Long before humans, dinosaurs, or even fish existed, one of our distant ancestors looked very different from anything alive today.
Researchers now say that all vertebrates—including humans—may trace their vision back to a tiny, worm-like creature with a single eye on top of its head.
This surprising discovery helps explain not only how our eyes evolved, but also why a small gland deep in our brain controls our sleep.
Scientists from Lund University in Sweden and the University of Sussex studied how eyes developed across animal evolution.
Their findings, published in Current Biology, suggest that about 600 million years ago, our distant ancestor was a small, simple animal that lived quietly on the seafloor.
It filtered plankton from the water for food and did not need to move around much. Over time, its lifestyle changed—and so did its eyes.
Earlier in its evolutionary history, this creature likely had a pair of eyes like many other animals. But as it became more sedentary, those eyes were no longer necessary and were gradually lost. Instead, it kept a small cluster of light-sensitive cells in the middle of its head.
These cells formed a simple “median eye,” similar to a single eye, which could detect light and dark and help the animal sense direction, such as what was up or down in the water.
Millions of years later, when descendants of this animal became more active swimmers again, they needed better vision. According to the researchers, new paired eyes evolved from parts of that central light-sensing structure.
This unusual evolutionary path may explain why vertebrate eyes are so different from the eyes of insects or squid. In vertebrates, the retina—the light-sensitive layer at the back of the eye—develops from brain tissue. In many other animals, eyes form from skin on the sides of the head.
The study also revealed something even more surprising. A remnant of that ancient single eye still exists inside our heads today. It has transformed into the pineal gland, a tiny structure deep in the brain that responds to light. The pineal gland produces melatonin, a hormone that controls our sleep-wake cycle and helps regulate the body’s internal clock.
In other words, the reason light affects when we feel sleepy or alert can be traced back to this primitive eye from hundreds of millions of years ago. The researchers say this discovery changes how scientists understand the evolution of the brain and vision, showing that our modern eyes developed through a complex series of changes rather than a straightforward path.
What may sound like science fiction is actually a reminder of how deeply connected we are to ancient life on Earth. The ability to see the world—and even to sleep at night—may owe its origins to a tiny one-eyed creature drifting in prehistoric seas.
