Scientists have discovered that a 'third eye' buried in the middle of the human head still plays a key role after millions of years of evolution. Researchers from the UK and Sweden called this ancient organ the composite ancestral median eye, and they now believe the species humans evolved from relied on this eye after losing their two side eyes roughly 500 million years ago.
Ancient Origins of the Third Eye
According to their study, these ancient non-vertebrate species, meaning creatures without a backbone, burrowed underground and lost the use of their normal eyes, leaving them to depend on this central eye for sensing light. Today, the remnants of the third eye can still be found inside human skulls. It no longer senses light, but still receives information about light and darkness from our eyes and uses this to help regulate when people sleep.
The Pineal Gland's Role in Sleep
Specifically, the third eye produces the hormone melatonin, the chemical signal that tells the body it is night and it needs to rest. This helps synchronize the circadian rhythms, the 24-hour cycle which promotes sleepiness and influences the body's other physiological processes during the day and night. This includes controlling the reproductive system, immune health, and some scientists theorize even affects our mood and ability to control body temperature.
This organ is now called the pineal gland. While it is no longer a real eye, it is made of special cells called pinealocytes that release melatonin. Scientists have unlocked the evolutionary timeline of the human 'third eye,' an ancient organ that non-vertebrate ancestors relied on and still exists in the human skull.
Evolutionary Insights from the Study
The goal of the new study, published in Current Biology, was to figure out how the human eye and retina evolved over hundreds of millions of years. Researchers led by Professor Thomas Baden, a neuroscientist at the University of Sussex, discovered that the world's earliest human ancestors had both side eyes and a middle light-sensing structure in their heads. When some of these early ancestors began burrowing and filter-feeding half a billion years ago, they lost their side eyes and came to rely mainly on this middle structure to sense direction and to know when it was day or night on the surface.
Much later, scientists believe parts of that third eye evolved and moved to the sides of the head, developing into the vital nerves called retinas we have in our eyes today. Baden told 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.' 'Therefore, the retina predates the eye, if that makes sense,' the scientist added.
Methodology and Findings
The international team did not perform new experiments or take any scans themselves. Instead, they carefully reviewed several existing studies and genetic data from animals such as fish and lampreys to see how this eye still serves a purpose in species related to humans. The results have changed what scientists understand about this mysterious eye by proving that our retinas and the pineal gland both came from the same ancient structure, rather than evolving separately.
Modern-Day Relevance and Cultural Beliefs
Some species still have a visible third eye, including a reptile from New Zealand called the tuatara, and the organ has a lens and retina just like their normal eyes. Its main job is not to see detailed images, but to detect changes in light, especially overhead light from the sky. This helps the tuatara regulate its circadian rhythms and manage daily behaviors like knowing when to bask in the sun or hide.
While there is no credible scientific evidence to prove it, there has also been a long belief that the human 'third eye' also has a connection to supernatural abilities, such as intuition, inner vision and psychic perception. This power has been strongly tied to traditions in Hindu and yogic culture, where the third eye is connected to the 'Ajna chakra,' an energy center in the human body. Ancient texts focusing on the practice of yoga described the awakening of this chakra as being able to unlock clairvoyance, telepathy and spiritual insight.
