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 and 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 600 million years ago, burrowing into the seabed. In becoming a filter feeder with no need to move around, the energetically expensive type of paired eyes was rendered useless and costly.
However, this lifestyle change left the light-sensing cells in the middle of its head unscathed, because the animal still needed to sense the time of day and distinguish between up and down. Although the paired eyes were gone, the light-sensing cells in the midline developed into a small midline eye.
Possibly within a few million years, this animal changed lifestyle again. A return to swimming reintroduced the need to control steering and measure its own body motion for efficient filter-feeding (sifting food out of water) and avoiding predators.
This pushed evolution to develop 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: our eyes.
The loss and regain of vision happened between 600 and 540 million years ago. Components of the midline eye remained and became the pineal organ in the brain, which produces and releases the sleep hormone melatonin.
In many vertebrates, the pineal organ receives light through a transparent (unpigmented) region in the middle of the head. However, in the mammalian lineage the pineal organ lost its light-sensing capacity – possibly because early mammals were active at night and hid during daytime. So the eyes, which were more sensitive, took over the light detection which drives melatonin release and sleep.
Eyes of all shapes and sizes
Those animals that did not lose the worm-like ancestor’s original paired light-sensing cells comprise most invertebrates around today, since they descended from a branch of the evolutionary tree that never adopted a static lifestyle. Such animals include crustaceans, insects, spiders, octopus, snails and many groups of worms. These animals still have modern versions of the original sets of light-sensing cells.
The paired eyes of insects and crustaceans are compound eyes, with an array of tiny and densely packed lenses per eye. Instead of compound eyes, octopus and snails have camera-type eyes with a single lens.
In fact, octopus and snails independently evolved the same eye design and visual performance as us vertebrates. However, our retina – the light sensitive layer at the back of our eyes – has over 100 types of neurons (mice have even more – 140), compared to a mere handful in octopus and snails. This makes it almost as complex as our cerebral cortex – the outer and largest part of our brain.
Scientists have thought that in the evolution of our eyes, this complexity 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. In our work, however, we argue that a lot of this complexity predates the retina.
As such, it is likely to have been present already in the “cyclops” ancestor eye. This has broad implications for the origin and wiring of neural circuits in our retina and brain alike.
For us vertebrates, the evolution of our eyes and brain is intimately linked. The emergence of new paired eyes is a fundamental part of this picture, since the eyes allowed for the complex behavior that call for cognition and large brains. Without the eyes, we would not just be humans without eyes; we would not exist at all, nor would any of the other vertebrates.
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