The parathyroid glands, four little nodules located in your neck, play a huge role in regulating extracellular calcium homeostasis. By releasing something called calcium-sensing receptor, they detect the calcium level in your blood and if need be, send out parathyroid hormone to open up internal calicum stores and equalize levels.

According to a new study by Okabe & Grahm in PNAS, detailed similarity in the expression of numerous genes suggests "the parathyroid gland of tetrapods and the gills of fish most likely share a common evolutionary origin."

A BBC report on all this can be found here, and interested parties can, as always, email me for a PDF of the paper itself.