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.
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, and 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 eventmillion 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.
Monotremes and marsupials
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 boundaryenvironmental 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.
Primates
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 and some primates—including humans. Primates, as an order of mammals, began to emerge around the beginning of the Paleogene Period. Primates 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 has other effects on primate and human behavior, as discussed in the color psychology article. Today, among simians, the catarrhines 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 are trichromats.
