Colour Vision: Evolution and variation in the animal kingdom; and the evolutionary significance of Deuteranomaly in humans.
-Abhibyanjana R. Thatal
In a bullfight, a bull is released in an arena where a matador is seen standing waving a red cloth that the bull charges on. A common misconception in this bullfight is that the bulls charge at the sight of the red color. Today, we have come to understand that the bull is not furious because of the red color but the waving of the cloth. In fact, bulls, like other cattle, are color-blind to red. This does not mean that they completely lack the ability to perceive colors; they can see only two of the three (Red, Green, and Blue) primary colors. This is also known as dichromatic vision. The way humans perceive red is not the same way a bull can perceive it, due to the anatomical nature of the eyes. Humans, on the other hand, are able to see all the three primary colors and are trichromatic in nature. In the paper titled The causes and consequences of color vision by Ellen Gerl and Molly Morris, they discuss the various kinds of color vision in humans and animals and the study of color perception in animals once we have come realize that animals don’t perceive the world the same way humans can. Humans and certain primates have evolved to have a trichromatic vision, this is due to the three classes of photopigments present in cones of the human retina which makes us able to perceive the wavelengths of the three primary colors, red, green and blue. The mixing of these three wavelengths through the filters in our photopigments enables us to the see the world in a wide spectrum of colors. Trichromatic vision in human beings has helped them process the visual information of the world through color, making it more comprehensible. There are many advantages of being able to see colors in trichromatic mode, humans and primates had the need to distinguish colors in nature in order to understand and communicate better with other human beings and nature. It is important that we have a color vision in order for us to notice subtle changes in expressions in the face while interacting with other people. (Changizi, 2010) Expressions that are not very salient like the light blushing of the cheeks when people are nervous or embarrassed, a trichromatic vision allows us to see that, improving our communication.
Another reason why color vision is important to human beings is for us to understand if certain fruits, berries, and even food are consumable. In the paper titled, Food color is in the eye of the beholder: the role of human trichromatic vision in food evaluation by Francesco Foroni, Giulio Pergola and Raffaella Ida Rumaiti, it is explained to the readers that humans and most apes and old primates with trichromatic vision distinguish and judge the nutrition value of food based on the brightness and color of the food. (Foroni et al, 2016) For instance, red indicates a higher protein content in the food.
In humans and their close ancestors, the trichromatic vision has been of evolutionary advantageous for the species to foraging, which was and still is an important way in which they get food and nutrition. (Caine et al 2010) This is perhaps also the reason why natural selection favored trichromatic vision for human beings. Among other species in nature, there are different kinds of vision which have been adaptive and useful for the survival of the species, for example, frogs and other reptiles are able to perceive ultraviolet rays which humans can’t see. Gerland Morris also explains in their paper The causes and consequences of color vision, that birds have the most sophisticated color vision as they have five classes of photopigment cones, as a result, they are able to see hues that we cannot see. (Gerl and Morris, 2008)The opsins gene which is responsible for the formation of photopigments generates five photopigments, out of which four are linked to the cone. (Yokoyama, 2000) The color vision and its importance are also linked with the complex brain activities of human and some primates in comparison to other mammals that enable them to interpret and us the color information in ways that other animals with limited central processing capacity couldn’t. (Jacobs, 2009) However, not all species require this ability to process information of the world. While it may seem that trichromatic vision is certainly more useful when compared to a dichromatic vision as it enables animals to distinguish only a limited number of colors, it may not entirely be true for all animals. For instance, most nocturnal animals would require a more sophisticated vision for the night when there is no light to process color information, hence they would not necessarily need a trichromatic vision. Most mammals have been found to have dichromatic vision, like dogs and horses, cattle and many more who rely on rod functioning more than the cone are able to see and process information better than humans or trichromats in low-light. (Gerl and Morris, 2008)
Humans though said to be trichromats have a certain percentage of people who are commonly called color-blind, this means that they are not able to perceive one of the three primary colors in nature, usually they cannot distinguish between red and green in nature, as a result, they have limited vision and are dichromatic in nature. For a long time, it was seen as a disadvantage and people with dichromatic vision were barred from certain professions like the navy and army. It is seen that human males have a higher chance of being born dichromatic than a female. (Gerla and Morris, 2008) One in twelve men experiences color-blindness, whereas one in two hundred women experience color-blindness. (“Types of Color Blindness – Color Blind Awareness”, 2018) This is due to the fact that in color-blindness, the defective gene that codes for opsin which is responsible for the formation of photopigments is an X-linked recessive trait. As a result when a female carrier produces offspring with a normal male and the offspring is a male with the recessive x-linked opsin gene defect they will be color-blind. This is the reason why males have a higher chance of being color-blind as compared to males.
