The human eye has simple anatomy, but it is a complicated structure whose evolution is also complicated. The complexity of the human eye is yet to be fully discovered. It is documented that Charles Darwin also wondered about the complexity of the human eye. Darwin believed that the small structure, the eye, with a high complexity developed through the naturalistic process of evolution. Darwin’s naturalistic theory of evolution suggests that the complex structures had gradual evolution, and since the eye is a complex structure, it is considered that it also had gradual evolution (Schwab, 2017). Modern scientists use Darwin’s concept of naturalistic evolution, gradual evolution, to study the evolution of the human eye. Darwin proposed a stepwise evolution of the human eye using simple eye structures from different organisms. He demonstrated the differences between eyes that were less complex, and the identified differences were arranged in a stepwise order from the minor complex to the most complex eye, the human eye.
The human eye evolved from a light-sensitive cell in a single-celled organism with a simple flat eyespot, for example, cnidaria. The eyespots could only tell whether it was dark or light and had no pigmented cells showing the directionality of light. The pigmented cell allows an organism to determine the direction of the light so that it can run away from or towards the light. The flat eyespot curved inwards, increasing visual acuity, and the eye could now sense the direction of the right. An organism with a curved eyespot is a flatworm (Schwab, 2017). Constriction of the curved rim forms an aperture. The curved pit stars fill with a clear jelly-like substance at this point of evolution. Mutation of the organism made it easy for the substance to be formed. The accumulated substance protects the light-sensitive cells from chemical damage and maintains the pit’s shape.
As the formed aperture continued to decrease in size, the visual acuity increased, and when aperture became small enough, it could shut out excess light into the eye. The shutting out of the light prevented the sensitive cells from being damaged (Schwab, 2017). As time went by, the aperture evolved to a perfect size. The big and the small aperture have poor eyesight. The perfect size aperture is the narrow-sided aperture, but the perfect size depends on the environment and the amount of lighting. A lens was formed after the aperture was developed to an ideal extent. The lens was created by an increased volume of a ball-shaped mass of cells formed on the eye’s refractive index. The refractive index position gave the eye an improved visual acuity.
After the lens was formed, the aperture had to be placed in another position allowing the lens to move more inward towards the center of the light-sensitive surface of the eye. With time, the lens moved and increased in refractive index (Kraan, 2017). The center of the lens had a greater index than the edges. Since the lens is formed with a mixture of proteins and the proteins are distributed unevenly, the lens can have a greater refractive index than other places. A biologically formed lens has a greater refractive index at the center than the edges. The refractive index improves the quality of the image formed. As the unicellular organisms continued to evolve, the eye also evolved.
The theory of evolution of the human eye has some issues. The morphological gaps are narrow that the process could be simple to move from one step to another, and there would be only one or two mutations involved. It is assumed that in a population with a flat eyespot, there are other organisms with curved eyespots while others have formed lenses (Kraan, 2017). In a group of similar organisms, the microorganisms could have eyespots with further developments. It is believed that the evolution process took place over several generations, and each generation had a selective condition for the evolution process. As the organisms evolve, their brains continue to develop and can interpret the information registered on the eye (Ågren, 2021). Scientists suggest that a flat eyespot can detect the direction of the light, and it only needs to locate the source of the light, and the brain should be able to interpret the intensity of the light. Another potential problem associated with the evolution of the human eye is that it would take a long time to get many light-sensitive cells that help form then curved eyespot.
In conclusion, the eye is a complex structure formed or evolved from simple structures or cells. And according to Charles Darwin, the naturalistic theory of evolution is associated with the evolution of complex structures from simple cells. Modern scientists use Darwin’s idea of eye evolution that the eye had several steps of evolution. The different parts of the human eye started as a simple flat eyespot, and as the organisms continued to evolve, more eye parts were formed, making the human eye a complex structure.
References
Ågren, J. A. (2021). The gene’s-eye view of evolution. Oxford University Press.
Kraan, A., P. (2017). How did eyes evolve? https://www.scienceworld.ca/stories/eyes-how/#:~:text=The%20first%20organisms%20with%20a,complex%20eyes%20we%20have%20today.
Schwab, I., R. (2017). The evolution of eyes: major steps. The Keeler lecture 2017. https://www.nature.com/articles/eye2017226
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