Our vision is responsible for perceiving colours and shapes in our environment. This is possible because of a special type of cells present in the eye retina* named cone cells. These cells contain pigments that absorb light at different wavelengths of the luminous spectrum. In simple terms, these pigments can sense different colours. The presence of different cone cells with different pigments is what supports colour vision. The greater the diversity of pigments, the more colours will be perceived. When the light interacts with the cone cells, these will transmit an electric signal to the brain. The brain will combine the information from all the different cells to construct a detailed picture of the outside world.
Colour vision in vertebrates* evolved 540 Mya in an early ancestor. This vertebrate ancestor carried four types of cone cells and therefore four types of pigments. These prehistoric cone cells were able to perceive light in the red, green, blue and violet or ultraviolet spectra. This type of colour vision is referred to as tetra-chromatic because the four different pigments enable the perception of four different colours. Vertebrates that evolved from this early ancestor, such as fish, birds and reptiles, therefore also possess tetra-chromatic vision. Diurnal* fish that live in shallow waters and are exposed to light, for example salmon, still have the ability to distinguish four colours. Similarly, modern-day reptiles and birds, such as crocodiles or sparrows, also have tetra-chromatic vision.
Mammals evolved from reptiles approximately 200 Mya in an era dominated by the most famous reptiles, the dinosaurs. In order to avoid predation, scientists believe that our mammalian ancestors were nocturnal and hid during the day. In the darkness of the night there was no need for colour perception. Thus, early mammals must have had tetra-chromatic vision but over time two of the four pigments degenerated. Consequently, mammals lost their ability of distinguishing four colours and their vision became di-chromatic. This is why the great majority of modern-day mammals, such as our beloved cats and dogs, can only see two colours.
Dinosaurs extinguished 65 Mya and so the lower predatory pressure permitted mammals to expand their habitats and acquire diurnal* habits. During that time, primates evolved an extra pigment which enabled them to distinguish blue green and red. These primates therefore had tri-chromatic vision. Scientists believe that the evolution of tri-chromatic vision in primates allowed them to detect ripe fruits and suitable leaves to eat. Just as the other primates, most humans can distinguish three colours: blue green and red. However, there are exceptions to this rule. In capuchin monkeys, for instance, only the females have tri-chromatic vision. In humans, not being able to distinguish three colours is called “colour blindness” or “colour vision deficiency”. This is quite common and occurs in 8% of men and 0.5% of women in the world.
Why do we have a mixture of di- and tri-chromatic vision in some primates?
Primates are social animals and live in groups. It is believed that tri-chromatic vision in some female primates makes them better at locating areas with ripe fruits and edible leaves. These females can then lead the rest of the group towards the food source. Di-chromatic individuals, on the other hand, are better at distinguishing patterns and hence quicker at detecting predators with camouflage patterns. These individuals may be responsible for raising the alarm to alert the rest of the group. Therefore, in a social context, having a mixed group of individuals with both tri- and di-chromatic vision may be more beneficial than a group with uniform vision.
Could it be possible that the same is true for humans? Maybe “colour blind” individuals in pre-historic times were responsible for detecting approaching predators, thus protecting the group.
To understand the text you might need the following concepts:
What is the retina?
Is a region of the eye normally located at the outermost layer ocular globe. This region contains the cells responsible from sensing the light from the outside. It also connects these cells with the neurons that will transmit the information to the visual areas of the brain.(1)
What are vertebrates?
Vertebrates are a subphylum of animals that include jawless fish, jawed vertebrates, tetrapods (reptiles, amphibians, birds and mammals) and fish with bones. They are characterised for having a back bone amongst other features.(2)
What are diurnal animals?
Diurnal animals are those that remain active during the day and are inactive or sleeping durin thte night.
Further reading and references:
Borges, Rui, Warren E. Johnson, Stephen J. O’Brien, Cidália Gomes, Christopher P. Heesy, and Agostinho Antunes. 2018. “Adaptive Genomic Evolution of Opsins Reveals That Early Mammals Flourished in Nocturnal Environments.” BMC Genomics 19(1):121.
Bowmaker, James K. 1998. “Evolution of Colour Vision in Vertebrates.” Eye 12(3):541–47.
Carvalho, Livia S., Daniel M. A. Pessoa, Jessica K. Mountford, Wayne I. L. Davies, and David M. Hunt. 2017. “The Genetic and Evolutionary Drives behind Primate Color Vision.” Frontiers in Ecology and Evolution 5.
Jacobs, Gerald H. 2009. “Evolution of Colour Vision in Mammals.” Philosophical Transactions of the Royal Society B: Biological Sciences 364(1531):2957–67.
Lucas, Peter W., Nathaniel J. Dominy, Pablo Riba‐Hernandez, Kathryn E. Stoner, Nayuta Yamashita, Esteban Lorí- Calderön, Wanda Petersen‐Pereira, Yahaira Rojas‐DurÁN, Ruth Salas‐Pena, Silvia Solis‐Madrigal, Daniel Osorio, and Brian W. Darvell. 2003. “Evolution and Function of Routine Trichromatic Vision in Primates.” Evolution 57(11):2636–43.
Saito, Atsuko, Akichika Mikami, Takayuki Hosokawa, and Toshikazu Hasegawa. 2006. “Advantage of Dichromats over Trichromats in Discrimination of Color-Camouflaged Stimuli in Humans.” Perceptual and Motor Skills 102(1):3–12.