The human eye is one of nature's most complex visual systems.
Yet its origins may trace back to a small, worm-like sea creature with a single, simple eye in the middle of its head.
According to a new evolutionary model published in * Current Biology*, the paired retinas of humans and other vertebrates arose through the repurposing of components from a single light-sensitive organ in a distant ancestor that lived around 600 million years ago.
This ancient median eye may not have disappeared entirely. Over time, the light-sensitive organ may also have given rise to the pineal system, located deep within the brain and involved in regulating the sleep-wake cycle.
The model, developed by sensory biologist Dan-Eric Nilsson of Lund University and George Kafetzis, Michael J. Bok, and Thomas Baden of the University of Sussex, is based on comparisons of light-sensitive cells, visual organs, and neural circuits across different animal groups.
No fossil of a 600-million-year-old, one-eyed animal has been discovered. Instead, the proposal is an evolutionary reconstruction drawing on evidence from living species.
The story begins with a small marine animal that lived before vertebrates emerged.
This distant ancestor probably lived in a burrow, filtered the surrounding seawater for food, and had a worm-like body. At an earlier stage in its evolutionary history, it may have had paired eyes – or at least clusters of light-sensitive cells – on either side of its head.
Even without forming detailed images, detecting day, night, or which way was up could have helped the animal survive.
However, the shift to a largely sedentary, filter-feeding lifestyle may have reduced the advantages of detailed vision. As a result, the light-sensitive structures on either side of the animal's head may have gradually disappeared.
The light-sensitive cells in the middle of its head, however, remained. Over time, they may have formed a simple light-sensing organ known as a "median eye".
The evolutionary story took a different turn when the descendants of this animal began swimming actively again.
A mobile lifestyle once more made a more advanced visual system capable of detecting food, obstacles, and predators advantageous.
According to the new model, rather than building this system from scratch, evolution repurposed the cells and neural circuits already present in the median eye.
The central light-sensitive organ may have diversified and expanded laterally, contributing to the formation of the left and right retinas, as well as the pineal and parapineal systems, of early vertebrates.
During embryonic development, the vertebrate retina forms as an outgrowth of the brain. By contrast, the eyes of insects and squid develop from tissues on the surface of the head and follow different evolutionary paths.
The model also offers an explanation for how the retina brought together cells with different evolutionary origins within a single complex circuit.
The rods and cones in our retinas belong to the ciliary photoreceptor lineage. Alongside these cells, which convert light into electrical signals, ganglion, amacrine, and horizontal cells with evolutionary links to rhabdomeric photoreceptors help process visual information and transmit it to the brain.
In many other animals, these two cellular lineages are associated with separate light-sensing systems. The ancestral median organ, however, may already have contained cells from both lineages.
The vertebrate retina may therefore have evolved by combining and reorganizing existing cells and neural circuits.
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A 2024 study published in * Nature Communications* supports the deep evolutionary roots of this circuitry. By comparing retinal cells in sea lampreys with those in mice, chickens, and zebrafish, researchers found that many cell types involved in visual processing were already present early in vertebrate evolution.
The pineal systems of many vertebrates can detect light directly. The human pineal gland does not see or directly detect light, but it produces melatonin, helping synchronize the body's internal clock with the day-night cycle.
The model is not a definitive evolutionary history confirmed by fossil evidence.
But if it is correct, the systems that allow us to see the world with two eyes and make us sleepy after dark may represent two separate legacies of the same primitive median eye that existed 600 million years ago.
The findings are reported in * Current Biology*.
This article was fact-checked by Rachel Garner and edited by Peter Dockrill. While we pride ourselves on our process, we are only human. If you spot a mistake, please let us know.