Goiter is usually not immediately lethal so for most of human history, people afflicted with it just kind of lived with it which likely meant there wasn't that

Goiter is usually not immediately lethal so for most of human history, people afflicted with it just kind of lived with it which likely meant there wasn't that much selective pressure against it. Consuming thyroid glands of terrestrial animals (usually ruminants) was an effective treatment of iodine dificiency before the modern nutritional sciences discovered the relationships between goiter and iodine deficiency: https://en.wikipedia.org/wiki/Goitre#History https://en.wikipedia.org/wiki/Goitre#History Ruminants are more efficient at absorbing nutrients from plant material due their digestive system, and this is how they get iodine in non-coastal areas. All vertebrates have thyroid glands and there is a very similar organ in marine chordates: https://en.wikipedia.org/wiki/Endostyle https://en.wikipedia.org/wiki/Endostyle. So this is an organ that likely emerged when our biological ancestors still lived in a iodine-rich environment.