Catecholamine Chemistry Dictated the Divergence of Melanization and Signaling Processes During Animal Evolution

Advanced vertebrates (gnathostomes) differ from the rest of the animal tree of life by their use of an elaborate endoskeleton and the deployment of the catecholamine neurotransmitters epinephrine and norepinephrine. Here we propose that the coordinated origins of these seemingly unrelated signature traits are explained by the varied propensities of different catecholamines to oxidize and polymerize into the light absorbent and structurally stable biopolymer melanin. Compared to dopamine and its precursor DOPA (L-3,4-dihydroxyphenylalanine), norepinephrine and epinephrine are much less susceptible to polymerization. The latter were therefore likely adaptively favored for peripheral signaling in vertebrates. In addition to the chemical evidence, we explore how peripheral nervous system development correlated with changes in gene family content, immune response organization, and internal support structure elaboration during early vertebrate evolution.