Humans and other vertebrates may share a tiny, worm-like ancestor that lived in the ocean nearly 600 million years ago and possessed a single eye centered on top of its head, according to a new evolutionary model published in Sciencealert. The research, developed by scientists at sciencedaily.com and the University of Sussex, suggests that the earliest vertebrate lineage passed through a cyclops-like stage before developing the paired eyes seen today.
New Evolutionary Hypothesis Traces Vertebrate Eyes to a Cyclops-Like Ancestor
The model was formulated by sensory biologist Dan-Eric Nilsson, alongside George Kafetzis, Michael J. Bok, and Thomas Baden. Because no 600-million-year-old fossil of a one-eyed animal has been discovered, the proposal is an evolutionary reconstruction built from animal anatomy, eye-development genes, light-sensing proteins, neural wiring, and single-cell gene-expression data across living species.
The results are a surprise. They turn our understanding of the evolution of the eye and the brain upside down,
Dan-E Nilsson, professor emeritus in sensory biology at Republic World, said in a press release.
From Stationary Filter Feeder to Active Swimmer
According to the researchers, the distant ancestor was a small, worm-like marine filter feeder that spent much of its life in one place, filtering plankton from seawater for food. Earlier in its evolutionary history, the organism may have possessed two eyes or light-sensitive cell groups on the sides of its head. However, as the creature adopted a stationary lifestyle, those paired structures offered little advantage and gradually disappeared over many generations.
While the side structures vanished, a cluster of light-sensitive cells in the middle of the head remained. This primitive central, or median, organ did not form detailed pictures, but it allowed the animal to distinguish night from day, determine upward direction, and gauge environmental exposure. Millions of years later, when the animal’s descendants began actively swimming again, a mobile lifestyle created a renewed need for vision to detect food, predators, and movement. Portions of the original median eye were repurposed, eventually expanding sideways to give rise to a new pair of image-forming eyes.
Origins of the Pineal Gland and Vertebrate Retina
The hypothesis also proposes that the ancient median visual system never completely disappeared. Instead, part of that ancient system survives inside the modern vertebrate brain as the pineal gland, an organ located deep within the brain that helps regulate sleep and synchronize the body’s internal clock with the day-night cycle. While some creature groups use the pineal gland as a third eye to directly detect light and shadow, the human pineal gland does not see light directly, but produces melatonin to manage nightly rhythms.
Furthermore, the model helps explain why vertebrate retinas are structured so differently from the eyes of insects, worms, and squid. The eyes of insects and squid develop from skin tissues on the sides of the head and rely heavily on rhabdomeric cells. In contrast, vertebrate rods and cones belong to the ciliary photoreceptor lineage, and the vertebrate retina develops as an outgrowth of embryonic brain tissue. The researchers suggest the vertebrate retina evolved by combining and reorganizing existing cells and neural circuits from the ancestral median organ, which already contained elements from both lineages.
Now we finally understand why the eyes of vertebrates differ so radically from the eyes of all other animal groups, such as insects and squid,
Nilsson noted, adding that the team also understands the origin of the neural circuits that analyze images in the human retina.
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