New Theory Explains Distinct Evolution of Vertebrate Eyes

A groundbreaking theory suggests that vertebrate eyes, including those of humans, may have evolved from a single light-sensitive organ rather than from the paired eyes of early bilaterian animals. This concept is proposed in a new synthesis published in Current Biology, led by researchers from the University of Sussex and Lund University. The study indicates that vertebrate eyes might have been “reinvented” through evolutionary processes.

Dan-Eric Nilsson, the senior author of the study and an expert in eye evolution, highlights the significant differences between vertebrate and invertebrate eyes. He explains, “The key difference is the identity of the main photoreceptor, which is of ciliary nature in the vertebrate eye but rhabdomeric in other animal groups, such as arthropods and cephalopods.”

Understanding these distinctions requires a closer look at the two primary types of light-sensitive photoreceptor cells: rhabdomeric and ciliary. While most invertebrates utilize rhabdomeric cells for vision, ciliary cells play roles primarily in regulating biological clocks. In contrast, vertebrates incorporate both types into their eyes, with ciliary photoreceptors—rods and cones—responsible for image formation, while rhabdomeric components help monitor ambient light and relay information to the brain.

The authors argue that the ancestral state of eyes can be traced back to the rhabdomeric arrangement found in invertebrates, which was inherited from a common ancestor shared among bilaterians. This raises the question of how vertebrates diverged from this evolutionary path.

After the bilaterian lineage split—one line leading to insects and crustaceans, and the other to deuterostomes, including vertebrates—one of their ancestors transitioned to a burrowing lifestyle. According to George Kafetzis, a research fellow at the University of Sussex, this change made lateral eyes less advantageous. “Neural tissue in general is very expensive to maintain and function,” he noted, suggesting that the lineage may have gradually lost its paired eyes.

Surprisingly, this ancestral deuterostome eventually returned to a free-swimming lifestyle, where paired lateral eyes became beneficial for navigation. In such animals, having two eyes allows for better steering by comparing light input from both sides. By this time, however, the original rhabdomeric eyes were no longer present.

To adapt, the study posits that the ancestral deuterostome retained a single, centrally located eye. Kafetzis explains that this “cyclopean” eye likely contained both ciliary and rhabdomeric cells, merging the two cellular lineages into a singular organ, which eventually evolved into the complex eyes seen in vertebrates today.

Interestingly, remnants of this evolutionary journey may still be found in the vertebrate pineal complex, often referred to as the “third eye.” There are notable similarities between the retina and the pineal organ, suggesting a shared ancestry. Many scientists believe that certain neurons in the retina, known as bipolar cells, represent a key evolutionary innovation. These cells connect rods and cones to ganglion cells, facilitating visual processing.

Kafetzis and his colleagues propose that bipolar-like cells may also exist in the pineal, albeit in a different form. They argue that these cells do not represent a completely new evolutionary development but instead showcase a blended origin from both rhabdomeric and ciliary cells.

While this new theory presents a compelling narrative about the evolution of vertebrate eyes, it also invites further research. The authors acknowledge that some aspects of their hypothesis remain contentious, particularly the idea that the ancestral chordate adopted a burrowing lifestyle. They propose various methods for testing their model, including molecular comparisons of pineal and retinal cells and developmental studies across different deuterostome species.

In the words of Kafetzis, “We want to put forward some literature-based and inspired hypotheses that are testable, and now we can go out and test them.” As researchers continue to explore these ideas, the evolution of vertebrate eyes may yield deeper insights into the complexities of animal biology.

Cell, 2026. DOI: 10.1016/j.cell.2025.12.056.