Evolutionary Significance of Iodine

The significance of inorganic and organic forms of iodine in the evolution of plants and animals is reviewed. Iodine is one of the most electron-rich atoms in the diet of marine and terrestrial organisms, and it enters cells via iodide transporters. Iodide, which acts as a primitive electron donor through peroxidase enzymes, has an ancestral antioxidant function in all iodide-concentrating cells from primitive marine algae to more recent terrestrial vertebrates. Similarly, thyroxine and iodothyronines show antioxidant activities through deiodinase enzymes. About 500-600 million years ago, in parallel with the evolution of the primitive brain in marine animals, thyroid cells originated from the primitive gut in vertebrates, migrated, and specialized in the uptake and storage of iodo-compounds in a novel follicular “thyroidal” structure, an adaptation that enabled the transition from the iodine-rich ocean to the iodine-deficient terrestrial environment.