A new study has revealed that our modern vision evolved from an ancient worm-like creature with one eye.
It’s easy to take our eyes for granted. But a study, recently in Current Biologyshowed that they went through an unbelievable evolutionary journey to reach their current family form.
It has long been known that our (vertebrate) eyes differ fundamentally from those of our distant relatives (invertebrates), due to their cellular composition and the way they develop before birth. However, answers to why or how these differences initially emerged remained elusive for a long time.
The new study suggests that our eyes descend from a worm-like ancestor that roamed the oceans 600 million years ago.
The same – the authors detail in an article in , – also applies to all bilateral animals, that is, animals whose bodies can be divided into left and right halves approximately in a mirror image.
The research looked at 36 large groups of living animals (encompassing almost all bilateral animals) to see where their eyes and light-sensitive cells are located and what they do.
A pattern emerged. It turned out that the eyes and light-sensitive cells are consistently found in two distinct locations: paired on both sides of the face, and in the midline of the head, at the top of the brain.
In the animals analyzed, cells in the paired position are used to guide movements, while their counterparts in the midline distinguish day from night and up from down.
Key change 600 and 540 million years ago
It was concluded that an ancient worm-like ancestor of all vertebrate animals lost its pair of “orienting” eyes when it adopted a mostly stationary lifestyle 600 million years ago, burrowing into the seafloor. By becoming a filter without needing to movethe energetically expensive type of paired eyes has become useless and costly.
However, this lifestyle change left the light-sensitive cells in the middle of its head intact, because the animal still needed to understand the time of day and distinguish between up and down.
Although the paired eyes are gone, the light-sensitive cells in the midline developed into a small central eye.
Possibly after a few million years, this animal changed its lifestyle again. A return to swimming has reintroduced the need to control orientation and measure one’s own body movement for efficient filter feeding (separating food from water) and to avoid predators.
This led to evolution to develop the central eye forming small eye sockets on each side. These eye sockets later separated from the central eye, moved to the sides of the head and formed new paired eyes: our eyes.
Vision loss and recovery occurred between 600 and 540 million years ago. Components of the central 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 has lost its ability to detect light —possibly because early mammals were active at night and hid during the day. Thus, the eyes, which were more sensitive, took over the light detection that regulates melatonin release and sleep.
There are eyes of all shapes and sizes
Animals that have not lost the original paired light-sensitive cells of their worm-like ancestor constitute the majority of today’s invertebrates, since they descend from a branch of the evolutionary tree that never adopted a static lifestyle. These animals include crustaceans, insects, spiders, octopuses, snails and many groups of worms. These animals still have modern versions of the original sets of light-sensitive cells.
The paired eyes of insects and crustaceans are compound eyes, with a set of small, densely packed lenses per eye. Instead of compound eyes, octopuses and snails have camera-type eyes with a single lens.
In fact, octopuses 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 — tin more than 100 types of neurons (rats have even more — 140)compared to only a small number in octopuses and snails. This makes it almost as complex as our cerebral cortex — the outer, largest part of our brain.
Scientists thought that in the evolution of our eyes, this complexity emerged relatively late. Similarities between light-sensitive cells in the brain and paired eyes supported previous hypotheses about a simple eye, similar to the pineal organ, in the early stages of its evolution. In the new work, however, it is argued that large part of this complexity precedes the retina.
Like this, it is likely that it was already present in the eye of the “cyclopean” ancestor. This has broad implications for the origin and connection of neural circuits in both our retina and our brain.
For us vertebrates, the evolution of our eyes and our brains are closely linked. The emergence of new paired eyes is a key part of this picture, as eyes enabled complex behavior that requires cognition and large brains.
Without eyeswe wouldn’t just be humans without eyes; we wouldn’t exist at all, nor any other vertebrate.
