THs as signaling molecules may have arisen opportunistically in
the process of metabolizing toxic iodocarbons or binding free and
reactive iodine species to organic molecules. After all, the basis
for thyroid hormone synthesis in vertebrates is the iodination of
tyrosine residues and the cross-linking of these iodinated tyrosines, reactions that can both be performed by a diversity of peroxidases from both plants and animals (Heyland et al., 2006a).
Therefore, the question of endogenous synthesis becomes less of
a problem to explain compared to the question of how THs evolved
into an endogenous signal transduction system. Below we will
briefly discuss one possible scenario involving the evolution of
THs as ligands for nuclear hormone receptors via genetic
assimilation.
Similarly