Why are vertebrate eyes built so differently from those of insects and squid? Researchers at Lund University and the University of Sussex, including Dan-E Nilsson, professor emeritus in sensory biology at Lund, reconstructed the deep history of vision by analysing light-sensitive cells across the animal kingdom, comparing their physiology and their placement in the body. Their conclusions, published in Current Biology, rewrite the standard account of how the eye and brain evolved.
The analysis points to an ancestor living almost 600 million years ago: a small, worm-like, sedentary animal that filtered plankton from seawater. It appears to have possessed paired eyes earlier in its lineage, lost them as its lifestyle became stationary, and retained light-sensitive cells that formed a single primitive eye on top of the head, enough to track day and night and sense up from down. When its descendants returned to active swimming, new paired, image-forming eyes developed from components of that median eye.
This repurposing explains a deep anatomical peculiarity: the vertebrate retina develops from the brain, whereas insect and squid eyes arise from skin on the sides of the head. It also offers the first account of where the retina's image-processing neural circuits came from. And it leaves a living relic in every human skull: the pineal gland, still light-sensitive and still governing circadian rhythm through melatonin, is the remnant of that ancient single eye.