One‑eyed creature gave rise to our modern eyes

A few species of vertebrates still retain the median eye on top of the head. In this frog, the median eye appears as a small light-blue spot between the regular eyes.
Photo Credit: TheAlphaWolf
(CC BY-NC 4.0)

Scientific Frontline: "At a Glance" Summary: Evolution of the Vertebrate Eye

• Main Discovery: All vertebrates evolved from a distant worm-like ancestor possessing a single median eye, which eventually gave rise to modern paired eyes and the brain's pineal gland.
• Methodology: Researchers conducted an extensive comparative analysis of light-sensitive cells across diverse animal groups, evaluating their specific physiological functions and anatomical placement within the body.
• Key Data: The identified ancestral organism lived approximately 600 million years ago, and its primitive median eye survives in modern vertebrates as the pineal gland, an organ that regulates sleep cycles via melatonin production.
• Significance: The findings explain why vertebrate retinas originate from brain tissue rather than the skin on the sides of the head, distinctly separating vertebrate optical evolution from that of invertebrates like insects and squid.
• Future Application: Tracing the evolutionary path of these optical structures provides a foundational framework for analyzing the neural circuits responsible for retinal image processing in modern neurobiology and ophthalmology.
• Branch of Science: Evolutionary Biology and Sensory Biology

Regal horned lizard
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.
Photo Credit: Bruno Frías Morales/iNaturalist
(CC BY-NC 4.0)

There is tiny cyclops among your oldest ancestors, and humans share these remarkable ancestral roots with all other vertebrates. This is according to new, surprising research on the evolution of the eye. 

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 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 Dan‑E 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 ancestors 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 concluded in the study. 

“Now we finally understand why the eyes of vertebrates differ so radically from the eyes of all other animal groups, such as insects and squid. The film of our eyes - the retina - developed from the brain, whereas the eyes of insects and squid originate in the skin on the sides of the head,” says Dan‑E Nilsson. 

In other words, vertebrate eyes constitute a more modern model that evolved thanks to this peculiar detour via a cyclops’ sedentary life. The conclusion that our modern eyes evolved through this specific evolutionary path, and not via some other ancient animal, is based on the researchers’ extensive analysis of light‑sensitive cells in all animal groups, as well as the physiology and placement of these cells in the body. 

“For the first time, we now also understand the origin of the neural circuits that analyse the image in our retina,” adds Dan‑E Nilsson. 

A fascinating fact is that remnants of the ancient parietal median eye from our distant ancestors remain in our heads today, transformed into the pineal gland. The pineal gland is a light‑sensitive organ in the vertebrate brain. It produces the hormone melatonin, which helps regulate the body’s circadian rhythm. 

“It’s mind‑boggling that our pineal gland’s ability to regulate our sleep according to light stems from the cyclopean median eye of a distant ancestor 600 million years ago,” concludes Dan‑E Nilsson. 

Published in journal: Current Biology

Title: Evolution of the vertebrate retina by repurposing of a composite ancestral median eye

Authors: George Kafetzis, Michael J. Bok,Tom Baden, and Dan-Eric Nilsson

Source/Credit: Lund University | Lena Björk Blixt

Reference Number: ebio022526_01

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