Researchers have traced the origin of human eyes to a surprising ancestor: a tiny creature with a single eye on top of its head, resembling a cyclops, that lived nearly 600 million years ago. According to a study from Lund University and the University of Sussex, this ancient median eye not only contributed to the development of paired eyes in vertebrates but also persists today as the pineal gland, which regulates sleep.
A Shocking Discovery in Eye Evolution
“The results are a surprise. They turn our understanding of the evolution of the eye and the brain upside down,” said Dan-E Nilsson, professor emeritus in sensory biology at Lund University. The findings suggest that the earliest vertebrate ancestors passed through a cyclops-like stage approximately 600 million years ago before evolving into the complex visual systems seen in vertebrates today.
The study proposes that the ancestor was a small, worm-like creature that lived in the ocean, spending most of its life stationary, filtering plankton. Initially, it likely had two eyes or light-sensitive cell groups, but because of its sedentary lifestyle, these paired eyes gradually disappeared over generations as they offered little advantage. However, a cluster of light-sensitive cells in the middle of its head persisted and developed into a primitive median eye.
From One Eye to Two: A Remarkable Transformation
Millions of years later, as descendants became more active swimmers, the need for vision re-emerged to detect food, obstacles, and predators. The researchers conclude that portions of the median eye were repurposed through evolution, eventually giving rise to a new pair of image-forming eyes. “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,” Nilsson added.
The retina, a thin layer of tissue at the back of the eye, converts light into electrical signals sent to the brain. In vertebrates, the retina evolved from brain tissue, unlike in insects and squid where it developed from surface tissue. This unique evolutionary detour explains why vertebrate eyes are structured differently.
The Ancient Eye Lives On: The Pineal Gland
Perhaps the most surprising aspect of the theory is that the ancient median eye never fully disappeared. Its evolutionary remains survive as the pineal gland, a small organ deep within the vertebrate brain. In humans, the pineal gland produces melatonin, a hormone that controls circadian rhythms and synchronizes sleep with daylight. Although it no longer forms images, it remains connected to light detection.
The study also sheds light on the neural circuits in the retina. “For the first time, we now also understand the origin of the neural circuits that analyse the image in our retina,” Nilsson said. These circuits process visual information like brightness, contrast, and movement before signals reach the brain.
The research is based on a comprehensive comparison of light-detecting cells across animal groups, examining their location, function, and connections to tissues and nerves. This evidence supports the theory that vertebrate eyes evolved through this specific sequence, rather than from the same structures as insect or squid eyes.
