Our cyclops-like ancestor, which humans share with all vertebrates, was a worm-like organism that did not have much use for a pair of eyes.
They were burrowing filter feeders, sitting still and filtering plankton from seawater. Back then, these paired organs that are actually there to control when you move and to know where you are going were not needed, says Dan-E Nilsson, professor emeritus of sensory biology at Lund University.
One eye on the head
Through the course of evolution, the little cyclops therefore lost its eyes - but retained a group of light-sensitive cells, a kind of eye, in the middle of its head.
They were needed to keep track of the circadian rhythm and to know which way was up and down.
Today, the eyes of vertebrates, including humans, differ greatly from those of other animals with two eyes - for example insects, crustaceans and cephalopods.
The light-sensitive cells that invertebrates have in their paired lateral eyes are not the kind of light-sensitive cells that we see with, says Nilsson.
“Fall in place”
When the researchers behind the study conducted a comprehensive analysis of light-sensitive cells in all animal groups, it became clear why our eyes are so different from those of invertebrates.
As millions of years passed and the cyclops evolved to swim again and lead an active life, it needed eyes again - and they evolved from the small cyclops' single eye in the middle of its head.
The human retina developed from the brain, while the eyes of insects and squids developed in the skin, on the sides of the head.
We didn't understand why things turned out the way they did, but now we do. The pieces fell into place, says Dan-E Nilsson.
The study has been published in the journal Current Biology.
