The human parathyroid glands, which regulate the level of calcium in the blood, probably evolved from the gills of fish, say researchers.
Fish use gills to breathe
The gills of ancestral marine creatures were used to regulate calcium levels.
A team from King's College London believe that they were internalised, rather than lost, when four-limbed, land-living animals evolved.
The research is published in Proceedings of the National Academy of Sciences.
Calcium plays a key role in many physiological processes, such as muscle contraction, blood coagulation and signalling by nerve cells.
In humans, calcium levels are regulated by the parathyroid glands - situated near the thyroid gland in the neck - which secrete parathyroid hormone if the calcium concentration in the blood falls too low.
This hormone then causes the release of calcium from bone, and increases its reuptake in the kidney, raising the calcium levels back to normal.
Fish do not have parathyroid glands.
Instead they increase their internal calcium concentration by using their gills to take up calcium from the surrounding water.
Researcher Professor Anthony Graham said: "As the tetrapod parathyroid gland and the gills of fish both contribute to the regulation of extracellular calcium levels, it is reasonable to suggest that the parathyroid gland evolved from a transformation of the gills when animals made the transition from the aquatic to the terrestrial environment.
"This interpretation would also explain why the parathyroid gland is positioned in the neck.
"If the gland had emerged from scratch when tetrapods evolved it could, as an endocrine organ, have been placed anywhere in the body and still exert its effect."
The researchers supported their theory by carrying out experiments comparing the parathyroid gland of chickens and mice and the gills of zebrafish and dogfish.
They found both develop from the same type of tissue in the embryo, called the pharyngeal pouch endoderm.
Both structures also express a gene called Gcm-2, which is crucial for their proper development.
The researchers also found a gene for parathyroid hormone in fish, and they discovered that this gene is expressed in the gills.
Professor Graham said: "The parathyroid gland and the gills of fish are related structures and likely share a common evolutionary history.
"This new research suggests that in fact, our gills are still sitting in our throats - disguised as our parathyroid glands."