Vision

Vision is a complex mechanism that exists in many forms across the animal kingdom. Planaria have very simple eyes, consisting of just a few light sensitive cells, detect whether light is present and allow them to perform phototaxis. More complex eyes have a retina with rods and cones as well as a lens which can be bent for vision at different focal lengths. These complex eyes have the capacity to produce fully formed images of the environment that appear different based on the rods and cones in an animal’s retina. Rods and cones are the visual cells that allow eyes to see different wavelengths of light. As the name suggests, rods have rod-like endings filled with opsins that can perceive more light under darker conditions than cones can, and are often found in the peripheral area of the retina. Nocturnal animals often have a preponderance of these in their retina in order to better see at night. Cones have cone-shaped endings that are also filled with opsins, however they can possess opsins of many different conformations. Opsins are a group of proteins that allow for light detection, with different opsin conformations allowing for the detection of different wavelengths of light. For example, humans have red, green, and blue cones, each with opsins keyed to those wavelengths of visible light. Our eyes receive light information from the environment and then our brain relates that to known ideas about colour, brightness, and other visual information. Different animals have different opsins, often those that best serve their survival. For example, rats can’t see red light. As nocturnal animals, rats have no need for red light since it’s longer wavelength and isn’t present at night, unlike blue light or UV. In their evolutionary past, rats no longer needed to see red, and so the opsin was free to mutate and disappear, but they did need to see UV, leading to selection for a UV-detecting opsin. Very rarely, humans will develop a mutation that leads to the development of a fourth opsin, and hence a new type of cone, that allows them to see more colours than non-mutant humans.