All mammals capable of perceiving color, with the exception of certain primates, have dichromatic color vision. This means that their retinas contain only two types of cone receptors instead of the three varieties of cones that enable trichromatic vision. The various types of cones in the eye are tuned to react to different wavelengths of visible light, which create nerve signals that the brain interprets as color. Dichromatic vision employs groups of "short" and "long" cones, which decipher the color of light based on where it falls between the high and low wavelength extremes. Trichromatic vision adds the benefit of "middle" cones, which allow the perception of color to become more finely tuned, particularly the ability to distinguish between red and green.
The vast majority of evolutionary anthropologists agree that primates evolved trichromatic color vision as a means of locating edible plants. The need to identify food species against a backdrop of dense foliage meant that color, as well as shape, became a crucial marker. The ability to locate food in this way provided the competitive advantage necessary to encourage the gradual development of trichromatic color vision. Scientists continue to debate, however, whether trichromatic color vision evolved specifically to allow primates to select the most nutritious leaves, many of which have red hues instead of green, or if it developed to enable them to select the most ripe and nutritious fruits available, which also tend to have a red hue.
Further complicating the "leaves-versus-fruits" debate is the inconsistent distribution of the full-color vision trait across primate species. All but one species of trichromatic mammals are so-called Old World (Eastern Hemisphere) primates (the exception being the New World howler monkey). The distribution of color vision in New World (Western Hemisphere) primates, however, is gender-specific, with only females possessing trichromatic vision. In these cases, the trichromatic trait requires two X chromosomes, which only occurs in females. The competitive advantage of the color vision trait should have been comparable in both New and Old World environments, but color vision did not develop simultaneously in both locations. While it is likely that contemporary primates use trichromatic vision to locate red, immature, protein-rich leaves, this may not have always been the case, and the possibility that different factors may have influenced the development of trichromatic color vision on separate continents has clouded the evolutionary debate.
Regardless of whether the identification of fruits or leaves compelled certain primate species to evolve trichromatic vision, there is mounting evidence that the development of extraordinary color perception resulted in a reduction of a sense of smell in those same species. Vision operates accurately at a much greater distance than smell, particularly when it comes to the detection of sexual maturity in potential mates. The rise of color vision likely allowed for more "long-range" sexual cues than pheromone detection did, which is why humans and our close primate cousins often possess a less acute olfactory sense than do other primates. Seeing red, it seems, is not without a sensory cost. Thus while humans and other primates enjoy the advantages of full color vision, the reasons why this trait developed have yet to be definitively determined.
