Scientists have finally uncovered how and why a mysterious “third eye” buried deep within the human head evolved, revealing its crucial role after millions of years of evolutionary change.
Researchers from the United Kingdom and Sweden have identified this ancient organ as the composite ancestral median eye. Their findings suggest that the species from which humans evolved depended heavily on this central structure after losing their two side eyes approximately 500 million years ago.
According to the study, ancient non-vertebrate species—creatures without backbones—burrowed underground and lost functionality of their normal eyes. This forced them to rely on the central organ for light detection.
The organ, known medically as the pineal gland, has been documented by physicians since ancient Greek times. However, it was previously thought to have evolved independently from regular eyes. The new research establishes a direct link to how our distant ancestors used their vision.
Though remnants of this third eye remain inside human skulls today, it no longer detects light. Instead, it receives light and darkness information from our regular eyes to help regulate sleep patterns.
The pineal gland produces melatonin, the hormone that signals to the body it’s nighttime and time to rest. This helps synchronize circadian rhythms—the 24-hour biological cycle that promotes sleepiness and influences various physiological processes throughout day and night.
These processes include:
While no longer a functional eye, the gland contains special cells called pinealocytes that release melatonin.
Published in Current Biology, the research aimed to understand how human eyes and retinas evolved over hundreds of millions of years.
Professor Thomas Baden, a neuroscientist at the University of Sussex who led the research, discovered that humanity’s earliest ancestors possessed both side eyes and a central light-sensing structure in their heads.
When early ancestors began burrowing and filter-feeding half a billion years ago, they lost their side eyes and depended primarily on the middle structure to sense direction and distinguish day from night.
Scientists now believe parts of this third eye later evolved and migrated to the sides of the head, developing into the retinas in our eyes today.
Professor Baden explained to BBC Science Focus: “The need to know what time of day it is, or where is up and down if you’re in deep water. That doesn’t go away. So, we speculate that that’s when we lost the original side eyes, but we kept the original median eye, because that’s what it’s good for.”
He added: “Therefore, the retina predates the eye, if that makes sense.”
The international team didn’t conduct new experiments or scans. Instead, they carefully reviewed existing studies and genetic data from animals including fish and lampreys to understand how this eye still serves purposes in human-related species.
The results transformed scientific understanding by proving that retinas and the pineal gland originated from the same ancient structure, rather than evolving separately.
Certain species still possess visible third eyes, including New Zealand’s tuatara reptile. This organ has a lens and retina similar to normal eyes.
The tuatara’s third eye doesn’t create detailed images but detects light changes, particularly overhead light from the sky. This helps regulate circadian rhythms and manage daily behaviors like sun-basking or hiding.
While no credible scientific evidence supports it, long-standing beliefs connect the human “third eye” to supernatural abilities including:
This concept is strongly tied to Hindu and yogic traditions, where the third eye links to the Ajna chakra, an energy center in the human body. Ancient yoga texts describe awakening this chakra as unlocking clairvoyance, telepathy, and spiritual insight.
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