary appearance of the aerobic metabolism.