Human Eyes May Trace Back to 600-Million-Year-Old One-Eyed Marine Worm

Evolutionary Study Traces Vertebrate Vision to Ancient Marine Ancestor

Rather than evolving two eyes from scratch, the researchers believe an ancient light-sensitive organ situated in the middle of the creature’s head was gradually adapted over millions of years. 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, comparative animal anatomy, eye-development genes, light-sensing proteins, neural wiring, and single-cell gene-expression data.

The results are a surprise. They turn our understanding of the evolution of the eye and the brain upside down, said Dan-E Nilsson, professor emeritus in sensory biology at thereport.live.

Lifestyle Shifts and the Rise of the Median Eye

The model outlines a scenario where a small marine organism with a worm-like body lived in the ocean, filtering seawater and plankton for food. Because it spent much of its life largely stationary in a burrow, complex vision became less important for survival.

The researchers believe the creature may originally have possessed two eyes or paired groups of light-sensitive cells. As the animal adapted to its stationary lifestyle, light-sensitive structures on either side of its head gradually disappeared. However, light-sensitive cells in the middle of its head remained.

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Although this primitive structure was incapable of producing detailed images, it likely enabled the creature to distinguish day from night, determine upward orientation, and maintain its position in the water.

From One Eye to Paired Retinas

Millions of years later, as the descendants of the creature gradually evolved into more active swimmers, the need for advanced vision returned to detect food, predators, and obstacles. Instead of creating an entirely new visual system, evolution repurposed parts of the ancient central eye, causing parts of that central organ to expand sideways and develop into paired retinas.

This unusual evolutionary path helps explain why vertebrate eyes develop as an outgrowth of the embryonic brain and differ radically from the eyes of insects and squid, which develop from skin tissue on the surface of the head. Furthermore, the vertebrate retina combines cells from different lineages—rods and cones belonging to the ciliary photoreceptor lineage, alongside downstream retinal cell classes with molecular features associated with the rhabdomeric lineage, linked by bipolar cells.

Survival in the Modern Human Brain

The evolutionary model also suggests that the ancient median eye did not disappear entirely, but lives on today deep within the human brain as the pineal gland.

While the human pineal gland does not see or directly detect light, it regulates the body’s internal biological clock by producing melatonin, helping synchronize sleep and wake cycles in response to light signals transmitted indirectly from the eyes. In many other vertebrates, the pineal organ can still detect light directly.

It’s mind-boggling that our pineal gland’s ability to regulate our sleep according to light stems from the cyclopean median eye of a distant ancestor 600 million years ago, Nilsson noted according to Republic World.