The thyroid, a vital endocrine organ in vertebrates, plays a key role in regulating metabolism and supporting growth. The first gland of both the nervous system and endocrine system to mature during an embryo's development, it initially evolved more than 500 million years ago out of a "primitive " precursor organ in chordates known as the endostyle. Now, using lamprey as a model organism, Caltech researchers have discovered how the evolutionary acquisition of a certain kind of stem cell, called