When it comes to a dichromatic colour vision in humans the limitation of colour vision may vary from person to person, to be specific, people suffering from protanopia are unable to distinguish the ‘red’ light. Those with tritanopia are unable to perceive ‘blue’ light and those with deuteranopia are unable to perceive the ‘green’ light in nature. It appears that the huge amount of colour information to the eye can cause us to miss the textural information of objects around us, however, a person with deuteranomaly (Colour-blind person) can detect and process textural information that a person with normal vision couldn’t. (Morgan, Adam, and Mollon,1992) Though it may seem obvious why trichromatic vision was favoured by nature in human beings, chromatic information is not the only kind of data that can be perceived from nature, as we can see in case of other animals like dogs etc. Dichromatic vision doesn’t rely on chromatic information as much and asses other visual cues or information of objects and surrounding like luminance, texture, depth etc. (Morgan et al 1992) this certainly shows a certain kind of advantage over a trichromatic vision. It was observed in an experiment carried out by Morgan, Adam, and Mollon where they took 16 subjects who had a trichromatic vision and seven male dichromats, that people who are colour-blind seemed to be unaffected by the use of camouflage to the target and area. Another experiment conducted by Nancy G.Caine1, Daniel Osorio and Nicholas I. Mundy where they take 15 marmosets out of which nine males and six females were exposed to low-light and normal light foraging, conclusively shows that even when it came to foraging dichromats had more advantage in low light intensity environment as compared to trichromats, in fact dichromatic capuchins had a higher chance of insect foraging than trichromats. (Caine, Osorio & Mundy,2009) All these experimentations and their result which indicate a certain advantage of dichromats when it comes to detecting texture, luminance, camouflage and detects objects in lowlight point to an advantage which may be causing deuteranomaly to be high among human beings. It also suggests that apart from the obvious disadvantage of having a dichromatic vision, it also enables them to perceive certain information which may be ignored by a heavily chromatic reliance on information of the surrounding by individuals with trichromatic vision, which is most likely the reason why we have a high frequency of colour-blindness in human beings.
- Caine, N., Osorio, D., & Mundy, N. (2009). A foraging advantage for dichromatic marmosets (Callithrix geoffroyi) at low light intensity. Biology Letters, 6(1), 36-38. doi: 10.1098/rsbl.2009.0591
- The Evolutionary Purpose of Color. (2018). Retrieved from https://upliftconnect.com/evolutionary-purpose-color/
- Gerl, E., & Morris, M. (2008). The causes and consequences of color vision. Springer Science + Business Media.
- Jacobs, G. (2009). Evolution of color vision in mammals. Philosophical Transactions Of The Royal Society B: Biological Sciences, 364(1531), 2957-2967. doi: 10.1098/rstb.2009.0039
- Foroni, F., Pergola, G., & Rumiati, R. (2016). Food color is in the eye of the beholder: the role of human trichromatic vision in food evaluation. Scientific Reports, 6(1). doi: 10.1038/srep37034
- Morgan, M., Adam, A., & Mollon, J. (1992). Dichromats detect color-camouflaged objects that are not detected by trichromats. Proceedings: Biological Sciences, 248(1323), 291-295.
- Types of Color Blindness – Color Blind Awareness. (2018). Retrieved from http://www.colorblindawareness.org/color-blindness/types-of-color-blindness/