New study suggests: Vertebrate eyes evolved from a single middle eye of a worm-like ancestor

A review article in Current Biology suggests that about 600 million years ago, vertebrate ancestors lost a pair of lateral eyes when they adopted a sedentary lifestyle, and then redeveloped paired eyes from a middle sensory organ, a remnant of which remains today in the pineal gland.

How vertebrate eyes, including humans, evolved is one of the most complex stories in the evolution of the nervous system. Now, George Kaptzis, Michael Bock, Tom Baden, and Dan-Erik Nilsson propose in a new review article published February 23, 2026 in the journal Current Biology, because the origin of the vertebrate retina does not stem from a direct and simple continuation of ancient lateral eyes, but from the "conversion" of an ancient and complex intermediate sensory organ that already included several types of sensory and processing cells. (PubMed)

The most important point in this news is that it is a new evolutionary model, and not the discovery of a single fossil that changed everything at once. According to the abstract of the article on PubMed, the researchers claim that the vertebrate retina is a unique structure, combining different lineages of photoreceptor cells within a multilayered circuit. In their opinion, this unusual combination is better explained if we assume that at a very early stage in evolution a complex middle sensory organ developed, and only later part of it "moved" aside and became the paired eyes of vertebrates. PubMed explicitly classifies the article as a Review, meaning a review article and a proposal of an interpretive framework, and not as a single experiment or field study.

The researchers surveyed 36 major groups of living animals, almost all bilaterians, and examined where the visual organs and light-sensing cells are located and what their function is. From this comparison, they say, a recurring pattern emerged: light-sensing systems appear repeatedly in two main locations—paired on the sides of the head, and central in the head, above the brain. According to their interpretation, the paired systems were used mainly for directing movement and navigation, while the central system helped distinguish between day and night and between up and down.

From this, the new evolutionary scenario was constructed. According to the researchers, about 600 million years ago, a worm-like ancestor of the vertebrate lineage lived, which adopted a relatively sedentary lifestyle, burrowing into the seabed and feeding by filtering particles from the water. In such a situation, a pair of "navigating" lateral eyes became too expensive energetically and less useful, and was therefore lost during evolution. In contrast, a group of light-sensitive cells that remained in the center of the head was preserved, because it was still necessary to detect light and dark cycles and the direction of the environment. According to the model, that middle system developed into a small middle eye, and only after a renewed transition to an active, swimming lifestyle did it gradually form lateral eye cups, which eventually evolved into the paired eyes of vertebrates. (phys.org)

One of the intriguing aspects of the paper is the link between that ancient middle eye and the pineal gland. According to the authors, remnants of the ancient middle sense organ were preserved in the vertebrate lineage and became part of the pineal/parapineal system, which is involved in the regulation of circadian rhythms and melatonin secretion. In many non-mammalian species, this region can still receive light through a relatively transparent area in the head; in mammals, according to the explanation proposed in the paper, direct sensitivity to light was lost, and the eyes themselves took on the role of transmitting the information needed to regulate melatonin secretion and sleep.

The researchers add that the new model may also explain why the vertebrate retina is so complex compared to the eyes of other animals. In the abstract of the article, they suggest that some of this complexity did not appear later, but rather preceded the retina itself. They also suggest that the origin of bipolar cells, which connect the photoreceptors and the processing circuits in the retina, is earlier than previously thought, and that they have a dual evolutionary origin. If this model is correct, the significance is broader than just the story of the development of the eye: it may also shed light on the development of neural circuits in the brain and the deep connection between the development of vision and the development of the vertebrate brain in general.

Along with the enthusiasm, it is important to remember the limitations of the claim. This is a synthetic model based on comparisons between animal groups, on anatomy, physiology and evolutionary interpretation, and not a "photographic reconstruction" of the ancestor itself. Therefore, it is more accurate to formulate the finding as follows: the study offers a new and compelling framework for understanding the origin of vertebrate eyes, and shifts the center of gravity from the idea of ​​a simple ancient lateral eye to the idea of ​​a complex intermediate sensory organ, from which both the retina and some of the light-related brain systems later emerged.

The bottom line is that if Kaptzis and his colleagues’ model holds up to further comparative testing, it could change not only the textbooks on eye development but also the way we think about the origins of the vertebrate brain. Instead of seeing the eye and brain as two systems that evolved separately and only later came together, the study suggests that they emerged together from the same ancient evolutionary solution—a solution that originated in a small, worm-like marine creature with a single, central eye.

More of the topic in Hayadan:

• In the end, only cockroaches will remain: half of the vertebrate population has become extinct since 1970
• Facial expressions
• Prehistoric animals breathed through their ears
• "Science in the people": the scientists raid the streets on the National Science Day
• Saran: The particle discovered about a year ago is indeed the Higgs boson or the 'God particle'