"Now we finally understand why the eyes of vertebrates differ so radically from the eyes of all other animal groups."
The Cyclops Origin of Your Vision
Researchers from Lund University and the University of Sussex report that all vertebrates share a common ancestor that had a single median eye on top of its head. This ancestor, a small worm-like filter feeder, lived approximately 600 million years ago.
An Evolutionary U-Turn
The process that led to this discovery is a remarkable story of adaptation. The paired eyes present in earlier ancestors were lost as the creature adapted to a stationary lifestyle. Later, a light-sensitive cell cluster in the center of its head formed a simple single eye.
When the ancestor returned to a swimming lifestyle, parts of the median eye gave rise to new paired image-forming eyes.
The Science Behind the Structure
This evolutionary path explains why vertebrate retinas develop from the brain, unlike other animals where eyes originate from skin tissue. The pineal gland in modern vertebrates is a remnant of that ancient median eye and regulates circadian rhythms via melatonin production.
The study, led by Dan-E Nilsson from Lund University, compared light-sensitive cells, structures, and their locations across different animal groups to reach this conclusion.
"The results are a surprise. They turn our understanding of the evolution of the eye and the brain upside down."
— Dan-E Nilsson
"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."
— Dan-E Nilsson