The human eye is an extraordinary organ made up of interconnecting cells and tissues that work together to give us sight. The cornea, lens, aqueous and vitreous refract light and focus it toward the light-sensitive part of the retina called the fovea, but as complex as vision is, our eyes are still imperfect. In nature, similar eye types have existed for millions of years and there are forms that are more advanced than ours. By looking into the eyes of other animals and exploring the development of the human eye in utero, we can see how the eye has evolved from a small patch of sensitive skin to the advanced visual system it is today.
To have a better understanding of what our eyes, how they function and how something so complex could have evolved in such a way over time, it is important to examine how the human eye develops prior to birth. The majority of the eye’s development begins during the third week in the embryonic stage and carries on until the fetal tenth week. Around the twenty-second day, bilateral grooves form on either side of the forebrain; called the optic vesicles, these grooves continue to grow into optic stalks and, in time, into the optic nerves.
While a portion of the vesicle develops what will eventually be the lens, another part forms into a c-shaped growth called the optic cup. Over the following weeks, the optic cup further progresses, developing the three cellular layers that make up the retina: the outer photoreceptor layer, the inner bipolar layer and the ganglion cell layer. After the lens and retina, the eye gradually forms the cornea, choroid and sclera before the uvea and vitreous.
When determining how the evolution of the structures within our eyes came to be, we must compare them to that of animals both closely related to homo sapiens as well as those who are rather different. In doing this, researchers had uncovered that organisms as distantly related as the lamprey have the same basic components of the eye.
As such, a multitude of organisms from lamprey to human have eyes with a three part retina, cornea and extraocular muscles to control them since our closest relative split apart over 500 million years ago. A relative of the lamprey with an even older common ancestor, the hagfish has quite different eyes from its eel cousin, as it lacks not only the components of the anterior chamber and the external supporting muscles but also only has two retinal cell layers instead of three.
In this, scientists feel they may have found a link. Prior to the version of eyes that any creature from lamprey to human has, researchers believe that the eye evolved in a similar manner as ours do during prenatal development. In the “proto-eye” that organisms had before the structure we have today, animals may have had a two layered, c-shaped eye that closely resembled that of embryos develop in the womb.
Early organisms are thought to have had a small patch of light-sensitive skin that helped them determine whether it was light or dark outside; in time those light sensitive cells formed the photoreceptors in the early retina. Researchers estimate that our modern day, camera-like eyes may have evolved from the little photosensitive patch of skin in as little as 364,000 years.
While humans are unique in several aspects, we share many similarities with other organisms throughout the Chordata phylum. What was once a mechanism to regulate a primitive circadian rhythm evolved into an incredibly complex visual system that helps animals see the environment around them. With an evolution that was once unexplainable, even by the great contributor to evolutional theories Charles Darwin, the origin of the eye can not only be explained, but ideas of intelligent design can be disputed.
The sense of vision has dramatically changed the course of natural history and people today are using the sense to do more and more to interact with the world around us than ever before. The c-shaped eye that develops in utero offers clues to unlock the prehistoric past of the intricate organ that we and trillions of other organisms use to see.
Throughout history of civilization, humans have prided themselves of their many differences from animals, often trying to do more to further distinguish themselves from fish, reptiles, birds and other mammals but a deeper look at ourselves reveals that we are closer to them than we may think.
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