The finding that helped clinch the case was the New World howler monkey. it's the only New World monkey with full trichromatic vision, and the researchers found that it also has the worst sense of smell among New World monkeys, with about 31 per cent of its olfactory receptor genes being nonfunctional. (Kleiner 12)
There is another interesting evolutionary difference between humans and our avian cohabitants. Even though birds are also trichromates, we do not use the same protein for detecting the color red. The primate version of this opsin apparently arose spontaneously in Old World primates from a mutation of the green opsin gene on the X chromosome some 30 million to 40 million years ago. (Travis 235) Perhaps another evolutionary clue that birds are in fact really the descendants of dinosaurs, but that is a topic for another paper. This is also the point, seen on the our chart in Appendix a, which shows the beginning of the divergence between trichromatic and dichromatic color vision. Figure 4 on the next page shows a snapshot of this development. Researchers have also concluded that the "evolutionary ancestor common to both had four distinct opsins. Early mammals then lost two of them, probably with little ill effect because these creatures were nocturnal and had a limited need to discern colors." (Travis 235)
Figure 4: A close up of the divergence and mutation of genes over time (History of biological Evolution 2007)
This is one of the amazing aspects of trichromatism, a few types of animal species posses it and that they are also widely separated phylogenetically. Among mammal phyla, only some primates have trichromatic vision. The wide phylogenetic separation of differing color perception abilities suggests that quite obviously color vision is a latent trait in almost all evolutionary groups, a genetic trait that can emerge when the conditions call for it. However, if you look at dichromatic species as opposed to trichromatic species you will discover that there is no direct line of descent linking these two species to a common ancestor with color vision. It would appear that color vision arose independently in these two different phyla, as opposed to having a linear origin to an earlier common ancestor. (Matthen 177) This certainly poses certainly difficulties for straight line Darwinists.
However, recent studies may contradict the separate development theory. Scientists have isolated the gene that controls the formation of the eye in flies. In fact they have successfully switched on this master gene and have grown "extra" eyes on the body of the fly. However this suggest an intriguing interconnection for the original development of the eye and perhaps even color-vision:
Although the human eye and the fly eye are vastly different, the similarities between the gene for a fly eye and that of a mammalian eye lead to a theory of a primordial eye that may have evolved only once and taken on different designs to accommodate the needs of the organism. (Silverston 14)
While trichromatism is one of the most important distinctions of human vision, there is another equally significant characteristic, the fact that humans experience highly predictable contrast and fatigue effects. (Gordon 79)
Colour contrast and fatigue effects are equally remarkable phenomena. If a red square is placed on a grey ground and fixated for a few moments, one comes to see a greenish tinge surrounding the red. An intense green light induces a reddish after-image; blue light induces yellow, and vice versa (note that red and green and blue and yellow are complementary hues in that they mix to form neutral greys). Anyone can experience these effects by simply staring at a coloured light (not the sun) for a few moments and then looking at a white surface. (Gordon 79-80)
While all this research has definitely increased our understanding of color vision, the speculation of why we have developed this sense is still a matter of debate.
The prevailing theory for many years as regarding trichromatic vision is that it enabled its inheritors to see edible fruits at a greater distance and thus increase their chances for survival. While this group believes that trichromatism was introduced because of a decline of a sense of smell, it is uncertain which came first. (Kliener 12; Travis 234) This is true of many evolutionary quandaries. There are of course the scientific hair-splitters in this category:
In a new wrinkle on this evolutionary mystery, Dominy and Lucas of the University of Hong Kong have recently challenged the dogma that trichromacy -- the scientific name for the form of color vision people have -- evolved for detecting ripe fruits. They argue that this color vision instead helped our primate ancestors find tender red leaves bursting with nutritional value. (Travis 235)
Others also believe that the reproductive drive has something to do with the increase to color sensitivity as well as the ability to pick up on emotional states of other primates. A study by Mark Changizi and colleagues at the California Institute of Technology in Pasadena looked at hemoglobin and its effect during blushing, anger, and sexual arousal on the skin surface.
Changizi and colleagues analysed data on the sensitivity of the colour-detecting cone cells in the eyes of 36 primate species. They found that primates with bare rumps and faces tended to be those that were most sensitive to the colours produced by haemoglobin. Primate vision may have evolved, says Changizi, not to help in foraging, but to pick up on visible signs of physiological changes in the body, such as blushing. (a Blushing Start 22)
In nature color is used for many purposes, from sexual indicators, used to attract the opposite sex or the buzzing bee, to warnings of poison which say to the color sensitive vertebrate, do not eat me. It is also used as camouflage for safety. (Savage 47) Do dichromates have an advantage over trichromates in these areas; this is certainly another voluntary possibility.
In one study researchers placed tamarins, a species of New World monkey, in an environment which mimicked the monkeys' natural one. Boxes filled with varying amounts of fudge colored to correspond with ripe as well as unripe fruit were placed among a background of green paper.
The "riper" the box, the more fudge it contained, mirroring the increased desirability of ripe fruit. Compared with their dichromatic relatives, trichromatic tamarins were both faster at finding the boxes among the artificial leaves and more efficient at picking the ripe ones, Smith, Mundy, and their colleagues report in Sept. 15 Journal of Experimental Biology. "The main finding is that trichromacy confers an advantage when selecting ripe fruits from those at various stages of maturity." This is the first time such an advantage has been demonstrated for primates using naturalistic stimuli," the researchers assert. (Travis 236)
Yet the question must be asked, as anyone would who has tried to discover a ripe melon at the grocers, would not a heightened sense of smell do the same thing? In fact evidence challenging the fruit theory has also appeared. Gerald H. Jacobs of the University of California, Santa Barbara and his colleagues discovered that howler monkeys, also New World primates, that eat primarily leaves and yet posses fully trichromatic vision. The howlers all have an additional opsin gene on their X chromosomes. Apparently an ancestor had apparently matched the gene duplication at the same time as Old World. But if finding ripe fruit was supposed as the evolutionary force in the development of trichromatic vision, why did the howlers who only eat green leaves, retain the trait?
It was a huge discovery that howler monkeys had independently evolved the same kind of color vision that monkeys from Africa and Southeast Asia had," says Dominy [of Yale university]. "Why would the only monkey in South America to evolve trichromatic vision be the one that eats the least amount of fruit? It didn't make a lot of sense." (Travis 236)
However, it was also later discovered that these simians would search out leaves that had a slight reddish tint to them that indicated that the leaves had more protein than their green counterparts. (Moffat 613) So the debate continues.
If color vision is a latent genetic possibility inherent across all species phyla, then why do not all species possess it?
While the retina had evolved from mere light sensitive lump of protoplasm to a more improved organ with photoreceptors and the ability to discern motion, color vision has seemed to have developed disproportionately and with a wide variation amongst all the species. Perhaps we must return to one of Darwin's theories here. The Principle of Divergence:
Darwin realized that speciation occurs when a sub-population of a species specializes. This sub-population distances itself from the ancestral population by developing the ability to exploit environmental resources in novel ways. By doing this it benefits from reduced competition. Thus, if a species is able to modify a pre-existing facility to help forge a new way of life, it is enabled…