The Evolution of the Human Eye


Hello, fellow honors Biology 3 students. My name is Zaid Peracha. My topic will focus on the evolution of the human eye. I am going to start my presentation by explaining why the eye is important. Now, let’s get started. The human eye is a complex and extraordinary organ. It has the ability to convert “light waves into visual images” and can detect anywhere from small amounts of photons to direct sunlight (Punzo). The eye’s key function of light conversion is crucial when it comes to seeing and understanding the world around us. Without your eyes, life would be a little more difficult. The picture shown shows a diagram of the human eye with its labeled structures. Now, I am going to get into the evolution of the eye and how our eyes became the complex structures we now know of today. Charles Darwin, a well-known nineteenth-century biologist, and naturalist, “considered the evolution of the human eye one of the toughest problems” his theory had to explain and believed that it seemed absurd to think the eye could evolve (Zimmer 1). Surprisingly, scientists have been able to trace the formation of the eye back five hundred million years ago. The evolution of the eye starts at light spots or pit eyes. A pit eye is a “single cup-shaped chamber” with an interior of photoreceptors that lie along the interior (Pankey 2). Pit eyes are usually found in “single-celled organisms,” such as bacteria, and these pit eyes help the bacteria find light where food is usually present (Pankey 2). Pit eyes or light spots usually form in clusters and are linked to the flagellum of specific bacteria. The light spot in single-celled organisms slowly developed into a cup-shape. The cup-shaped eye “are sensitive to detecting movement” which helped organisms hide from predators (Thompson 1). The picture shown is the structure of a single-celled organism called a Euglena. Euglena’s contain light spots and use that light spot to search for food. Euglena can be found in soil or still water. Over the millennia, the light cup became larger with a smaller opening. The small opening “improved image resolution over pit eyes” but, was only helpful when viewing simple images (Pankey 2). The picture shown is Professor Guzman reminding you that each step in the evolution of the eye is more beneficial, meaning that these organisms survived with those traits and then passed on that specific trait to the next generation. One of the key steps in the evolution of the eye was the development of the cornea. The cornea allowed the inside of the eye to not only be protected from infection but caused the inside of the eye to fill with fluid. The fluid within the eye helped with “maximizing the number of photons captured in dim light conditions” and optimized the light sensitivity (Pankey 2). The diagram illustrates the developing eye and points out where the cornea developed. The light blue represents the fluid within the eye. Not long after, translucent proteins formed at the surface of the eye, also known as the lens. The lens helps focus light on the retina. The focusing of light helps an organism develop far and near vision. The diagram depicts light entering the eye through the lens which is focused onto the retina. Due to the development of near and far vision, the iris formed and helped maintain how much light enters the eye. This helps by making sure the eye is accepting enough light in certain conditions in order to provide proper vision. The sclera and tear glands did not form long after the iris. The sclera helps provide structure for the eye. The Tear glands provide a protective film for the eye that helps with protecting the eye from infection and from foreign objects that enter the eye. The diagram shows where the iris, sclera, and tear glands are located in the human eye. With the evolution of the eye, the brain also evolved in order to give the eye the ability to fully gain and use its complex features. “The visual cortex in the occipital lobe of the cerebrum of the brain” expanded to aid the eye to detect sharper and colorful images (Thompson 1). Each part of the eye has an important role and in the next slide, I will go through the process of how light enters the eye in the developed human eye we know of today. Light enters the eye by first going through the cornea. The cornea is the “transparent layer at the front of the eye” that protects the eye from infection (Thompson 1). Light will then pass through the pupil. The pupil is the opening in the center part of the eye called the iris, which is the colored part of the eye. Light will then go “through a lens behind the iris” (Thompson 1). This will allow the lens to focus the light onto the retina which the retina will then turn into electrical impulses. These electrical impulses will “then [be] carried [into] the brain via the optic nerves” (Thompson 1). The brain interprets these electrical impulses as images. Due to evolution, the process of light conversion developed from a simple process to the complex process that is known today. In comparison to other creatures with smaller brains, such as reptiles, fish, and insects, their eyes’ development went through different biological changes due to the “historical, physical, and functional constraints” of the environment they live in (Pankey 6). As an example, due to different visual needs, “vertebrates vary in the proportion of rod and cone cells” (Pankey 3). The “tokay gecko retinas have only rod cells” and this gives them an advantage when it comes to seeing objects in the dark (Pankey 3). Humans contain both rod and cone cells because humans have evolved to see in both light and dark environments, with a little adjustment. Our eyes cannot see well in the dark as well as the tokay gecko because these geckos have a significant number of rod cells in comparison to humans. In addition, cephalopods, best known as squid or octopus, have a pretty large advantage in comparison to humans when it comes to vision. Most cephalopods have front-facing retinas which gives them the ability to not have a blindspot. Humans, on the other hand, have an inverted retina which created a blindspot. Due to the cephalopod’s ability to not have a blind spot, they are able to see predators at different angles which helps them escape quicker. Humans have to turn their heads if they want to see where their blindspots do not allow them to. Today, scientists are developing new ways to make our eyes better and want to help the less fortunate who do not have the privilege of vision at all. The eye took five hundred million years to develop into the way we know of today. Scientists may be able to speed up the evolution process in order to make our lives better. If it will make our lives better, the world just has to wait and see.

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