Background:

Almost all of the important hormones that play a critical role in the human endocrine system are made from either fats or proteins, This is useful because the metabolic precursors required to build these hormones are found within plants and/or animals which are typically available in most human environments.

The thyroid hormones, T3 and T4, on the other hand are unique. While they do have a proteinaceous component they are largely made up of three or four iodine atoms respectively.

It's surprising to me that the regulation of one of the most crucial aspects of human physiology, cellular metabolism, is dependent on the availability of the heavy metalloid element iodine.

So what does regulating metabolism actually mean? Wikipedia says T3 (the active thyroid hormone present at the thyroid hormone receptor in target tissues):

Increases cardiac output
Increases heart rate
Increases ventilation rate
Increases basal metabolic rate
Potentiates the effects of catecholamines (i.e. increases sympathetic activity)
Potentiates brain development
Thickens endometrium in females
Increases catabolism of proteins and carbohydrates^[54]

Despite its important role in regulating the above functions, it turns out iodine just isn't super common in the modern diet. The natural sources are sort of random - the overwhelming leader is kelp followed up by cod, and yogurt/milk products.

If you don't get enough iodine in your diet, you can develop thyroid disease. Because thyroid hormones have such pervasive effects on human physiology, symptoms of disease vary greatly. If your thyroid hormone levels are consistently too low, it can make you gain weight feel tired, lethargic, depressed, and generally awful. When they are consistently too high, you'll probably lose weight, have trouble sleeping, and feel full of nervous energy.

It turns out the availability of the element iodine plays a huge role in human health. Unfortunately, iodine just is not all that common in modern diets. As a public health measure to reduce the incidence of thyroid disease, most countries have (with wildly successful results) authorized the iodination of table salt to help their citizens ingest a sufficient quantity of iodine to keep their thyroid hormones functioning properly.

Finally, and most interestingly - if you look back on the phylogenetic tree of life the first organisms to evolve iodine-regulated metabolic processes were the phylum protochordate, an ancestor to chordates and therefore all vertebrates (Dumont, 2011)

Question:

Why is iodine a required component of all life that has a spinal cord?

I will give a 1000 RSC bounty to whoever can help me understand - 1.) what evolutionary advantage, worth the cost of being dependent on a relatively rare element, is unlocked by the incorporation of iodine into the regulation of cellular metabolism, and 2.) how the development of iodine-regulated metabolism impacted life's evolution from protochordate to modern mammals.

Question

Question

Why is human metabolism dependent on iodine?