The pineal evolved from photoreceptive organs: in lampreys, fish, frogs, and many reptiles the parietal or pineal eye sits atop the skull with a lens-like structure and retina-like cells, reading light to set daily and seasonal rhythms. In mammals it lost direct light sensitivity but still converts darkness into melatonin, running circadian timing from inside the skull. The tuatara's parietal eye remains the textbook case. The eye-ancestry is standard comparative endocrinology.
The folk arc: Descartes, in The Passions of the Soul (1649), named the pineal the seat of the soul, the one unpaired structure where a two-sided brain could meet a unitary spirit. Twentieth-century Theosophy fused that with the ajna chakra of Hindu tradition: the third eye as the organ of intuition and vision, calcified by modernity, fluoride in the popular version, reopened by meditation. The fusion is modern; the experiential traditions are old.
The claim, from Blavatsky's Secret Doctrine (1888) through the Monroe and prison-planet literature: the third eye is the one sensor aimed inward, the organ that perceives the architecture of the trap or the exit, and its age-related calcification reads as engineered blindness, a reducing valve installed in hardware. The gland calcifies; the rest is the claim.
Vestige, symbol, or sleeping organ: what exactly did we lose when the midline eye went inside?