It is all too easy to take our eyes for granted in daily life. However, groundbreaking research reveals they embarked on an extraordinary evolutionary journey to achieve their current, familiar form. A study suggests that modern eyesight traces its origins back to a worm-like ancestor that inhabited the oceans approximately 600 million years ago.
Fundamental Differences in Eye Evolution
Scientists have long recognised that vertebrate eyes differ fundamentally from those of invertebrates, due to variations in cell composition and developmental processes before birth. Yet, the reasons behind these differences and how they first emerged remained elusive for many years.
Our investigation indicates that all bilateral animals—creatures whose bodies can be divided into roughly mirror-image left and right halves—descend from this ancient worm-like ancestor. To uncover this, we surveyed 36 major groups of living animals, covering nearly all bilateral species, to analyse the location and function of their eyes and light-sensing cells.
A Consistent Pattern Emerges
A clear pattern emerged from the data. 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, positioned on top of the brain. Across the animals examined, cells in the paired position are utilised for steering movements, while their midline counterparts distinguish between day and night and orient up from down.
We concluded that an ancient worm-like ancestor of all vertebrate animals lost the "steering" pair of eyes when it adopted a mostly stationary lifestyle around 600 million years ago, burrowing into the seabed. As a filter feeder with no need to move around, the energetically expensive paired eyes became useless and costly, leading to their loss.
Midline Eyes and Evolutionary Adaptations
However, this lifestyle change left the light-sensing cells in the middle of its head unscathed, as the animal still required the ability to sense the time of day and distinguish between up and down. Although the paired eyes disappeared, the light-sensing cells in the midline developed into a small midline eye.
Possibly within a few million years, this animal changed its lifestyle once again. A return to swimming reintroduced the necessity to control steering and measure body motion for efficient filter-feeding and predator avoidance. This evolutionary pressure prompted the development of the midline eye by forming small eye cups on each side.
These eye cups later separated from the midline eye, moved out to the sides of the head, and formed new paired eyes—the eyes we possess today. The loss and regain of vision occurred between 600 and 540 million years ago, with components of the midline eye remaining and evolving into the pineal organ in the brain, which produces and releases the sleep hormone melatonin.
Complexity in Eye Development
In many vertebrates, the pineal organ receives light through a transparent region in the middle of the head. However, in the mammalian lineage, the pineal organ lost its light-sensing capacity, likely because early mammals were active at night and hid during daytime. Consequently, the eyes, which were more sensitive, assumed the role of light detection to drive melatonin release and regulate sleep.
Animals that did not lose the worm-like ancestor's original paired light-sensing cells comprise most invertebrates today, as they descended from an evolutionary branch that never adopted a static lifestyle. This group includes crustaceans, insects, spiders, octopus, snails, and many worms, which retain modern versions of the original light-sensing cells.
Diverse Eye Structures Across Species
The paired eyes of insects and crustaceans are compound eyes, featuring an array of tiny and densely packed lenses per eye. In contrast, octopus and snails have camera-type eyes with a single lens. Interestingly, octopus and snails independently evolved the same eye design and visual performance as vertebrates.
Yet, our retina—the light-sensitive layer at the back of our eyes—contains over 100 types of neurons, compared to a mere handful in octopus and snails. This complexity makes it almost as intricate as our cerebral cortex, the outer and largest part of our brain.
Scientists previously believed that this complexity in eye evolution emerged fairly late. Similarities between light-sensing cells in the brain and paired eyes informed earlier hypotheses about a simple, pineal organ-like eye early in its evolution. However, our work argues that much of this complexity predates the retina, likely present already in the "cyclops" ancestor eye.
Implications for Neural Circuitry
This finding has broad implications for the origin and wiring of neural circuits in both our retina and brain. For vertebrates, the evolution of eyes and brain is intimately linked. The emergence of new paired eyes is a fundamental aspect of this picture, as they enabled complex behaviours that necessitate cognition and large brains.
Without the eyes, humans and other vertebrates would not exist at all. The evolutionary journey from a worm-like creature to modern eyesight underscores the intricate and dynamic processes that have shaped life on Earth.
About the authors: George Kafetzis is a Research Fellow in Neuroscience at the University of Sussex. Dan Nilsson is a Professor emeritus of Zoology at Lund University. This article is republished from The Conversation under a Creative Commons license.
