The rice-sized pineal gland is the leftover of an ancient eye
Descartes thought the pineal gland was where soul and body met. Evolution tells a stranger story. This rice-grain structure deep in the brain appears to descend from a light-sensing organ on top of the head, and some lizards and frogs still carry a working version, often called a third eye.
In humans the gland is reddish-grey, cone-shaped and roughly 5 to 8 millimetres long, sitting near the brain's centre where the two halves of the thalamus meet. Galen, in the 2nd century, could find no purpose for it and assumed it propped up surrounding tissue; he named it after a small pine cone, a name later rendered in Latin as pinealis. Unusually, it sits outside the blood-brain barrier, and its blood supply is second only to the kidney's.
Its main job is making melatonin, a hormone derived from serotonin that helps set sleep patterns. Light suppresses production and darkness releases it. The signal begins with light-sensitive ganglion cells in the retina, passes through the hypothalamus and a relay in the neck called the superior cervical ganglion, and only then reaches the gland. The organ grows until about age one or two, and the high melatonin levels of childhood are thought to hold back sexual development; output falls at puberty.
Calcium deposits, nicknamed brain sand, build up in the gland and often show on skull X-rays. Estimates put calcification at about 40 percent of Americans by age seventeen, and it has been seen in children as young as two. Calcification impairs melatonin production, though whether it causes sleep problems remains unsettled.
The evolutionary trail is the most surprising part. Early vertebrates probably had a pair of light sensors on top of the head, as lampreys do today; some Devonian fish skulls show two openings for them, and the fossil Haikouichthys may have had four eyes. In living four-limbed animals, the parietal eye and pineal gland likely descend from the left and right halves of that pair. Pineal cells in many non-mammals closely resemble retinal photoreceptors. Some lineages lost the organ entirely, including crocodilians, sloths, manatees and pangolins, while walruses and some seals have exceptionally large ones.
Source: Pineal gland
