Eye evolution roadmap
Throughout history, many creatures have developed increasingly complex eyes in response to different selective pressures.
However, not all organizations experience the same pressures. This is why some creatures today still have fairly simple eyes, or why some have no eyes at all. These organisms are examples of eyes “frozen” in time. They provide snapshots of the past, or “checkpoints” of how the eye has transformed along its evolutionary journey.
Scientists are studying the genes, anatomy, and vision of these creatures to determine a roadmap for the birth of the eye. And so, we constructed an evolutionary graphical timeline of the different stages of the eye using several candidate species.
Let’s see how the eye formed over time.
Where does the vision come from?
The retina is a layer of nerve tissue, often at the back of the eye, that is sensitive to light.
When light hits it, specialized cells called photoreceptors convert light energy into electrical signals and send them to the brain. Then the brain processes these electrical signals into images, creating vision.
The first form of vision appeared in unicellular organisms. Containing simple nerve cells that can only distinguish light from dark, they are the most common eye in existence today.
The ability to sense shape, direction, and color comes from all the add-ons that evolution introduces into these cells.
Two main types of eyes
Two main types of eyes dominate all species. Despite differing shapes or specialized parts, the improvement in vision in both eye types is the product of small, incremental changes that optimize the physics of light.
Simple eyes are actually quite complex, but get their name from the fact that they consist of an individual unit.
Some molluscs and all higher vertebrates, such as birds, reptiles or humans, have simple eyes.
Simple eyes evolved from a cup of pigment, slowly folding inward over time into the shape we recognize today. Specialized structures such as the lens, cornea, and pupil have emerged to help improve the focusing of light on the retina. This helps create sharper, clearer images for the brain to process.
Compound eyes are formed by repeating the same basic units of photoreceptors called the ommatidia. Each ommatidium is similar to a simple eye, made up of lenses and photoreceptors.
Grouped together, the ommatidia form a geodesic pattern commonly seen in insects and crustaceans.
Our understanding of the evolution of the compound eye is a bit murky, but we do know that rudimentary ommatidia have evolved into larger clustered structures that maximize light capture.
In environments like caves, deep underground, or the ocean floor where there is little or no light, compound eyes are useful for producing vision that gives the least advantage over other species.
How will the vision evolve?
Our increasing reliance on technology and digital devices could usher in the advent of a new form of eye.
The muscles around the eye stretch to move the lens when looking at something nearby. The round shape of the eye lengthens in response to this muscle tension.
Screen time with cellphones, 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 from afar. Since the pandemic, cases have increased by 17%, affecting nearly 37% of school children.
Other evolutionary possibilities for our eyes are currently less obvious. It remains to be seen whether advanced corrective therapies, such as corneal transplants or visual prostheses, will have a long-term evolutionary impact on the eye.
For now, colored contacts and wearable technology may be our glimpse into the future of vision.
Fernald, Russell D. “Shedding Genetic Light on Eye Evolution.” Science, vol. 313, no. 5795, September 29, 2006, p. 1914-1918
Gehring, WJ “New Perspectives on Eye Development and the Evolution of Eyes and Photoreceptors.” Journal of Heredity, vol. 96, no. 3, January 13, 2005, p. 171–184. Accessed December 18, 2019.
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Land, Michael F and Dan-Eric Nilsson. Animal eyes. Oxford; New York, Oxford University Press, 2002.
“The main themes of the research work of Prof. Dan-E. Nilsson: Vision-Research.eu – the gateway to European vision research. Accessed October 3, 2022.
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