Numerous aminoacyl-tRNA synthetase sequences have been aligned by comp
uter and phylogenetic trees constructed from them for the two classes
of these enzymes. Branching orders based on a consensus of these trees
have been proposed for the two groups. Although the order of appearan
ce can be rationalized to fit many different scenarios having to do wi
th the genetic code, the invention of a system for translating nucleic
acid sequences into polypeptide chains must have pre dated the existe
nce of these proteins. In the past, a variety of schemes has been prop
osed for matching amino acids and tRNAs. Most of these have invoked di
rect recognition of one by the other, whether or not the anticodon was
involved. Often ignored is the possibility of a non-protein (presumab
ly RNA) matchmaker for bringing the two into conjunction. If such had
been the case, then the contemporary aminoacyl-tRNA synthetases could
have entered the system gradually, each specific type replacing its ma
tchmaking RNA counterpart in turn. A simple displacement scheme of thi
s sort accommodates the existence of two different families of these e
nzymes, the second being introduced well before the first had undergon
e sufficient genetic duplications to specify the full gamut of amino a
cids. Such a scheme is also consistent with similar amino acids often,
but not always, being the substrates of enzymes with the most similar
amino acid sequences.