Imagine discovering that hidden deep inside your brain is something that may once have been a real eye. Not a spiritual ‘third eye’ from movies and myths, but an actual light-sensing organ connected to your ancient ancestors. Scientists now believe the tiny pineal gland inside the human brain could be the remnant of a real third eye that existed hundreds of millions of years ago.
A new theory published in Current Biology is trying to explain how this strange organ evolved, and why humans and other vertebrates ended up with such unusual eyes compared to the rest of the animal kingdom. And surprisingly, a reptile from New Zealand may hold the biggest clue. Meet the reptile with a real third eye
Tuatara may look like an ordinary lizard at first glance, but it has something extraordinary on top of its head, a functioning third eye. This special eye, called the ‘parietal eye,’ has its own lens, retina, and nerve connections to the brain, much like the two regular eyes on the sides of its head. Young tuataras are even born with the eye clearly visible before it later becomes covered by scales as they grow older. Scientists say humans also carry a version of this ancient structure. Ours is called the pineal gland, a tiny organ buried deep inside the brain that helps control sleep cycles and how the body reacts to light and darkness. Why are human eyes so unusual? Most animals on Earth follow a fairly similar eye design. Creatures like flies, crabs, and octopuses use one type of light-detecting cell in their main eyes, while another type remains inside the brain for simpler tasks like sensing day and night. But vertebrates, including humans, birds, reptiles, and fish, are different. Our eyes are built using a strange mix of these systems, and for years, scientists struggled to understand why evolution took such an unusual path. Thomas Baden said: What is the original solution to vision, and to what extent have different species just copied or modified it to make it their own?
“What really are the patterns? As you do this over time, you start to wonder, what is the original eye?” Also read: Your iPhone may soon lock itself if someone snatches it: Apple may use multiple signals
Strange ancestor that may explain everything According to the new theory, the answer goes back around 575 million years. At that time, our distant ancestors were tiny worm-like creatures living on the ocean floor.
Scientists believe these creatures originally had two side eyes for navigation and a simpler eye-like structure on top of the head to detect light and direction. Researchers think these ancestors began burying themselves in sand and mud to feed. Since they no longer needed to navigate much, their side eyes slowly disappeared over time. However, the light-sensing structure on top of the head remained useful because it could still help detect day and night or determine which direction was upward. One eye may have evolved into three
Later, these ancestors returned to open water and needed vision again for movement and survival. But there was a problem; they had already lost their original side eyes. Scientists believe evolution solved this by modifying the remaining light-sensitive organ on top of the head. Over millions of years, this structure slowly became more advanced and eventually formed extensions that moved toward the sides of the head. These later developed into the complex eyes that vertebrates have today. The original structure itself never fully disappeared. Instead, it remained behind as the pineal gland found in almost all vertebrates today. In some animals, like tuataras, it still resembles a proper eye. In fish, it can directly sense light. In mammals like humans, it lost direct contact with sunlight but still controls biological rhythms through signals received from the eyes. Scientists say the retina may be older than the eye itself
One of the most fascinating parts of the theory is the idea that the retina, the light-sensitive tissue inside the eye, may actually be older than the eye as we know it. Baden says the original structure on top of the head was probably not a complete eye, but rather several groups of light-sensitive cells working together. “The retina predates the eye, if that makes sense,” Baden said. “I always thought that was a cute tagline.” Also read: AI disrupted the budget of tech giant companies: Microsoft cuts licenses, Uber’s year-long fund cleared in just 4 months
Humans may once have had four eyes As if the story were not strange enough already, another recent study published in Nature suggests our ancient ancestors may once have had four fully developed eyes. Scientists are still testing these theories, and many questions remain unanswered. Studying evolutionary history from more than half a billion years ago is incredibly difficult. Still, researchers believe future studies could soon confirm whether these ideas are correct. Your hidden ‘third eye’ still exists today Even though humans no longer have a visible third eye, the remains of that ancient system may still be sitting quietly inside the brain. The pineal gland no longer sees sunlight directly, but it still helps regulate sleep, body clocks, and responses to light.
So, buried deep inside your skull is a tiny organ that may once have looked toward the sky hundreds of millions of years ago, long before humans even existed.
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