The convergence of ancestral sequences independently constructed from different branches of a phylogenetic tree can be used as a test of homology of data sequences. This criterion has shown that all phenylalanine tRNAs are related to a common ancestor, whereas eukaryotic and prokaryotic tyrosine tRNAs may have independent origins. All glycine tRNAs share a common ancestor. The glycine tRNA family splits according to the purine or pyrimidine nature of the first anticodon base prior to the divergence of eukaryotes and prokaryotes. The structural similarity between some prokaryotic glycine and and valine tRNAs is the result of their derivation from a common ancestor that existed previous to the divergence of the different glycine tRNAs. These results support models of genetic code evolution involving the incremental elaboration of earlier, simpler codes.