Evolution of color vision
Color vision requires a number of opsin molecules with different absorbance peaks, and at least three opsins were present in the ancestor of chelicerates and pancrustaceans; members of both these groups 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 tetrapods and amniotes (~360 million years ago) had tetrachromatic vision—the ability to discern four different wavelengths of light—and thus at least this many (and probably far more) visual "colors." Thus the wide variety of colors and shades exhibited in tropical fish on a coral reef is explained by their color vision—such shallow-water fish see colors as well or better than humans do. The many subtle colors in birds have similarly developed to signal other birds, not mammals.
Today, most mammals possess dichromatic vision, corresponding to red–green color blindness. They can thus distinguish between violet, blue, green and yellow, but cannot distinguish ultraviolet, reds and oranges. 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 rats; this small size would have helped them to find shelter in protected environments. In addition, 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 (K–Pg 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.
Since the beginning of the Paleogene Period, surviving mammals enlarged, moving away by adaptive 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. They have re-developed trichromatic color vision since that time, by the mechanism of gene duplication, being under unusually high evolutionary pressure to develop color vision better than the mammalian standard. Ability to perceive red 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 orange carotenoids have not yet been masked by chlorophyll.
Another theory is that detecting skin flushing and thereby mood may have influenced the development of primate trichromate vision. The color red also have other effects on primate and human behavior as discussed in the color psychology article.
Today, among simians, the catarrhines (Old World monkeys and apes, including humans) are routinely trichromatic—meaning that both males and females possess three opsins, sensitive to short-wave, medium-wave, and long-wave light—while, conversely, only a small fraction of platyrrhine primates (New World monkeys) are trichromats.
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