A one-eyed sea creature from 600 million years ago could be the reason humans have eyes

Some of the earliest roots of the human eye appear to lie in a small marine animal that lived nearly 600 million years ago. Researchers at Lund University and the University of Sussex report that all vertebrates evolved from an ancestor that carried a single light-sensing organ on the top of its head. Their findings, published in Current Biology, suggest that this median eye later gave rise both to the paired eyes seen in vertebrates today and to the pineal gland deep within the brain. The work revisits long-held assumptions about how complex vision evolved and proposes a more indirect path than previously thought. It centres on light-sensitive cells, their position in the body, and how repeated lifestyle changes may have altered their function over time.The distant ancestor described in the study "Evolution of the vertebrate retina by repurposing of a composite ancestral median eye" was small and soft-bodied, living in ancient seas. It fed by filtering plankton from the water and appears to have led a mostly sedentary life. Earlier forms in this evolutionary branch likely had paired eyes, although researchers cannot say whether those eyes formed clear images or simply detected light.At some point, the paired structures were lost. A quieter lifestyle made them less necessary. What remained was a cluster of light-sensitive cells positioned along the midline of the head. Over time, this cluster formed a simple median eye. It could register light and dark and perhaps detect orientation, but it was not built for detailed vision.Later evolutionary shifts brought a return to more active movement. Swimming required better spatial awareness. According to the researchers, parts of the median eye were adapted again, giving rise to new paired eyes capable of forming images.This route differs from what is seen in insects or squid. In vertebrates, the retina develops from brain tissue. In many other animals, eyes originate from skin cells at the sides of the head. The distinction has long been noted, though its origin was less clear.Dan E Nilsson, professor emeritus in sensory biology at Lund University, described the findings as unexpected. The study suggests that vertebrate eyes did not simply evolve in parallel with other complex eyes but followed a detour shaped by changing behaviour and environment.The median eye did not disappear entirely. Its remains are thought to persist as the pineal gland. This small structure sits near the centre of the brain and remains sensitive to light signals, though indirectly.The pineal gland produces melatonin, a hormone involved in regulating circadian rhythms. In effect, it still responds to day and night cycles. The researchers argue that this function can be traced back to the ancient light-sensing cells of the median eye.The idea that sleep regulation in modern vertebrates connects to a primitive cyclops-like ancestor may seem distant. Yet the cellular evidence, drawn from comparisons across animal groups, points in that direction. The evolutionary path appears less direct than once assumed, shaped by loss, adaptation and return.