Tetrapod vertebrates originally had four cone opsin genes that allowed them to see ultraviolet light. Over time, mutations caused some lineages to lose the ultraviolet and one other cone, resulting in two cone vision. Later, a gene duplication in old world monkeys led to the three cone system of trichromacy found in humans and other primates, allowing distinction of red and green wavelengths. This helped primates detect ripe foliage. While humans have three cones, some species have more, and one woman was even found to have four cones, seeing more colors than typical humans.

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Taylor Gut
Evolution of Trichromatic Vision in Vertebrates
The eye allows vertebrates to experience the vibrant hues of a fall landscape, a summer
sunset, and even poisonous markings on a coral snake. Unfortunately, not all species are as
proficient in their ability to sense color. While the anatomy of the eye itself is an evolutionary
marvel, it is the cone opsins of photoreceptors in the retina that allow for color vision (Shichida
and Matsuyama 2009). Objects reflect certain wavelengths of light which determine the colors
that individuals may see. Humans can see millions of colors because of short, medium, and long
cone opsins. These three specific cone opsins allow most humans to see wavelengths between
390-700nm. However, throughout evolutionary history, the number of cones and their ability to
absorb wavelengths of light has undergone some drastic changes (Judson, 2009). Although
common ancestors of vertebrates relied on two or four opsins to observe colored, a series of
evolutionary adaptations have resulted in the three opsin vision system of trichromacy found in
humans and other primates.
Tetrapods Originally Exhibited Four Cone Opsins
Species that rely on a fourth cone opsin for vision can sense color at wavelengths of 300-
400nm, a range known as ultraviolet light. For example, some bees, birds, and other reptiles
have the capacity to sense ultraviolet light reflected off of flowers, potential mates, and their prey
(Greenwood, 2015). Surprisingly, evidence from gene sequencing demonstrates that the common
ancestor of tetrapod vertebrates possessed this ultraviolet cone opsin in addition to three others
(Figure 1). Conservation of long and short wavelength absorption in almost all tetrapods is
another indication of common lineage (Nathans, 1999). The presence of all four cone opsins is
thought to have been established approximately 360 million years ago (Jacobs, 2009). While the

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lineage of opsins ends here in some species, the evolution of trichromacy in humans required an
intermediate step that took an immense amount of time.
Four Cone Opsins to Two Cone Opsins
Before trichromats were present, the nocturnal lifestyle of mammalian ancestors drove
evolution of only two functioning cone opsins that absorbed short and long wavelengths. These
species are classified as dichromats and are excellent at detecting movement (Mollon, 1999).
Mutations in the ultraviolet cone opsin caused it to function analogous to short wavelength cone
opsins resulting in mammalian ability to sense blue and not ultraviolet light (Greenwood, 2015).
A series of seven distinct amino acid mutations has been identified as the most likely explanation
leading to the switch from ultraviolet to short wavelength. Interestingly, when these amino acid
mutations were experimentally implemented, the order in which they were applied was essential
and there were no effects when the mutations were applied independently (Yokoyama, 2015).
The gene for blue pigment, located on the short cone opsin, is on a non-sex chromosome
(chromosome 7); therefore, deficiencies observing this wavelength are rarely seen and this gene
is highly conserved throughout mammalian phyla. Along with the loss of ultraviolet sensation,
the ability to distinguish between red-green wavelengths is missing in modern day dichromats
such as dogs, cats, and elephants (Nathans, 1989).
Two Cone Opsins to Three Cone Opsins
To better illustrate the distinction between dichromats to trichromats, it is important to
discuss vision in New World monkeys compared to Old World monkeys. Dichromatic New
World monkeys include species such as marmosets, howlers, and capuchins. Old World
monkeys encompass species such as baboons, macaques, and guenons. Data displayed in Figure
2 shows evidence that New World monkeys’ lack relative sensitivity to medium wavelength light

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which is observed at approximately 540nm (Mollon, 1999). Color vision has been widely
studied in New World monkeys so we can generalize this lack of medium wavelength to other
mammalian dichromats (Bowmaker, 1990).
Trichromatic vision in Old World monkeys has been tightly linked to that of humans.
More specifically, experiments utilizing suction electrode techniques show similar characteristics
of spectral sensitivity in macaques and humans. This could be a result of analogous cone opsins
developing divergently in multiple lineages that ultimately resulted in trichromacy (Bowmaker,
1990). A duplication of the cone gene on the X chromosome continued to mutate to form long to
middle photopigments which lead to the three cone system. This directed the ability of
trichromats to distinguish between red and green wavelengths. This idea is supported because
the genes of the red opsin differ between humans and birds (Travis, 2003). The most strongly
supported theory as to why species adapted this difference in red and green wavelengths is so
that ripe foliage could be detected during scavenging (Greenwood 2015). Unfortunately,
trichromacy may have come at a cost for humans. It is hypothesized that around the time
trichromatic vision evolved, humans lost their ability to sense pheromones although the exact
mechanism is not well understood (Travis, 2003).
As the aforementioned information states, it was the evolutionary adaptations from four
opsins, then two opsins, that led to three opsin trichromacy as seen in humans. While this overall
mechanism is understood fairly well, there is room for further research in color vision. One
woman was found to have four color opsins and, therefore, could distinguish colors that no other
known human can see (Greenwood, 2015). Additional exploration on her condition would be
extremely insightful for understanding color vision. There are other species, such as the mantis
shrimp, that have up to 16 color opsins. However, it is unclear whether or not an increased

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number of cone opsins necessarily indicates the ability to sense more colors. Perhaps humans
will never be able to fathom the true level of subjectivity between species with color vision.
Figure 2. Relative sensitivitytocolorof New WorldMonkeys(Upper) thathave dichromaticvision
and OldWorldMonkeysand Humans(Lower) thathave trichromaticvision.
Tetrapod vertebrates
Birds and other reptilesMammals
Placental Mammals
S, S2, M, L
S, LS, LS2, L
Figure 1. Phylogenictree of fourcoloropsingenes(S,S2,M, L) that leadto placental mammals
withonlytwocolor opsingenes(S1,L) ~140 millionyearsago.

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