Evolution of color vision

Color vision, a proximate adaptation of the vision sensory modality, allows for the discrimination of light based on its wavelength components. There are two physiological requirements for a visual system to discriminate wavelengths. There must be at least two different classes of photoreceptors sensitive to different ranges of the electromagnetic spectrum, and their outputs must be compared or contrasted with each other. This is because of the principle of univariance states that a single photoreceptor cannot unambiguously determine either the wavelength or intensity of incident light. But through comparisons of different photoreceptor classes, the ambiguity can be resolved. The evolutionary process leading to color vision in both humans and animals involves both the differentiation of photopigments to create different classes of cone cells, and the comparison between different cones in subsequent stages of visual processing.

The evolutionary process of switching from a single photopigment to two different pigments would have provided early ancestors with several advantages.

In one way, adding a new pigment would allow them to see a wider range of the electromagnetic spectrum. Secondly, new random connections would create wavelength opponency and the new wavelength opponent neurons would be much more sensitive than the non-wavelength opponent neurons. This is the result of some wavelength distributions favouring excitation instead of inhibition. Both excitation and inhibition would be features of a neural substrate during the formation of a second pigment. Overall, the advantage gained from increased sensitivity with wavelength opponency would open up opportunities for future exploitation by mutations and even further improvement.

Color vision requires a number of opsin molecules with different absorbance peaks, and at least three opsins were present in the ancestor of arthropods; chelicerates and pancrustaceans today possess color vision.

Researchers studying the opsin genes responsible for color-vision pigments have long known that four photopigment opsins exist in birds, reptiles and teleost fish. This indicates that the common ancestor of amphibians and amniotes (≈350 million years ago) had tetrachromatic vision — the ability to see four dimensions of color.

Today, most mammals possess dichromatic vision, corresponding to protanopia red–green color blindness. They can thus see violet, blue, green and yellow light, but cannot see ultraviolet or deep red light. This was probably a feature of the first mammalian ancestors, which were likely small, nocturnal, and burrowing.

At the time of the Cretaceous–Paleogene extinction event 66 million years ago, the burrowing ability probably helped mammals survive extinction. Mammalian species of the time had already started to differentiate, but were still generally small, comparable in size to shrews; this small size would have helped them to find shelter in protected environments.

It is postulated that some early monotremes, marsupials, and placentals were semiaquatic or burrowing, as there are multiple mammalian lineages with such habits today. Any burrowing or semiaquatic mammal would have had additional protection from Cretaceous–Paleogene boundary environmental stresses. However, many such species evidently possessed poor color vision in comparison with non-mammalian vertebrate species of the time, including reptiles, birds, and amphibians.