Humans and other vertebrates may trace their visual systems and sleep cycles back to a tiny worm-like sea creature that possessed a single, central eye 600 million years ago, according to an evolutionary model published by researchers at Lund University and the University of Sussex.
The origin of human eyesight has long puzzled biologists, particularly given how differently vertebrate eyes are structured compared to those of insects and squid. A new evolutionary reconstruction published in Current Biology proposes that the vertebrate lineage passed through a cyclops-like stage long before the arrival of the first true vertebrates (ScienceAlert). Rather than developing two distinct eyes from scratch, our distant ancestors repurposed an ancient light-sensitive organ located in the middle of the head (The Report).
From Filter-Feeder Retreat to a Single Central Eye
The evolutionary model, developed by researchers including Dan-Eric Nilsson of Lund University alongside colleagues at the University of Sussex, draws on comparative anatomical data, light-sensing proteins, and gene expression analysis (ZME Science). No fossil evidence of a 600-million-year-old one-eyed animal exists; instead, the reconstruction relies entirely on evidence from living species (ScienceAlert).
Long before vertebrates emerged, a small marine animal with a worm-like body lived in the ocean, filtering seawater for plankton and food (Republic World). Earlier in its lineage, the creature likely possessed paired eyes or clusters of light-sensitive cells on the sides of its head (ScienceAlert). However, adopting a sedentary, burrowing lifestyle made complex lateral vision largely unnecessary (ScienceAlert).
“We don’t know whether the paired eyes in our branch of the evolutionary tree were just light-sensitive cells or simple image-forming eyes.”
Dan-E Nilsson, professor emeritus in sensory biology at Lund University (Republic World)
While those side structures faded away, a cluster of light-sensitive cells in the middle of the head remained (ScienceAlert). This simple median eye did not capture detailed pictures, but it allowed the creature to distinguish day from night and maintain orientation underwater (Republic World).
How Evolution Rebuilt Paired Retinas and Neural Circuits
The trajectory changed when descendants of this stationary organism returned to active swimming (ScienceAlert). A mobile existence restored the evolutionary advantage of advanced vision for tracking food, navigating obstacles, and spotting predators (Republic World). Instead of building a visual apparatus from scratch, evolution expanded and repurposed parts of the surviving median organ laterally, forming the left and right retinas (ScienceAlert).
This developmental pathway accounts for long-standing anatomical differences between animal groups. While insect and squid eyes originate from surface skin tissue, vertebrate retinas develop directly as outgrowths of the embryonic brain (ScienceAlert). Furthermore, the vertebrate retina merges two distinct cellular lineages—ciliary photoreceptors found in rods and cones alongside rhabdomeric-linked downstream cells (ZME Science).
“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.”
Dan-E Nilsson, professor emeritus in sensory biology at Lund University (ScienceAlert)
The Pineal Gland Connection Inside the Modern Human Brain
The ancient median eye did not vanish entirely (ScienceDaily). Researchers suggest that its evolutionary remnants survive today as the pineal gland, situated deep within the vertebrate brain (ScienceDaily).
While certain other vertebrates retain pineal organs capable of detecting light directly, the human pineal gland does not see (ScienceAlert). Instead, it receives indirect light signals from the eyes and produces melatonin to regulate sleep-wake cycles (The Report).
Researchers stress that while the model requires further confirmation, it upends conventional views on neurological and visual development (ScienceAlert).
“The results are a surprise.”
Dan-E Nilsson, professor emeritus in sensory biology at Lund University (ZME Science)
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