Biology:Evolution of human colour vision

Short description: Overview of the evolution of human colour vision

The evolution of human colour vision produced a trichromatic view of the world in comparison to a majority of other mammals that only have a dichromatic view. Early pre-primate mammals are believed to have viewed the world partly using ultraviolet (UV) light around 90 million years ago, but in the lineage leading to humans their spectral sensitivity shifted toward blue via a succession of mutations. It is thought that the shift to trichromatic color vision evolved in primates as an adaptive trait over time.

Primitive ancestral vision

It is believed that due to a number of environmental factors, ancient mammals lived with limited colour vision. This is believed to have also been influenced by life styles, including being predominantly nocturnal. There is little data indicating the advantages of UV vision in early mammals.

Early mammal ultraviolet vision, or ultraviolet sensitivity, included sensitivity in the wavelength ranges between 350 nm  and 430 nm . These wavelengths are shorter than visible light but longer than X-rays. In some rare cases, some modern day humans can see within the UV spectrum at wavelengths close to 310 nm .

In other animals that possess UV vision such as birds, ultraviolet sensitivity can be advantageous for courtship and reproductive success. This is because some birds have feathers with certain favourable colourations that can not be distinguished by human vision outside of the UV spectrum.

Opsins and colour vision

Opsins function by acting as enzymes that are activated and change shape when light absorption causes chromophores to isomerize. Opsins are responsible for adjusting wavelength dependence of the chromophore light induced isomerization reaction. Therefore, opsins act by determining chromophore sensitivity to light at any given wavelength. Opsins that have different amino acid sequences but are bound to identical chromophores result in different absorption values at each wavelength.

Opsin genes are used to encode the photoreceptor proteins responsible for color vision and dim light vision. The photoreceptor proteins created can be further categorized in to rhodopsins, which are found in rod photoreceptor cells and assist with night vision; and photopsins, or cone opsins, which are responsible for colour vision and expressed in cone photoreceptor cells of the retina.

Cone opsins are categorized further by their absorption maxima λ max which is the wavelength when the greatest amount of light absorption takes place. Further categorization of cone opsins also depends on the specific amino acid sequences each of the opsins uses, which may have an evolutionary basis.

Evolution of cone opsins and human colour vision

Recent studies have shown that primitive nocturnal mammalian ancestors had dichromatic vision consisting of UV–sensitive and red–sensitive traits. A change occurred approximately 30 million years ago where human ancestors evolved four classes of opsin genes, which enabled vision that included the full spectrum of visible light. UV–sensitivity is said to have been lost at this time.

Mutagenesis experiments involving the Boreoeutherian ancestor to humans have shown that seven genetic mutations are linked to losing UV vision and gaining the blue light vision that most humans have today over the course of millions of years. These mutations: F46T, F49L, T52F, F86L, T93P, A114G and S118T, include 5040 potential pathways for the amino acid changes required to create genetic changes in the short wavelength sensitive, or blue opsin. Of the 5040 pathways, 335 have been deemed as possible trajectories for the evolution of blue opsin. It has been discovered that each individual mutation has no effect on its own, and that only multiple changes combined following an epistatic pattern in a specific order resulted in changes in the evolutionary direction of blue vision.

Incomplete trajectories, or evolutionary pathways, are shown to be caused by T52F mutations occurring first because T52F does not have a peak for the absorption of light within the entire visible region. T52F mutations are deemed to be structurally unstable, and the evolutionary path is immediately terminated. Having any of the other stable mutations occur first, including F46T, F49L, F86L, T93P, A114G or S118T, opens up the possibility of having 1032 out of 5042 potential trajectories open up to evolution. This is because having any of the other mutations occur first would allow for 134, 74, 252, 348, 102 and 122 potential pathways for mutations involving each of the remaining 6 mutants, equal to 1032 potential pathways for the evolution of short wavelength sensitive opsins to take place.

Studies using in vitro assays have shown that epistatic evolution took place in ancestral Boreoeutherian species with the 7 mutations on genetically reconstructed Boreoetherian short wavelength sensitive opsins. λmax values were shifted from a value of 357 nm  to 411 nm , an increase which indicated that human short wavelength sensitive opsins did indeed evolve from ancestral Boreoeutherian species using these 7 mutations.

Further analysis has shown that 4008 out of the 5040 possible trajectories were terminated prematurely due to nonfunctional pigments that were dehydrated. Mutagenesis results also reveal that ancestral human short wavelength sensitive opsin remained UV-sensitive until about 80 million years ago, before gradually increasing its λmax by 20 nm  75 million years ago and 20 nm  45 million years ago. It eventually reached the current λmax of 430 nm  30 million years ago.

It is believed that middle and long wave sensitive pigments appeared after the final stages of short wavelength sensitive opsin pigments evolved, and that trichromatic vision was formed through interprotein epistasis.

Scientists believe that the slow rate of evolution of human ancestral vision can also be attributed to slow environmental changes.

Evolutionary pathway of short wavelength opsins

It has been theorized that λ max-shifts might have been required as human ancestors started to switch from leading nocturnal lifestyles to more diurnal lifestyles. This caused their vision to adjust to various twilight settings over time. To identify the path from which short wavelength opsins evolved, increases in absolute max values were used by researchers with a limitation of approximately |Δλmax|<25 nm  per step. This allows for subdivision of the 1032 potential pathways that were generated by analysis of first mutations beginning with any of the stable mutants: F46T, F49L, F86L, T93P, A114G or S118T to be narrowed down to 335 potential pathways.

Evolutionary pathway of the medium and long wavelength opsins

It was found that the last two mutations, F46T and T52F, occurred between 45 million and 30 million years ago as the absolute max for short wave length opsins was increasing from 400 nm  to 430 nm . During this time, ancestral Boreotherian had two long wave length opsins created by gene duplication, with one retaining the absolute max value of 560 nm , equal to the ancestral value. This led to the creation of the modern human long wavelength sensitive, or red opsin.

The other short wavelength sensitive opsin increased its absolute max value to 530 nm  and became the middle wavelength sensitive, or green-sensitive opsin. This occurred through mutations involving S180A, Y277F and T285A. The order that these mutations took place in the ancestral Boreotheria is currently not fully known. It is hypothesized that T285A was one of the first two mutations because absolute max values would be between 532-538 nm  which is close to the absolute value found in human middle wavelength opsins.

See also

• Color vision
• Opsin
• Opponent process
• Color blindness

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