The light spot in the middle of the head forms the median eye in this lizard. The animal’s regular eyes are not visible because the picture is taken from behind. Credit: Bruno Frías Morales / Wikimedia Commons
Deep inside the human brain, a small gland in the middle of the forehead helps keep time. As darkness arrives, the pineal gland helps set the body’s nightly rhythm. But some creatures use the pineal gland as a sort of “third eye”, helping them detect light and shadow, though the organ doesn’t form pictures. Our human lineage lost this ancestral ability millions of years ago.
But a new evolutionary hypothesis suggests that this unassuming organ is the surviving central remnant of the eye from which our two image-forming eyes ultimately arose.
In a review published in Current Biology, researchers from Lund University and the University of Sussex argue that the lineage leading to vertebrates passed through a cyclops-like stage roughly 560 million years ago. The animal was not a one-eyed giant, but probably a tiny, wormlike marine filter feeder with a light-sensitive organ on top of its head. As its descendants returned to active swimming, parts of that central organ expanded sideways and were repurposed into paired retinas.
The researchers assembled a historical reconstruction from animal anatomy, eye-development genes, light-sensing proteins, neural wiring and single-cell gene-expression data. Their model may help explain why the vertebrate retina is built so differently from the eyes of insects, worms and squid.
“The results are a surprise. They turn our understanding of the evolution of the eye and the brain upside down,” Dan-E Nilsson, a sensory biologist at Lund and an author of the review, said in a press release.
The Eye That Remained
Repeated lifestyle changes drove the unique evolution of vertebrate eyes. Credit: G. Kafetzis et al. 2026
Most animals with left and right sides appear to have inherited two broad light-sensing systems. One uses “rhabdomeric” cells, which gather light with dense, brushlike folds. These cells dominate the side-facing eyes of insects, many worms and mollusks. The other uses “ciliary” cells, built around a modified hairlike structure. In many invertebrates, those cells sit near the middle of the head and monitor broad changes in illumination rather than producing detailed images.
Vertebrates combine the systems in an unusual way. Our rods and cones belong to the ciliary family, while several downstream retinal cell classes have molecular features associated with the rhabdomeric lineage. Bipolar cells link the two. The retina is therefore less like a single evolutionary invention than a merger of ancient sensory parts, arranged in a layered circuit that was already present in the earliest vertebrates.
The authors propose that an ancestral deuterostome — the broad lineage that would later split into vertebrates, sea squirts, lancelets, acorn worms, starfish and their relatives — adopted a slow, partly buried, suspension-feeding life. Side-facing eyes that helped steer an actively moving animal became less useful and disappeared. A central light sensor remained because it could still distinguish day from night, gauge whether the animal was exposed and help it maintain the correct orientation.
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The exact nature of the lost paired eyes remains unknown. They may have been little more than light-sensitive patches, or they may have formed simple images. What matters to the hypothesis is that the median system survived the ecological retreat and became raw material for a second visual beginning.
When later members of the same lineage returned to active swimming, evolution did not rebuild the lost lateral eyes from scratch. Instead, it modified and expanded parts of the surviving median organ, eventually producing the paired retinas of early vertebrates.
From One to Two Eyes
When later descendants began swimming freely again, they needed more than a clock that detects day/night. Light changes underwater with depth, weather, time and body angle. A central organ containing receptors with different sensitivities could compare those signals, helping an animal tell whether it was rolling, sinking or approaching open water.
That’s not to say “one eye became two” like a round eyeball neatly cleaving in half. The authors of the review propose that side regions of a composite light-sensitive organ formed shallow cups, gained directional sensitivity and gradually shifted toward opposite sides of the head. Lenses and higher-resolution vision arrived later, as early vertebrates became more active and began pursuing prey or avoiding obstacles.
This scenario would explain why the vertebrate retina develops as an outgrowth of the embryonic brain and why it contains an improbable mixture of cell lineages. The review goes further, proposing that bipolar cells — the relay neurons that carry signals from rods and cones deeper into the retina — may have evolved from two different ancestral cell types, as explained earlier. If so, the retina’s central wiring was assembled by connecting circuits that had once operated separately in the median eye.
Lampreys offer one clue to how all of this may have panned out. Their pineal organ contains distinct light-sensing circuits that resemble pieces of a retina but remain partly disconnected. Gene-expression studies in zebrafish also find pineal cells resembling rods, cones, ganglion cells and retinal support cells. To the researchers, these similarities look like traces of the components that existed before the paired retina became a unified organ.
A separate fossil study published in Nature seems to add evidence ot the claim. Researchers examining 518-million-year-old jawless vertebrates reported two large lateral eyes and two smaller central eyes, all apparently equipped with light-absorbing pigment and lenses. Those animals may represent a transitional stage when vertebrates retained both their new side-facing eyes and image-forming pineal eyes on top of the head.
The ‘Third Eye’ That No Longer Sees
A few species of vertebrates still retain the median eye on top of the head. In this frog, the median eye appears as a small light-blue spot between the regular eyes. Credit: TheAlphaWolf / Wikimedia Commons
In some living fish, amphibians and reptiles, a light-sensitive parietal or “third” eye still lies near the top of the skull and connects with the pineal complex. In mammals, the pineal gland moved deeper into the brain and lost direct access to light. Signals arriving from the ordinary eyes now help govern its release of melatonin, the hormone that primes you for sleep after dusk. Evolution often preserves an organ while changing its job, so this is not surprising.
Nilsson has spent decades asking how complex eyes could evolve. In a 1994 mathematical model developed with Susanne Pelger, he showed that a flat patch of light-sensitive tissue could, under sustained natural selection, become a focused eye in a few hundred thousand years.
However, the case remains provisional. The Current Biology paper repeatedly frames its reconstructed history as a hypothesis, and many of the crucial transitions occurred in soft-bodied animals that left an uncertain fossil record. “It’s a compelling new idea, but the jury is still out,” Karthik Shekhar, a computational biologist at the University of California, Berkeley, who was not involved in the research, told The New York Times.
Researchers will want to test the model by comparing pineal and retinal cells across many vertebrates, mapping their wiring and looking for deep molecular similarities. Tom Baden, a neurobiologist at Sussex and a co-author, told The Times that his group had already begun such comparisons in zebrafish. In the meantime, the authors also call for more detailed genetic and anatomical studies of the pineal organs of lampreys and other early-diverging vertebrates.
“This is the start, not the end,” Baden said.