If you eat ALL of the organs, and I mean all of them, there is iodine in the meat and particularly the thyroid glands depending on the local soil conditions. I hear the Plains Indians would divide the thyroid of a killed animal up equally so everyone got some. It's possible we evolved in a place where soil iodine was very rich and the animals carried it. There is also evidence hominids have consumed shellfish, another possible source, for 1.94 million years. Plus evidence we were consuming other shore-based foods. The ideal environment for human evolution may have been open grassland with some coastal areas. No other living hominid has the dramatic need for iodine that our brain does, it's one of the unusual things about humans.

Many believe that single celled organisms evolved from the sea to live in collectives on land. These collectives are multicellular organisms, like us! Maybe cavemen wouldn't all have access to sea foods and thus sea minerals, but maybe our original cells did?

"An interesting fact to note is that the mineral composition of modern animals, such as ourselves, is similar to that of these ancient organisms, who originated in the sea. Thus, we can deduce that as animals, and life in general, evolved from the sea."

Endosymbiotic theory maybe? We evolved from something.. so perhaps that is where our need came from?

Like.. a chicken soup. Individually the components for chicken soup: Chicken. Onion. Carrots. Alone they are just that. A single component. But make a soup out of all of these and then it becomes rich, it all comes together. Each element has a contribution just like each cell has an contribution to the body. So all together it works. Maybe way back when something, some organism or bacteria, that used iodine got taken into the body and it was accepted and not rejected. The body thought "hey we can use this, we like what this does for us" so it built itself around it.

There's going to be a million answers for this.. great question!

I think historically it was in our soil in small amounts but over time has been depleted.

These guys attribute it to the soil having been "leached of this element by glaciation, floods or snow water." I could see that being the case, and that we've been just scraping along with suboptimal amounts for a while. Luckily, it's easy to supplement with some kelp here and there and the excess is readily excreted.

If you really want to be pissed at evolution, you should focus on our lack of vitamin C synthesis and our inability to convert acetyl-CoA into pyruvate. I shake my fist at Evolution every day because of these.

Our iodine requirements are one of the supporting arguments for the Aquatic Ape Theory. This theory is that we evolved in coastal regions and ate a lot of seafood such as mussels which are easily gathered at low tide, this would have provided a lot of iodine. The long chain omega-3 fatty acids such as EPA and DHA would have been easily available through gathering shellfish and aided the enlargement of our brains. Once our brains grew enough we could hunt and fish to add more animal foods to our diet, such as ruminants.

The Aquatic Ape Theory gets criticised a lot, mostly poking fun at the name, that it theory is we were practically dolphins. This is wrong, a more accurate name would be the Littoral Ape Theory. Littoral meaning coastal.

Animals that are adapted to being inland, such as sheep hold on to iodine very well but not to minerals such as copper which is readily available in the environment. Where-as humans loose iodine easily and hoard copper, which is why we can have copper toxicities such as Wilson's disease. Excess iodine on the other hand is urinated out. In fact the test for if you have enough iodine in your body involves drinking a set amount of iodine then collecting your urine for 24 hours to measure the iodine in it.

Indigenous hunter-gatherers do fine without iodized salt. Seems reasonable that there is generally enough iodide naturally present to support healthy thyroid function. Or that their diets also free from compounds that interfere with iodine adsorption.

Also look at the African Rift Valley lakes, high concentration of minerals, and also iodide. Likely nearby where significant amounts of human evolution took place. There's never been significant evolutionary pressure to evolve away from iodide requirements.

When pregnant women get their weird cravings, I think it has a lot to do with stockpiling rare minerals like this, make them accessible to the fetus, and it could be that the child is more or less "set for life" after that, comparatively speaking. It could be that no iodine gets excreted from primitive people adhering to a healthy paleo diet - how would we know? We're told by our western doctors that we need more iodine, but they only study a populace of poisoned patients.