It’s easy to take our eyes for granted. But our recent research shows they took an incredible evolutionary journey to reach their current familiar form.

It has long been known that our (vertebrate) eyes differ fundamentally from the ones of our distant relatives (invertebrates), because of their cell composition and how they develop before birth. However, answers to why or how these differences first emerged long remained elusive.

Our study suggests that our eyes descend from a worm-like ancestor that was roaming the oceans 600 million years ago. The same also applies to all bilateral animals, meaning animals whose bodies can be divided into roughly mirror-image left and right halves.

As part of our study, we surveyed 36 major groups of living animals (covering nearly all bilateral animals) to see where their eyes and light-sensing cells are located and what they do.

A pattern emerged. We discovered that eyes and light-sensing cells are consistently found at two separate locations: paired on both sides of the face, and at the midline of the head, on top of the brain. Across the animals we looked at, cells in the paired position are used to steer movements, while their midline counterparts tell day from night........

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