  Of the twenty amino acids used in proteins, ten were formed in Miller's
atmospheric discharge experiments. The two other major proposed sources of
prebiotic amino acid synthesis include formation in hydrothermal vents and
delivery to Earth via meteorites. We combine observational and experimental
data of amino acid frequencies formed by these diverse mechanisms and show
that, regardless of the source, these ten early amino acids can be ranked in
order of decreasing abundance in prebiotic contexts. This order can be
predicted by thermodynamics. The relative abundances of the early amino acids
were most likely reflected in the composition of the first proteins at the time
the genetic code originated. The remaining amino acids were incorporated into
proteins after pathways for their biochemical synthesis evolved. This is
consistent with theories of the evolution of the genetic code by stepwise
addition of new amino acids. These are hints that key aspects of early
biochemistry may be universal.
