A new paper has been published in a magazine current biology The complex pair of eyes of humans and other vertebrates suggests that they evolved from a single central eye on the top of the head of our ancient ancestors. The authors propose that the light-sensing tissue inside our eyes is older than the eyes themselves, and that remnants of this original visual system are still functioning deep within the human brain. This study provides evidence that vision in vertebrates has followed a fundamentally different evolutionary path compared to the rest of the animal kingdom.
Animal eyes generally rely on two different types of light-detecting cells known as photoreceptors. Bilaterally symmetrical animals, that is, animals with distinct left and right halves of the body, typically have both groups of these cells. The first group is called striated photoreceptors. These cells traditionally make up a pair of eyes on the sides of the invertebrate head and are used for visual navigation and image processing.
The second group is called ciliary photoreceptors. These cells are usually located deep in the brain or in a single location at the top of the head. Rather than forming images, these help animals regulate their daily biological rhythms and track light levels in their surroundings. Insects, crabs, and octopuses all follow this standard biological blueprint.
Vertebrates, including humans, birds, reptiles, and fish, completely break this law of evolution. The human eye uses ciliary cells to capture light, but then passes those signals to neurons with rhabdomeric properties to process the resulting images. This unique mixture of two different cellular systems is found nowhere else in nature.
The scientific community lacks a comprehensive explanation of how the human eye acquired this unusual hybrid structure. “What was the original solution to vision, and to what extent have different species copied or modified it to make it their own?” Thomas Baden, a neuroscientist at the University of Sussex and co-author of the study, told BBC Science Focus. “What is the pattern actually? When you do this over time, you start to wonder what the original eye was.”
To solve this evolutionary puzzle, scientists analyzed the arrangement and function of light-sensing cells across 36 major animal groups. They mapped out an evolutionary timeline and identified patterns that point to an ancient insect-like ancestor that lived about 600 million years ago. This small creature probably had both a pair of side eyes on the sides of its head and a central eye in the middle of the top.
“We don’t know whether the pair 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,” Dan Erik Nilsson, professor emeritus of sensory biology at Lund University, said in a press release.
The authors hypothesize that vertebrate ancestors eventually adopted a sedentary lifestyle. They began burrowing into the sediments of the ocean floor to filter food particles from the water. In this scenario, they were no longer swimming around, so maintaining a complex pair of eyes for navigation became an unnecessary biological expense.
As a result, the researchers suggest that the side eye disappeared over time. The only visual system that remained was a single patch of light-detecting cells at the top of the head. “You need to know what time it is, and if you’re in deep water, you need to know where is up and down. That doesn’t go away,” Baden said. “My guess is that the original side eye was lost at that time, but we kept the original center eye, because that’s a good thing.”
After millions of years, these creatures abandoned their burrows and began swimming in the open sea, the newspaper said. Navigating the seas once again required complex vision. Because the animal had already lost its side eyes, the researchers propose that evolution may have reused the only available light-sensing device it had left.
This model suggests that a single central eye gradually became more complex, forming a cup-shaped extension that could detect the direction of incoming light. These cups eventually split and moved to the sides of the head. This migration likely led to the formation of a new pair of eyes that all vertebrates use today.
“We now 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, but the eyes of insects and squids originate from the skin on the side of the head,” Nilsson said.
Researchers argue that this evolutionary detour explains the strange cellular makeup of the human eye. The original median eye is thought to have been a mixed system containing both ciliary cells and striated cells. When it split to form the modern eye, this hybrid circuit was probably incorporated to form the multilayered retina.
“For the first time, we also understand the origin of the neural circuitry that analyzes images in the retina,” Nilsson added. A key connecting part of this new system was the bipolar cell. Bipolar cells serve as a structural bridge between two ancient photoreceptor types.
The authors suggest that this retinal complexity developed long before the eyes themselves were fully formed on the sides of the head, and that the bipolar cells themselves have two distinct evolutionary origins. “What’s on the head isn’t really an eye, but rather a series of sensors, more like patches of photoreceptors,” Baden explained. Because of this, “The retina is older than the eye, if that makes sense. I always thought that was a cute tagline.”
The authors argue that the original median eye did not disappear completely. Instead, it persists today as the pineal gland, a tiny organ embedded deep within the human brain. Although the pineal gland no longer directly detects light in mammals, it still uses light signals relayed from the eyes to produce melatonin and regulate sleep cycles.
This ancestral third eye structure is still visible in some modern animals. The tuatara, a lizard-like reptile from New Zealand, actually has a functioning third eye with a lens and retina on the top of its head. The pineal gland in fish remains a simple organ that can directly detect light passing through the skull.
“It’s surprising that the human pineal gland’s ability to regulate sleep in response to light originates from our distant ancestor’s round mesian eye 600 million years ago,” Nilsson said. “The results are surprising; they turn our understanding of eye and brain evolution upside down.”
Although this study presents a detailed hypothesis for the evolution of vertebrate vision, it relies heavily on comparisons of cellular and genetic characteristics in modern animals to reconstruct ancient history. The fossil record from 500 million years ago is sparse, meaning scientists cannot directly observe the exact sequence of structural changes in the soft tissues of these extinct ancestors.
Researchers note that it is difficult to neatly categorize all modern retinal cells into strict evolutionary lineages. Over millions of years, some of these cells appear to have mixed characteristics of both ancient groups. This is a process known as chimerization. This mixture makes it difficult to trace the exact origins of all neural circuits in the modern human eye.
Future research may focus on collecting more genetic data from a wider range of animals to test these hypotheses. Scientists hope to use advanced mapping techniques to compare the ultrastructure of the pineal gland with that of the retina in more detail.
“The core testable parts of our proposal, I think with some funding and a few years, we can get a yes or no answer,” Baden said. By studying the genetic profiles of simpler marine animals, the researchers aim to determine whether these early light-sensing systems first integrated and then eventually split, resulting in the vision we rely on today.
The study, “Evolution of the vertebrate retina through compound ancestral median eye reuse,” was authored by George Kafetzis, Michael J. Bok, Tom Baden, and Dan-Eric Nilsson.
