Visualization of the evolution of sight and eye
Roadmap of the evolution of the eye
Throughout historynumerous creatures have evolved increasingly complex eyes in response to various selective pressures.
However, not all organisms are under the same pressures. This is why some creatures today still have eyes that are quite simple, or why some have no eyes at all. These organisms are an example of eyes that are “frozen” in time. They provide snapshots of the past or “checkpoints” of how the eye has transformed during its evolutionary journey.
Scientists study genes, anatomy, and the vision of these creatures to discover the road map of how the eye came to be. And so we put together an evolutionary graphical timeline of the different stages of the eye using several candidate species.
Let’s see how the eye has formed over time.
Where does the vision come from?
The Retina is a layer of nerve tissue, often in the back of the eye, that is sensitive to light.
When light hits it, specialized cells called photoreceptors convert the light energy into electrical signals and send them to the brain. The brain then processes these electrical signals into images, creating vision.
The earliest form of vision appeared in single-celled organisms. They contain simple nerve cells that can distinguish only light from darkness, and are the most common eyes that exist today.
The ability to detect shape, direction, and color comes from all the additions that evolution introduced into those cells.
Two main types of eyes
Two main types of eyes are dominant among species. Despite the different shapes or specialized parts, improved vision in both types of eyes is the product of small, gradual changes that optimize the physics of light.
Simple eyes are actually quite complex, but they get their name because they consist of one single unit.
Some molluscs and all higher vertebrates, such as birds, reptiles or humans, have simple eyes.
Simple eyes developed from a pigment cup, slowly bending over time into the shape we recognize today. Specialized structures such as the lens, cornea, and pupil emerged to improve the focus of light on the retina. This helps create sharper, clearer images for the brain to process.
Compound eyes are created by repeating the same basic units of the so-called photoreceptors ommatidia. Each ommatidium is similar to a simple eye, composed of lenses and photoreceptors.
Grouped together, ommatidia form a geodesic pattern commonly seen in insects and crustaceans.
Our understanding of the evolution of the compound eye is somewhat unclear, but we do know that rudimentary ommatidia evolved into larger, clustered structures that maximize light capture.
In environments such as caves, deep underground, or the bottom of the ocean where there is little or no light, compound eyes are useful for creating vision that gives even the slightest advantage over other species.
How will the vision develop?
Our increasing dependence on technology and digital devices may be ushering in a new eye shape.
The muscles around the eye stretch to move the lens when you look at something close. The round shape of the eye elongates in response to this muscle strain.
The time spent in front of mobile phones, tablets and computers has increased dramatically over the years, especially during the COVID-19 pandemic. Recent studies are already reporting an increase in childhood myopia, the inability to see at a distance. Since the pandemic, the number of cases has increased by 17%, affecting almost 37% of school children.
Other evolutionary opportunities for our eyes are currently less obvious. It remains to be seen whether advanced corrective therapies, such as corneal transplants or visual prosthetics, will have any long-term evolutionary effect on the eye.
For now, colored contacts and wearable technology may be our glimpse into the future of vision.