Scientists trace human vision back to ancient one-eyed ancestor living 600 million years ago

Animal/ File: wikipedia

 According to Science daily, humans share an unexpectedly unusual ancestor with all other vertebrates. Recent research suggests that deep in evolutionary history, one of our earliest relatives may have had a single eye-similar to a tiny cyclops-located on the top of its head.

Scientists from Lund University and the University of Sussex argue that all vertebrates can be traced back to this ancient one-eyed organism. Their findings indicate that the remnants of this original “median eye” still exist today, but in a transformed form: it has evolved into the pineal gland, a small structure deep within the brain.

“The results are surprising. They overturn our understanding of how the eye and brain evolved,” says Dan-E Nilsson, professor emeritus of sensory biology at Lund University.

A worm-like animal from 600 million years ago

This distant ancestor lived around 600 million years ago. It was a small, worm-shaped creature that likely stayed in one place most of the time, feeding by filtering plankton from seawater. Earlier in its evolutionary history, it probably had a pair of eyes, like many other animals.

“We don’t know whether those paired eyes were simple light-detecting cells or more advanced image-forming organs. We only know that they were eventually lost in our lineage,” Nilsson explains.

As the organism adapted to a more sedentary lifestyle, two eyes were no longer necessary, and over time they disappeared.

The rise of a single central eye

Even after losing its paired eyes, the creature retained a group of light-sensitive cells in the middle of its head. These gradually developed into a simple single eye capable of sensing light and darkness, helping it orient itself in its environment.

Millions of years later, the animal became more active again and returned to swimming. This shift created evolutionary pressure for improved vision. Researchers believe that parts of this central eye later contributed to the development of new paired eyes capable of forming images.

Why vertebrate eyes are unique

This evolutionary history helps explain why vertebrate eyes are structurally different from those of insects, squid, and many other animals.

“Now we finally understand why vertebrate eyes are so distinct from those of other groups like insects and squid. In vertebrates, the retina developed from brain tissue, whereas in insects and squid, eyes originate from skin on the sides of the head,” Nilsson says.

In other words, the retina in vertebrates is essentially an extension of the brain, unlike in many other species where eyes form from external tissues.

An unusual evolutionary pathway

Scientists describe this as an unconventional evolutionary route. Instead of directly evolving from earlier eye structures, vertebrate vision appears to have been rebuilt after the loss of earlier paired eyes, using this single median eye as a foundation.

The conclusion is based on comparisons of light-sensitive cells across species and their anatomical structures and positions.

“For the first time, we can now also understand the origin of the neural circuits that process images in the retina,” Nilsson adds.

The pineal gland: a remnant of an ancient eye

One of the most striking findings is that a part of this ancient eye still exists in humans today. It has transformed into the pineal gland, a small light-sensitive organ located in the brain.

The pineal gland produces melatonin, a hormone that regulates the body’s internal clock, including sleep cycles.

“It is remarkable that our ability to adjust sleep patterns according to light comes from the median eye of a distant 600-million-year-old ancestor,” Nilsson concludes.