Color vision, a proximate adaptation of the visionsensory modality, allows for the discrimination of light based on itswavelength components.
The evolutionary process of switching from a singlephotopigment to two different pigments would have provided early ancestors with a sensitivity advantage in two ways.
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 aneural 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.[1]
Color vision requires a number ofopsin molecules with different absorbance peaks, and at least three opsins were present in the ancestor ofarthropods;chelicerates andpancrustaceans today possess color vision.[2]
Researchers studying theopsin genes responsible for color-vision pigments have long known that fourphotopigment opsins exist in birds, reptiles andteleost fish.[3] This indicates that the common ancestor ofamphibians andamniotes (≈350 million years ago) hadtetrachromatic vision — the ability to see four dimensions of color.[4]
Today, most mammals possessdichromatic vision, corresponding toprotanopia red–green color blindness. They can thus see violet, blue, green and yellow light, but cannot see ultraviolet or deep red light.[5][6] This was probably a feature of the firstmammalian ancestors, which were likely small, nocturnal, and burrowing.
At the time of theCretaceous–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 toshrews; this small size would have helped them to find shelter in protected environments.
It is postulated that some earlymonotremes,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 fromCretaceous–Paleogene boundary environmental stresses.[7] However, many such species evidently possessed poor color vision in comparison with non-mammalian vertebrate species of the time, including reptiles, birds, and amphibians.
Since the beginning of thePaleogene Period, surviving mammals enlarged, moving away byadaptive radiation from a burrowing existence and into the open, although most species kept their relatively poor color vision. Exceptions occur for some marsupials (which possibly kept their original color vision) and some primates—including humans.Primates, as an order of mammals, began to emerge around the beginning of the Paleogene Period.
Primates have re-developedtrichromatic color vision since that time, by the mechanism ofgene duplication, being under unusually highevolutionary pressure to develop color vision better than the mammalian standard. Ability to perceive red[8] and orange hues allows tree-dwelling primates to discern them from green. This is particularly important for primates in the detection of red and orange fruit, as well as nutrient-rich new foliage, in which the red and orangecarotenoids have not yet been masked bychlorophyll.
Another theory is that detecting skinflushing and thereby mood may have influenced the development of primate trichromate vision. The color red also has other effects on primate and human behavior, as discussed in thecolor psychology article.[9]
Today, amongsimians, thecatarrhines (Old World monkeys andapes, includinghumans) are routinely trichromatic—meaning that both males and females possess three opsins, sensitive to short-wave, medium-wave, and long-wave light[4]—while, conversely, only a small fraction ofplatyrrhine primates (New World monkeys) are trichromats.[10]
A new study published inBiological Reviews proposes that animal color vision emerged approximately 500 million years ago. This timeline precedes the emergence of many brightly colored organisms, such asflowering plants, colorfulvertebrates, andarthropods. The study suggests that the ability to perceive color developed before the widespread appearance of colorful stimuli in the environment.[11]
This discovery has generated interest and discussion among scientists because it raises important questions about the evolutionary pressures that led to the development of color vision. Early forms of color vision may have been utilized for activities such asforaging,mate choice, or avoiding predators, even in a less colorful world.[11]
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