A collection of ancient enzymes was studied using Bayesian Phylogenetics methodology to estimate the distribution of amino acids at the root of the tree of life under the assumption that the most conserved regions should best reflect the prebiotic availability of amino acids. Different measures of conservation were compared with the putative order of appearance of the amino acids on earth, and a generally positive association was found. This supports other independent lines of research on this topic indicating that more conserved sites contain older amino acids, such as glycine and alanine.