Deep in the center of your brain sits a rice-sized, pine cone shaped organ that once looked up at the prehistoric sky. This episode follows the pineal gland from ancient medicine to modern chronobiology. Herophilus imagined it as a valve controlling the flow of vital spirits, or pneuma, through the brain’s ventricles. Galen, who called it konarion, argued it could not move and so was just a support for blood vessels. In the 17th century René Descartes noticed it was the brain’s only unpaired structure and named it the principal seat of the soul, and later Madame Blavatsky linked it to the third eye.

Biology turned out stranger than mysticism. The pineal gland began as a light-sensing organ, and in some frogs, lizards and the tuatara of New Zealand it still forms a parietal eye with a cornea, lens and retina. In 1958 Yale dermatologist Aaron B. Lerner, chasing a skin disease cure, isolated melatonin. The episode traces the looping nerve route that tells the gland when it is dark, and why this barely protected organ slowly calcifies.

• H.P. Lovecraft’s story From Beyond imagined a machine that stimulates the pineal gland to reveal other dimensions
• Fossil Devonian fishes show two holes in the skull, evidence of paired eyes that fused into one structure
• A 1917 finding that cow pineal extract lightened frog skin set Lerner on the path to melatonin
• The signal runs from the retina to the suprachiasmatic nucleus, down through the spinal cord and superior cervical ganglia, and back up
• Brain sand, or corpora arenacea, shows on skull X-rays and is estimated in about 40 percent of Americans by age 17