Trichromatic colour vision in humans
All primates have trichromatic colour vision, a trait that other eutherian mammals do not possess (Surridge, Osorio & Mundy, 2016). Jacobs (2012) has provided a conventional definition of colour vision, terming it as “the ability of animals to reliably discriminate objects and lights based solely on differences in their spectral properties” (p. 156). Primates are thought to have developed colour vision as an aid in locating food within the forest. This is demonstrated in recent research on the foraging abilities of various trichromatic and dichromatic primates. Additionally, several studies have also been aimed towards determining the chemical attributes of different wild foods to further shed light on how primates have evolved in their trichromatic vision. The focus of this essay is to look at the adaptive benefits of trichromatic visions and how we, as humans, have over the years evolved this ability. To do so, there is need to examine human colour vision form an evolutionary perspective.
The evolution of trichromatic vision can be traced back between 80 and 30 million years ago when humans are believed to have developed an ability to differentiate between blue light and UV Light. Thereafter, humans also developed sensitivity for the colour green (Yokoyama, et al., 2014). These abilities were added to the red sensitivity that was already present in our early ancestors.
The development in these abilities has a genetic basis to it. According to Yokoyama et al. (2014), humans are believed to have developed sensitivity to the blue visual pigment in the Boreoeutherian era over the duration of seven mutations. This new ability can in turn be traced from the UV-sensitive pigment with more than 5,040 likely evolutionary paths beings associated with this relationship.
Our ancestors are believed to have been predominantly nocturnal, an environmental factor that could possibly explain their limited colour vision. The evolution in colour vision therefore was an important adaptive trait in terms of survival.
As early as the nineteenth century, scientists had identified that fruit colouration tended to evolve with an evolvement in animal colour vision (Jacobs 2007). In this case the main purpose of colour vision was to enable primates to distinguish bold coloured targets alongside a green background full of foliage. In such as a scenario, lightness tends to differ. Based on this finding, visual modelling studies indicate that trichromats should have enhanced discrimination of the red-green pigment. This is because trichromats express two Medium/Low Opsins and hence their ability to detect fruits conspicuously (Kawamura et al., 2014). Both prosimians and New World primates are associated with poly-allelic X-linked polymorphism, and this may be due to a heterotic process.
Unlike other mammals, primates have trichromacy adaptability because of possessing three different cones in the retina of their eyes. Each of these cones expresses a different visual pigment (Carvalho et al., 2017). The differences in cone types to express different visual pigment are because of variations in their sensitivity (Cavarlho et al., 2017). There is a widely held assumption among scientists that primates only had dichromatic pigments in their ancestral lineage namely, long wave sensitivity and short wave sensitivity. Some primate species are believed to have evolved further and developed trichromacy through a replication of the Long Wave Sensitivity pigment. Consequently, these primates underwent a mutational drift, effectively generating trichromacy capabilities. Jacobs and Deegan (20019) report that of all the anthropoid primates, only humans, monkeys and apes demonstrate routine trichromatic vision in both males and females. Chromosome 7 carried the genetic code for the short wavelength pigment opsins, whereas the long and middle wavelength pigment opsins are carried by neighbouring sites located on the X-chromosomes. The fact that not all primates show sensitivity to the three cones is indicative of geographic differences in regard to primate colour vision (Dominy, Svenning & Li, 2003).
The development of gene sequencing nearly three decades ago was instrumental in isolating and sequencing human cone opsins. Through gene sequencing results, scientist can now confirm that the cone opsin genes came about as early as 540 Ma. The four opsin genes that underlie cone pigments have been identified in the major vertebrate groups, but mammals appear to diverge from this evolutionary trend. Monotremes and eutherians lineages shows signs of variation from other mammals as early as 166 Ma and 148 Ma, in that order (Jacobs, 2009). Investigations of opsin genes and cone pigments in marsupials and monotremes could perhaps give us an insight into the evolutionary norm of mammals in terms of their colour vision. Studies on earliest mammals reveal that they possess three main cone pigments namely, LWS, SWS1, and SWS2; however, the sws1 gene was seen to degenerate in the ensuing monotreme evolution, thereby losing its functionality.
As humans, we evolved the trichromatic vision from our primitive ancestors through various genetic mutations that spanned across millions of years. Through years of research scientists have finally managed to put the pieces together in a study that explicitly details how humans developed the capability to see blue light. The research, which was published in the PLOS Genetics journal, was conducted by biologists from Emory University under the leadership of lead researcher Shozo Yokoyama. From the study, Yokoyama et al. identified five categories of opsin genes that predetermine visual pigments for colour vision, as well as dim-light. Further, the researchers revealed that the opsin genes tend to vary with a subsequent adapting of vision, in line with changes in the species environment. Yokoyama et al. further note that nearly 90 million years ago our then primitive ancestors indulged in nocturnal behaviour. They were also characterised by sensitivities to red colour and UV light. Consequently, these primitive mammals viewed the world around them from a bi-cromatic perspective. However, about 30 million ago, these primitive mammalian ancestors had evolved considerably; they now possessed four categories of opsin genes. As a result, they could now see in both during the day and at night.
The ability of trichromatic primates to develop the red-green visual capability is associated with foraging benefits. Primates with trichromatic vision have an advantage over those with dichromatic vision in that they are capable of discriminating between green and red. Consequently, primate trichromacy is seen as a desirable adaptation trait in that it enables the primates to detect foods amidst a background full of mature foliage. In this case, this evolution enables trichromatic primates to readily detect young leaves or ripe fruits from other mature foliage (Lucas et al., 2003). This foraging capability was put to the test by Caine and Mundy (2000) in an experiment in which trichromatic marmosets were shown to have an improved ability to detect yellow/red fruits amidst a green foliage environment, relative to the dichromatic marmosets. The primates were kept in a naturalized captive setting with the trichromatic marmosets showing a high capability to identify more orange Kix in comparison with dichromatic marmosets. This was after the researchers had scattered food items in the cage where these animals were being held. This was the case even when less noticeable food items were left on the floor. Based on these results, trichromats were seen to have an enhanced foraging capability.
Trichromatics tend to exhibit perceived brightness characteristic, which is due to the combined effect of the MWS and LWS cones (Sharpe et al., 2005). This perceived brightness or luminance is believed to aid in the detection of movement and pattern. There are several benefits associated with trichromatic vision the ability to identify red targets when confronted with fluctuating lightning conditions. Besides, trichromatics tend to exhibit better detection abilities of predators and sexual signals (Pessoa et al., 2014). A modelling study conducted by Osorio et al. (2004) showed that trichromats preferring dim lighting conditions as they searched for fruits. This is due to the fact that cones that detect short wavelengths exhibit a lower photon catch in comparison with the cones sensitive to either long or medium wavelengths.
In conclusion, humans are characterised by a trichromatic colour vision, a trait that has come about through numerous genetic mutations of our primitive ancestors, spanning millions of years. The evolution started with sensitivity for blue light, followed by a green sensitivity. Various scientists have been instrumental in tracing the evolutionary development of trichromatic colour vision in mammals, key among them being Shozo Yokoyama et al. From their study, sensitivity to the blue visual pigment can be traced back to the Boreoeutherians. An analysis of the genetic mutations that occurred along with the historical sequence of events is vital in terms of helping to shed more light on molecular evolution and its implications on functional change, including the choice of mating partners, detection of prey and location of food.