Evolution of the human eye

The human eye is indeed a delicate and complex organ responsible for vision, functioning similar to a camera by focusing light and converting it into electrical signals that the brain can then translate into clear images. This extraordinary mechanism has the potential to observe almost anything from just a few photons to direct sunlight and can shift focus from a screen close by to a distant landscape in just a fraction of a second. In fact, the structures present in the human eye are so incredibly complex that Charles Darwin himself stated that the idea of them having evolved appeared to be “absurd in the highest possible degree”. Furthermore, in his 1859 book, Origin of Species, Darwin elaborated on his theory of evolution by natural selection and explained how it would be bizarre to assume that the eye formed through natural selection. However, despite this, Darwin still believed that the human eye did indeed evolve this way, regardless of the lack of evidence present at the time. The first eyes appeared approximately 541 million years ago in a group of now extinct animals called trilobites, which bore a correspondence to large marine woodlice. These creatures possessed compound eyes composed of various lenses, each capable of distinguishing light. This development allowed the trilobites to have enhanced prey and predator detection, increasing their chances of survival and reproduction. Their sudden appearance in the fossil record is unusual, given that trilobite ancestors from 544 million years ago didn’t have eyes. Surely the intricate interconnection of the retina, lens, pupil and optic nerve in eyes suggests that their complexity could not have arisen abruptly. A crucial development within the evolution of the human eye was the formation of the lens. The lens of the eye allows light to be successfully focussed on photoreceptive cells that are capable of absorbing light and converting it into electrical signals through a process called phototransduction. The earliest lens had a quite simple structure and supposedly consisted of transparent proteins. Gradually, the lens became more and more advanced which enabled many organisms to detect changes in light within their environment, further strengthening their survival possibility. This also led to humans having improved defensive abilities which is deemed helpful when in situations that contain potential dangers. One of the fundamental features that distinguishes between our camera-like eyes providing us with striking vision from those of other organisms is that the human eye contains light-sensing tissues located in the retina. These primarily depend on the retinoids which are responsible for moving the tissues themselves across various cells. A protein known as interphotoreceptor retinoid-binding protein (IRBP) carries out this transportation and according to recent research, this protein originated from a particular bacterial gene that suddenly appeared in the vertebrate lineage over 500 million years ago. This research disclosed how this protein played a critical role in the development of the eyes by transporting retinoids between photoreceptors and the retinal pigment epithelium. In short, without this protein, this risk of catching retinal diseases would be significantly increased. Because of the presence of the light-sensing tissues, humans are able to adapt to environments that have different light intensities, allowing us to function in both dim and light conditions. Despite all this, some scientists still believe that it was just a simple light-sensitive spot which developed into the retina, and over time formed a lens at the front of the eye. This light-sensitive spot has undoubtedly given humans a microscopic survival advantage and once upon a time changes occurred that caused a pit to form in the light-sensitive spot which caused our vision to become sharper. Over time, this pit’s opening became more narrow, allowing light to enter through small slits, mimicking those of a pinhole camera. The evolution of the human eye has therefore been vital to the survival of humans. Through allowing enhanced prey and predator detection as seen with the trilobites, improving defensive abilities, and enabling humans to work in both dim and light conditions, it is evident that the evolution of the human eye has significantly impacted the success of survival for humans.