What do we have in common with an ancient one-eyed worm?

Human eyes are amazing. They can detect light, colour, movement and detail, helping many people see the world around them.

But scientists think the story of how our eyes evolved may have begun with something much simpler: a small, worm-like sea animal that lived around 600 million years ago.

Research, published in the scientific journal Current Biology, suggests that some early ancestors of vertebrates (animals with a backbone) may once have had simple light-sensitive patches on the sides of their bodies.

These were not eyes like ours. They probably could not see faces, fish or seaweed. Instead, they may only have helped the animal tell light from dark, or up from down.

Scientists think this ancient animal may have lived in a burrow under the sea and filtered tiny bits of food from the water.

If the animal spent much of its time sitting still and filter-feeding, sharp vision may not have been very useful.

So, over many generations, some side light-sensors may have been lost. But one light-sensitive area in the middle of the head may have remained. Scientists call this kind a median eye.

Later, when the descendants of this animal became more active swimmers, better vision would have helped them spot food, obstacles and predators. Scientists think evolution may have reused parts of that ancient middle eye to help form left and right retinas - the retina is the light-sensitive layer at the back of the eye that sends signals to the brain.

Researchers worked this out by studying living animals and their light-sensitive cells. And in 2024 an earlier study of sea lampreys (an eel-like fish), mice, chickens and zebrafish found that many important retina cell types were already present early in vertebrate evolution - way before humans came along.

And that the ancient light-sensing system may also be linked to the pineal system, deep in the brain. In humans, the pineal gland helps control sleep by making melatonin - a natural chemical made by the body that helps control when we feel sleepy and awake.