A global team of biologists has published a study in the journal Current Biology that offers a drastically new perspective on the origin of vertebrate eyes. The scientists propose that our paired eyes, along with the unique structure of the retina, may have evolved from a single central visual organ possessed by a small, worm-like ancestor that lived approximately 600 million years ago. This discovery helps to resolve a long-standing mystery: why the photosensitive system of vertebrates is fundamentally different from that of the vast majority of other life forms on Earth.
As the authors explain, there are two main types of photoreceptors in the animal kingdom: rhabdomeric and ciliary. In insects, mollusks, and crustaceans, the primary eyes are constructed from rhabdomeric cells, while ciliary cells are solely responsible for biological rhythms and light sensitivity. In vertebrates, the situation is reversed: in our retinas, light is perceived by ciliary photoreceptors, but the signal from them is transmitted to neurons that show clear signs of rhabdomeric origin. This hybrid structure is not found in any other animal group.
By analyzing the arrangement and types of light-sensitive cells across 36 major taxonomic groups, the researchers reconstructed the appearance of a distant ancestor. According to their theory, this was a small creature with three eyes: two on the sides and one on the upper part of its head. Evolution then led these creatures to a sedentary, burrowing lifestyle on the seabed, where complex side eyes became unnecessary and gradually disappeared. However, the central eye was preserved, as even when buried in sediment, it was necessary to distinguish day from night and understand orientation.
When these organisms later returned to active swimming, the need for full vision re-emerged. The scientists believe that evolution then repurposed the preserved resource: the single central eye became more complex, divided into two halves, and shifted to the sides of the head, giving rise to the modern paired eyes of all vertebrates, including humans. The authors suggest this also explains the peculiar “inverted” architecture of our retina, where nerve fibers lie in front of the photoreceptors.
Furthermore, the researchers hypothesize that the ancient central eye did not disappear entirely but transformed into the pineal gland, a tiny organ deep within the brain responsible for melatonin production and circadian rhythm regulation. As living evidence supporting their hypothesis, the scientists cite the tuatara, an ancient reptile found in New Zealand. On the upper part of its head, it still possesses a parietal “third eye” with its own lens and rudimentary retina—an evolutionary remnant from those ancient times.
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