Deep in human evolutionary history, before anything recognizable as a vertebrate existed, one of our oldest relatives had a single eye sitting on top of its head.
Now, according to new research, this ancient, cyclops-like creature may represent the evolutionary origin of every vertebrate eye on Earth, including humans. In fact, an evolutionary descendant of this early light-sensing organ still exists inside your skull today.
A new study from Lund University and the University of Sussex, published in Current Biology, traces how vertebrate vision evolved from a worm-like filter feeder that lived about 600 million years ago to the complex eyes humans rely on today. The results reframe how scientists understand the evolution of eyes across the animal kingdom.
“The results are a surprise,” said Dan-E Nilsson, professor emeritus in sensory biology at Lund University. “They turn our understanding of the evolution of the eye and the brain upside down.”
An Ancient Cyclops-Like Creature
The animal at the center of this research was a small, stationary filter feeder that lived in the sea. Earlier in its evolutionary history, it likely had two eyes, but as it adapted to a sedentary lifestyle, it no longer needed paired vision and eventually lost it.
“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,” Nilsson said. “We only know that the organism later lost them.”
This animal kept a group of light-sensitive cells at the center of its head, forming a single median eye. The eye could detect light and dark, which was enough for the animal to orient itself while remaining stationary.
Over time, this lineage became more active and returned to a swimming lifestyle. Increased mobility required improved vision, so the median eye, the only remaining light-sensing structure, likely provided the foundation for the raw material that forms the paired, image-forming eyes found in all vertebrates today.
Vertebrate Eyes Are Wired Differently
This evolutionary path through a single-eyed ancestor helps explain why vertebrate eyes differ from those of insects, squid, and most other animals. In insects and squid, eyes develop from skin tissue on the sides of the head. In vertebrates, the retina is an outgrowth of the brain itself.
“Now we finally understand why the eyes of vertebrates differ so radically from the eyes of all other animal groups, such as insects and squid,” Nilsson said. “The film of our eyes — the retina — developed from the brain, whereas the eyes of insects and squid originate in the skin on the sides of the head.”
This difference is due to vertebrate vision being reconstructed through a different developmental pathway after the original side eyes were lost. Other major animal groups have not taken this evolutionary route. Researchers reached this conclusion by comparing light-sensitive cells across different animals and examining their structure, function, and location in the body to trace how eye types evolved.
“For the first time, we now also understand the origin of the neural circuits that analyze the image in our retina,” Nilsson added.
The Eye That Never Fully Disappeared
One of the most significant findings is that the original median eye did not disappear as traditional eyes evolved over time. In humans, it evolved into the pineal gland, a small structure deep in the brain that produces melatonin and controls the body’s circadian rhythm, regulating sleep and wakefulness in response to light. What was once a simple light-detecting structure evolved into a regulator of human sleep patterns.
“It’s mind-boggling that our pineal gland’s ability to regulate our sleep according to light stems from the cyclopean median eye of a distant ancestor 600 million years ago,” Nilsson said.
In some modern vertebrates, such as certain lizards and frogs, the median eye never fully retreated inward and can still be found near the surface of the skull as a small, pale spot. This eye is sensitive enough to detect light. In humans, the lineage followed a different evolutionary path, but still persists deep in the brain, still performing a similar function after hundreds of millions of years.
Austin Burgess is a writer and researcher with a background in sales, marketing, and data analytics. He holds an MBA, a Bachelor of Science in Business Administration, and a data analytics certification. His work focuses on breaking scientific developments, with an emphasis on emerging biology, cognitive neuroscience, and archaeological discoveries.