“An eye for an eye makes the whole world blind” could have been quite true 600 million years ago (MYA), when a single-eyed ancestor of ours—a “cyclops” worm — crawled on the planet. Scientists from Lund University and the University of Sussex report that all vertebrates arose from this ancient one-eyed worm and that the remains of this “median eye” still exist. Many call it the third eye, some suggest it can be ‘activated’ to amp up the senses, but in scientific terminology, it’s just the pineal gland.
How the Worm Ended up with One Eye
This distant ancestor spent most of its time in one place, filtering plankton from seawater. While having one eye may seem primitive, the worm actually has two eyes, like many other animals. As the creature adapted to its stationary lifestyle, two eyes weren’t necessary, and they disappeared. “We don’t know whether the paired eyes in our branch of the evolutionary tree were just light-sensitive cells or simple image-forming eyes. We only know that the organism later lost them,” says Dan-E Nilsson, professor emeritus in sensory biology at Lund University.
However, this is not the last time we encounter paired eyes on this unique worm. Turns out, its evolutionary route had other plans.
Back to Square One: A Pair of Eyes Again
Although it lost its paired eyes, the organism retained a group of light-sensitive cells at the center of its head. These cells eventually formed a simple single eye that could detect light and darkness, aiding in sensing its orientation. Millions of years later, this ancestor became active again and returned to a swimming lifestyle. No longer stationary, the worm needed to develop better vision, and researchers believe that this shift gradually transformed parts of the median eye into paired eyes capable of forming images.
How did the Modern Eye develop?
This unusual back-and-forth in evolution helps explain why vertebrate eyes differ from those of invertebrates such as squids and insects. While our retina, which can be called the ‘film’ of our eyes, is where the light from an object falls to form an image, developed from the brain, the eyes of insects and squid originated in the skin on the sides of their heads.
Rather than evolving directly from earlier eye structures, vertebrate vision has taken an evolutionary detour—rebuilt from the solitary median eye after the loss of the earlier paired eyes. This is inferred from detailed comparisons of light-sensitive cells across different animal groups, including their location and structure within the body. The results also give us our first look into how neural circuits analyze the image in our retina.
The Third Eye: A Memoir or a Myth?
Now, let’s cut to the present.
Believe it or not, a portion of the median eye exists in our body to this date, but it doesn’t impart psychic powers. It is rather a very humble gland, called the pineal gland, located in the brain. It is light-sensitive and produces melatonin, a hormone that regulates the body’s circadian rhythm, which can be summed up as a clock that regulates digestion, temperature, and the sleep-wake cycle. It gets calcified as we age, but a good routine can keep it healthy.
Remember, early to bed and early to rise makes a man healthy, wealthy, and wise. Benjamin Franklin said this in 1735, and he didn’t know he would be the face on a hundred-dollar bill then. Get good sleep, and that keeps you sharp, a lot like Benjamin Franklin. On a side note, know that you have more in common with a cyclops than you ever imagined.
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