  Organisms leave a distinctive chemical signature in their environment because
they synthesize those molecules that maximize their fitness. As a result, the
relative concentrations of related chemical monomers in life-bearing
environmental samples reflect, in part, those compounds' adaptive utility. In
contrast, rates of molecular synthesis in a lifeless environment are dictated
by reaction kinetics and thermodynamics, so concentrations of related monomers
in abiotic samples tend to exhibit specific patterns dominated by small, easily
formed, low-formation-energy molecules. We contend that this distinction can
serve as a universal biosignature: the measurement of chemical concentration
ratios that belie formation kinetics or equilibrium thermodynamics indicates
the likely presence of life. We explore the features of this biosignature as
observed in amino acids and carboxylic acids, using published data from
numerous studies of terrestrial sediments, abiotic (spark, UV, and high-energy
proton) synthesis experments, and meteorite bodies. We then compare these data
to the results of experimental studies of an evolving digital life system. We
observe the robust and repeatable evolution of an analogous biosignature in a
digital lifeform, suggesting that evolutionary selection necessarily constrains
organism composition and that the monomer abundance biosignature phenomenon is
universal to evolved biosystems.
