"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.

Over the following millions of years, our distant ancestor once again began to live an active, swimming life, increasing the need for paired eyes. From parts of the small median eye, new image-forming eyes in pairs developed, the researchers conclude in the study.