not joined. however, there is only ever a single thyroid gland in most mammals, and the shape found in humans is common to many other species. in larval lampreys, the thyroid originates as an exocrine gland, secreting its hormones into the gut, and associated with the larva's filter-feeding apparatus. in the adult lamprey, the gland separates from the gut, and becomes endocrine, but this path of development may reflect the evolutionary origin of the thyroid. for instance, the closest living relatives of vertebrates, the tunicates and amphioxi (lancelets), have a structure very similar to that of larval lampreys (the endostyle), and this also secretes iodine-containing compounds, though not thyroxine. thyroxine is critical to metabolic regulation, and growth throughout the vertebrate clade. iodine and t4 trigger the change from a plant-eating water-dwelling tadpole into a meat-eating land-dwelling frog, with better neurological, visuospatial, smell and cognitive abilities for hunting, as seen in other predatory animals. a similar phenomenon happens in the neotenic amphibian salamanders, which, without introducing iodine, don't transform into land-dwelling adults, and live and reproduce in the larval form of aquatic axolotl. among amphibians, administering a thyroid-blocking agent such as propylthiouracil (ptu) can prevent tadpoles from metamorphosing into frogs; in contrast, administering thyroxine will trigger metamorphosis. in amphibian metamorphosis, thyroxine and iodine also exert a well-studied experimental model of apoptosis on the cells of gills, tail, and fins of tadpoles. iodine, via iodolipids, has favored the evolution of terrestrial animal species and has likely played a crucial role in the evolution of the human brain.