Humans share a surprisingly bizarre ancestry with all other vertebrates. A new study suggests that, going back far into our evolutionary history, one of our earliest relatives had a single eye, much like a small cyclops, located on the top of its head.
Scientists from Lund University and the University of Sussex report that all vertebrates can be traced back to this ancient one-eyed creature. According to their findings, remnants of that original “median eye” still exist today, but in a very different shape. It became the pineal gland, a small structure deep in the brain.
“The results are surprising and turn our understanding of eye and brain evolution upside down,” said Dan E. Nilsson, professor emeritus of sensory biology at Lund University.
An insect-like creature from 600 million years ago
This distant ancestor lived about 600 million years ago. It was a small insect-like animal that spent most of its time in one place, filtering and feeding on plankton from seawater. Early in their evolutionary history, they likely had two eyes, like many other animals.
“We don’t know whether the pairs of eyes on this branch of the evolutionary tree were just light-sensitive cells or simple image-forming eyes. All we know is that organisms later lost them,” says Dan E. Nilsson.
As this creature adapted to a more stationary lifestyle, it no longer needed to have two eyes. Over time, that pair of eyes disappeared.
How a single “median eye” took over
Even after losing its pair of eyes, the creature retained a cluster of light-sensitive cells in the center of its head. These cells gradually formed a simple single eye that could sense light and darkness and help the animal recognize direction.
Millions of years later, this ancestor became active again and returned to a swimming lifestyle. This change has created new pressure to develop better vision. Researchers believe that part of the original median eye eventually gave rise to a new pair of eyes capable of forming images.
Why are vertebrate eyes so different?
This unusual evolutionary path helps explain why vertebrate eyes have a different structure from the eyes of other animals, such as insects and squid.
“We finally understand why the eyes of vertebrates are fundamentally different from the eyes of all other animal groups, such as insects and squids. The membrane of our eyes, the retina, developed from the brain, while the eyes of insects and squids originate from the skin on the sides of the head,” Dany-Nilsson says.
In vertebrates, the retina is essentially an extension of the brain. In contrast, many other animals develop eyes from surface tissues of their bodies.
The detour that shaped our modern vision
Researchers describe this history as an unusual evolutionary detour. Rather than evolving directly from an early eye structure, vertebrate vision appears to have been rebuilt from this single median eye after the loss of an early pair of eyes.
This conclusion follows from detailed comparisons of light-sensitive cells across different animal groups, as well as their structure and location within the body.
“For the first time, we also understand the origins of the neural circuitry that analyzes images in the retina,” added Dan E. Nilsson.
The pineal gland: the living remains of an ancient eye
One of the most interesting discoveries is that parts of this ancient eye still exist in humans today. It evolved into the pineal gland, a light-sensitive organ in the brain.
The pineal gland produces melatonin, a hormone that helps control the body’s circadian rhythms, including sleep patterns.
“It is surprising that our pineal gland’s ability to regulate sleep in response to light originates from the circular median eye of our distant ancestors 600 million years ago,” concludes Dan-E. Nilsson.
