How a one-eyed creature gave rise to our modern eyes

[erronis]

https://phys.org/news/2026-02-oneeyed-creature-gave-modern-eyes.html
by Lena Bjork Blixt, Lund University

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/iNaturalist/Creative Commons

There is a tiny cyclops among your oldest ancestors, and humans share these remarkable ancestral roots with all other vertebrates. Researchers from Lund University and University of Sussex have found that all vertebrates evolved from a distant ancestor that had a single eye located at the top of its head. The study, published in Current Biology, also reveals that the remnants of this so-called median eye have today become the pineal gland in our brains.

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

This cyclops-like creature, which is our very distant relative, existed almost 600 million years ago. It was a small, worm-like organism that had adopted a sedentary lifestyle and fed by filtering plankton from seawater. Previously, this creature had some form of paired eyes, like most other animals.

"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. We only know that the organism later lost them," says Nilsson.

The increasingly calm lifestyle meant that the worm-like creature no longer needed paired eyes, and therefore that function was lost over the course of evolution. However, the animal kept a group of light-sensitive cells in the middle of its head. These cells developed into a small, primitive median eye that could keep track of night and day, and sense what was up and down.

. . .