Humans have a ‘third eye’ in the brain: Study links it to a 600-million-year-old Cyclops ancestor

Here’s a science story that sounds straight out of Greek mythology, except it’s real, and it’s about us. A new study by researchers at Lund University and the University of Sussex suggests that humans, along with every other creature with a backbone, may descend from a tiny worm-like animal that had just one eye, sitting right on top of its head, much like a cyclops.

And that’s not even the strangest part. According to the research, remnants of this ancient “third eye” may still be sitting inside our brains today, in the form of the pineal gland, the organ that helps control our sleep cycle.

Meet Our Cyclops Ancestor

Nearly 600 million years ago, long before dinosaurs or even fish existed, a small, worm-like creature lived in the ocean. It didn’t move around much, instead, it stayed in one spot and filtered plankton from the water for food.

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Scientists believe this creature originally had two eyes, or at least two light-sensing patches, like most animals need to judge distance and direction. But because it lived such a still, stationary lifestyle, it simply didn’t need that kind of vision anymore. Over generations, those two eyes faded away.

What it kept instead was a single patch of light-sensitive cells right in the middle of its, a simple “median eye.” This wasn’t sharp enough to see actual images, but it was enough to tell day from night, and which way was up.

So How Did We End Up With Two Eyes Again?

Here’s the twist: millions of years later, descendants of this creature started swimming actively again. Suddenly, spotting food, dodging predators, and sensing direction mattered a lot more, and vision became valuable again.According to the study, evolution didn’t build a fresh pair of eyes from scratch. Instead, it repurposed parts of that old central “cyclops eye” to construct a brand-new pair of image-forming eyes, the ancestors of the eyes humans and other vertebrates use today.

This, researchers say, is why our eyes are built so differently from those of insects or squid. Dan-E Nilsson, professor emeritus in sensory biology at Lund University, explains that our retina actually develops from brain tissue, while in insects and squid, eyes grow from skin cells on the head, a completely different evolutionary path.

The Real Twist: Part of That Ancient Eye Is Still Inside You

Perhaps the most fascinating claim in the study is this: the original cyclops eye never fully disappeared. Researchers believe it survives today as the pineal gland, a small structure deep inside the vertebrate brain.

We no longer use it to see, but it hasn’t stopped reacting to light and dark. The pineal gland produces melatonin, the hormone that makes us feel sleepy at night and alert during the day, helping to regulate our internal body clock.

In many animals, the pineal gland can sense light directly. In humans, our eyes handle that job instead, sending light signals to the brain so melatonin levels can rise and fall on schedule.

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Nilsson sums up the discovery as genuinely startling, noting how strange it is that the part of our brain regulating sleep today can be traced back to a single primitive eye from a creature that lived some 600 million years ago.

This research doesn’t just rewrite a chapter of biology textbooks, it reshapes how scientists understand the origins of the eye and even parts of the brain itself. It suggests that evolution doesn’t always move forward in straight lines; sometimes it takes detours, loses features, and later repurposes old leftover parts for entirely new jobs.

So the next time you feel sleepy after sunset, you might just have a one-eyed ancestor from the ocean floor to thank for it.