Evolution Of The Eye
The eye is perhaps one of the most sophisticated products of evolution. The eye comes in all shapes and sizes, but its purpose remains the same for all organisms that possess it – to help the organism become more aware of its surroundings using light to create visuals. The eye has become a widespread success evolutionary speaking with most animals today possessing one or more eyes. It is an excellent example of convergent evolution and tells us how the need for eyes and visual stimuli was advantageous for animals despite many living in completely different types of habitats.
Structure of a camera eye
There are of course many versions of the eye. Depending on what evolutionary path you follow, organisms will have evolved different types of eyes, the main ones being the compound eye, the cup eye and the camera eye. The eye of a crab, for example, differs from the eye of an eagle. In this essay, I will be focusing on the camera eye and the steps relevant to the evolutionary course that led to its creation. Some key features of the camera eye include the lens, the pupil and the retina that contains photoreceptor cells. Figure 1 shows a very basic structure of a camera eye. Cambrian explosion
Life is thought to have begun around 3.7 billion years ago. It was during the Cambrian explosion (approx. 5.4 million years ago) that most of the present-day phyla arose. From the study of fossil records, it is believed that the eye started its evolution journey during the Cambrian period. It was recently discovered that the earliest eye to have evolved may have been the eye of an extant trilobite species called Schmidtiellus reeta (Schoenemann, B et al). The eye is relatively simple compared to some species alive today such as the human eye but was a revolutionary tool for facilitating movement and navigation of the terrain 530 million years ago.
Based on data from a study conducted by Zhao, F et al, over a third of the 9941 fossil records studied possessed eyes (Zhao, F et al 2013). Despite eyes being relatively common among the early metazoans, the record of these soft tissue structures is unfortunately limited. This is because they are not an ideal specimen for fossilisation due to, as mentioned previously, the soft tissue that makes them (Zhao, F et al 2013).
It has been hypothesised that the evolution of the eye could have been one of the key elements responsible for the Cambrian explosions vast and rapid diversification (Zhao, F et al 2013). The eyes most basic ability, to detect light and dark, consequentially introduced new behavioural systems to metazoan animals. These will be briefly discussed later in the essay. It was these new behavioural systems that perhaps propelled the evolution of these animals, leading to the Cambrian explosion (Parker, A.R., 1998.)
From eyespot to eye
If we look at some species alive today, we can be taken back in the time of the evolutionary journey of the eye, giving us plausible theories of how to eye came to be. Invertebrates of today have relatively simple eyes, Dugesia japonica, for example, is a flatworm that possesses eyespots. These eyespots refer to a layer of photosensitive cells that contain a pigment that reacts to light. The brain then interprets this information to form an image.
Eyespots lack most of the essential components that our eyes need to form a detailed image, such as a lens. Although the image formed is far less detailed than our own, it would have allowed the creature to distinguish between light and dark, as well as shadows which is important for escaping prey. As a result, those organisms fortunate enough to have the mutation that was responsible for this ‘prototype’ of an eye would have a higher chance of survival and consequentially a higher chance of reproducing and passing on this gene. It would have also meant that the animal could distinguish between night and day (Nilsson, D.E., 2009). The cycle of night and day is relevant to most animals. It can act as an indicator of whether plants are photosynthesizing and thus provides an ideal feeding schedule for animals who feed on these autotrophs. (Nilsson, D.E., 2009). Therefore, it can be deduced that the eye played some part in the diverse range of nocturnal and diurnal behaviours in animals today. Therefore, the eyespot itself – despite being relatively simple- would have been advantageous for individuals who had it and thus may have increased the likelihood of an animal to survive and reproduce, passing on its genes. This would explain how even the slight resemblance of an eye millions of years ago would become successful, evolutionary speaking.
The next step of generating a more detailed image through the eye would have been the indentation of the photosensitive cells, forming an eye cup structure (Schwab, I.R., 2018) as shown in figure 2. This step is important because now the animal can tell which direction the light is coming from and provides significant spatial information, thus making it useful in the determination of what direction a predator is coming from. To detect the direction light is coming from, multiple photocells need to be facing different directions (Dawkins, R. and Ward, L. 1996). One way of achieving this is by creating a concave shape by which the photocells follow (Dawkins, R. and Ward, L. 1996). The brain can distinguish the direction of light by registering what photocells are active and what ones are not (see figure 2). Natural selections favouritism over a slight indentation of the photocells would give rise to the camera and compound eyes we see today. After many generations, the steepness of the cup would increase to the point that a pinhole eye is formed (Schwab, I.R., 2018). An example of an animal today with a pinhole eye is Nautilus pompilius (Schwab, I.R., 2018). The eye of this animal lacks a lens with the pinhole doing most of the light focusing. The pinhole eye is useful because now a slightly more detailed image can be formed. A comparatively large pinhole results in more light entering the eye and penetrating the retina, but light from the source is spread out making the image blurry (Wandell, B.A., 1995). On the other hand, with a smaller pinhole more of the light from the source is focused onto a smaller area of the retina, generating a sharper image, however the quality of the image is limited by diffraction and a smaller pinhole means little light can penetrate the photosensitive cells (Wandell, B.A., 1995).
To solve this problem the lens evolved. By introducing the lens, it meant that an increase in light sensitivity and resolution could be achieved while also maintaining a very small pinhole (Nilsson, D.E. and Pelger, S., 1994). The main purpose of the lens can be summarised as refracting light and protection of the eye against light damage (Dejong et al. 1989). By being transparent, light can enter the eye and reach the retina with little distortion (Bassnett, S. et al 2011). Furthermore, the region of the eye detecting light can be moved from the area of the receptor cells to the area of the lens (figure 1 b). This is useful as the diameter of a lens is larger than that of the receptor cells. The lens can range from millimetres to centimetres in diameter compared to the 5–10 µm diameter of the receptor cells (Nilsson, D.E., 2009). In short, the lens increases the light detection area. One suggested theory of how the lens evolved is that a progressive increase in the concentration of proteins inside the tissue of the eye, in front of the retina formed a structure slightly resembling the lens as we know it (Nilsson, D.E. and Pelger, S., 1994). A random mutation that involved a translucent membrane or piece of tissue to form in front of the retina may have been the foundation on which the lens evolved. It is this process of random luck with mutations that is core to natural selection. Evolution does not have any intentions of building an eye, it simply chooses the best-adapted organism through the process of natural selection, resulting in organisms with the best-equipped tools for survival.
The complexity of the eyes of some present-day animals can be seen as a miraculous phenomenon. However, when considering that each small step, from the eyespot to the eyecup for example, took many years it doesn’t seem as miraculous. Natural selection is an expert at selecting the most seemingly unimpressive mutation, a slight indentation of photoreceptive cells for example, and over millions of years transforming a series of very slight mutations into a new feature of an animal. Half of an eye is better than none, the same is true for 49% or 2%. Any structure leading up to the eye as we know it acts as the foundation upon which the eye is modelled.