However, not all organizations experience 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 exemplify eyes that are “frozen” in time. They provide snapshots of the past or “checkpoints” of how the eye has transformed throughout its evolutionary journey. Scientists are studying the genes, anatomy and vision of these creatures to figure out a road map of how the eye came to be. And so, we created an evolutionary graphic timeline of the different stages of the eye using several candidate species. Let’s take a look at how the eye has evolved 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 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. Containing simple nerve cells that can only distinguish light from dark, they are the most common eye in existence today. The ability to detect shapes, direction and color comes from all the add-ons inserted into these cells.
Two main types of eyes
Two main eye types dominate the species. Despite having different shapes or specialized parts, the improved vision in both types of eyes is the product of small, incremental changes that optimize the physics of light.
Just Eyes
Simple eyes are actually quite complex, but they get their name because they are made up of a single 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 arose to help improve the focus of light on the retina. This helps create sharper, clearer images for the brain to process.
Compound Eyes
Compound eyes are formed by repeating the same basic photoreceptor units called ommatids. Each ommatidium is similar to a simple eye, consisting 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 little fuzzy, 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 ocean floor where there is little to no light, compound eyes are useful in producing vision that gives even the slightest advantage over other species.
How will Vision evolve?
Our increasing reliance on technology and digital devices may be leading to the emergence of 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. Screen time with 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 far. Since the pandemic, cases have increased by 17%, affecting nearly 37% of students. Other evolutionary opportunities for our eyes are currently less obvious. It remains to be seen whether advanced corrective treatments, such as corneal transplants or optical prostheses, will have any long-term evolutionary effect on the eye. For now, color contacts and wearable technology may be our glimpse into the future of vision. Full sources Fernald, Russell D. “Casting a Genetic Light on the Evolution of Eyes.” Science, vol. 313, no. 5795, 29 Sept. 2006, pp. 1914–1918 Gehring, WJ “New perspectives on eye development and eye and photoreceptor evolution”. Journal of Heredity, vol. 96, no. 3, 13 Jan. 2005, pp. 171–184. Accessed 18 December 2019. “The Evolution of Vision | PHOTO.” 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 research on the European Vision. Accessed 3 Oct. 2022.
