English: Over three billion years ago, blue-green algae were the first living Prokaryota to produce toxic oxygen in the atmosphere. About 700 million years ago (m/y/a) thyroxine (T4) is present in fibrous exoskeletal scleroproteins of the lowest invertebrates (sponges and corals) without showing any hormonal

action. About 500-400 m/y/a some primitive marine fishes started to emerge from the iodine-rich sea (60

ug/L) and transferred to iodine-deficient fresh water (5-0.2 ug/L) and about 400-300 m/y/a these vertebrates

evolved in amphibians and reptiles and transferred to I-deficient land. Living organisms were confined to the sea-water for more than 3 billion years. When about 500 million years ago plants and animals began to transfer from the sea to estuaries, rivers and land, environmental deficiency of antioxidant minerals (iodine, selenium, etc.) was a challenge to the evolution of terrestrial

life. In iodine-deficient mainland, the two types of animal and plant cells have followed two different and

opposite paths: the animal cell has used the " thyroid follicle " as reservoir of iodine. On the contrary,plant cells eliminated iodine from its own metabolism. Therefore the vertebrates needed a

new follicular organ: the thyroid gland, as reservoir of antioxidant iodide. These vertebrates started to

use its primitive T4 as transporter, into the peripherical cells, of antioxidant iodide and T3. The

remaining T3 became the active hormone in the metamorphosis and thermogenesis for a better adaptation to

terrestrial environment: fresh water, atmosphere, gravity, temperature and diet.(Venturi, 2011)