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.