Term Paper on "Evolution of Color Vision in Vertebrates"

Term Paper 12 pages (3313 words) Sources: 10

[EXCERPT] . . . .

Color Vision

Evolution of Color Vision in Vertebrates

Color Vision is one of the most striking and somewhat mysterious developments in the evolutionary progress of vertebrates. While most of us take it for granted and probably view it as a rather straight forward developmental step in evolution, it is anything but a liner step-by-step progression. The development of the eye itself does seem to have been a basic progressive evolutionary move forward. Probably starting out as a light sensitive pigment spot on smaller organisms and then proceeded to become differentiated into more light sensitive cells which gradually coated the inside of a slowly forming eyeball to finally become what we now know as the retina. The early retina was probably at first sensitive to movement and changes in light intensity, but eventually the ability to see color was acquired. (Seashore 35) the retina-eye configuration is generally a standard evolutionary development in all vertebrates and many non-vertebrate species as well, see figure 1, page 2.

However, at this point the story of color vision development becomes as varied as the rainbow itself. Color vision is not something that evolved over time homogenously over all species phyla. Instead at first glance it almost seems a random development, appearing in some species while ignoring others. Some species ancestor's possess one type of color vision while their descendants seem to posses a completely different type or loose color vision altogether. Divergences and parallel developments seem to be the norm regarding the evolution of color vision. There are many theories and changing view on this development from hardcore Darwinists citing ecosystem survival strategies to Creationists hold the eye up as only being possible through the process of intelligent design.

Figure 1: (Evolution of the Eye 2008)

It is helpful to review the eye and color vision in its current state of development in vertebrates, humans among them. Strait line evolution would seem to suppose that all vertebrates, being higher in the evolutionary scale, would all have developed a somewhat identical color vision strategy. This is not the case. Instead there are a wide variety of perceptional thresholds in vertebrates across the phyla, from seeing only in black and white, to even more color distinctions in higher and lower wavelengths in other than human species. "Considered across biological taxa in all of its occurrences, colour is a heterogeneous collection of perceptual concepts generated from wavelength-sensitive data for a variety of specialized purposes by cognitive systems with different neurocomputational structures and evolutionary histories." (Matthen 186)

Also, according to Matthen, color vision development is a "disunity" (186) rather than a unified field of development that progresses evenly in all life forms. He goes on to further observe that:

There is neither a single phenomenology of colour vision nor a set of shared concepts that defines colour wherever it may occur. There is a commonality in the informational material from which colour concepts are constructed; this is inherited from the opsins that constitute the basis for any colour-vision system. Consequently, there is a functional commonality in the mechanisms that are needed to gather this information, but, as the Disunity of Colour Thesis stated at the start of Chapter 6 implies, no one mind-independent property that all colour perceivers track or detect, no one ecological problem they all try to solve. (Matthen 186)

In trying to discover the evolutionary necessity of color vision, researchers have always had to generalize about the survival benefits of that perception.

At present Humans, other apes as well as Old World monkeys have trichromatic vision. This means that we have eyes containing three color receptors that are sensitive to blue, green and yellow-red. These receptors let us distinguish approximately 2.3 million colors. Most of the other mammals on the planet only have receptors for blue and green, and can subsequently distinguish far fewer colors. (Kleiner 12) "New World primates' vision changed, too, but not to full trichromacy. In most of these animals, some of the females discern reds and yellows from greens, but males don't." (Travis 236)

Each light-sensitive cell of the human eye responds to a specific wavelength of visible light. Interestingly, the same chemical component is used in the detection of each particular color, a molecule called 11-cis-retinal, is the single chemical component that absorbs light in any receptor cell. It is the larger protein molecule that this molecule is attached to that determines the specific wavelength of light it absorbs. (Color Vision 427)

The human retina contains two classes of light-sensing cells: "rods" (which perceive shades of grey and are most useful in twilight and shade) and three kinds of "cones," each sensitive to a particular wavelength. Interpreted by the brain, these three wavelengths become the three primary colours of vision: red, blue and green. When the information received by the cone cells is combined, the world is revealed to us in all the subtlety and splendour of its rainbow colour scheme. (Savage 47)

Furthermore, we perceive only a very narrow band of the electromagnetic spectrum, in the small band range of 400 to 700 nanometers.

Figure 3: Human Color Vision spectrum, rods and cones (Color Vision a)

Initially it was thought that vertebrates see color by the brain comparing the strength of the signals from each of the receptor rods, thus separating the color wavelengths. (Interestingly, this theory does not explain the perception of colors such as gold, silver and brown). The more recent opponent-process (O-P) theory states that we perceive colors in opposing pairs such as blue and yellow, red, and green. In O-P theory, too much blue light is though to cut ones awareness of yellow and too much green cuts the awareness of red and vice versa. Studies are leaning towards a combination of theories that speculate that the brain further processes these signals in terms of these opposing pairs. (Color Vision 57; Chatterjee & Callaway 668) Now when did this perception of color arise in vertebrates?

Vertebrates began to appear between 530 and 510 million years ago during what is called the "Cambrian Explosion," a time at which the fossil records seem to show a rapid outcropping of differentiated species development. "The first tetrapod (an animal that has four limbs, along with hips and shoulders and fingers and toes) crawled out of the Earth's oceans some time between 375 and 350 million years ago." (Seashore 427) the species at this stage could see light as back and white or yellow blue and all of the varieties this range begat. Some vertebrate could also perceive ultra-violet and had an orange-red cone receptor as well, but these subsequently vanished around 200 million years ago. As the ages passed and dinosaurs ruled the earth, most vertebrates possessed limited color vision, having only blue cone, yellow cones and black white rods, "Back in the late Cretaceous, early placental mammals saw the world in limited colors, much like humans with red-green color blindness do," (Moffat 613)

Then, about 40 million years ago, a divergence in the way the retina and color vision was forming had developed. This was shortly after mammals and then primates had developed in the evolutionary scheme of things. (Reference Appendix I) Two specific branches of color vision development took place, Dichromatic (two-color) and Trichromatic (three-color) vision.

When it comes to their color vision, people fall between birds and most mammals. People generally have three opsins [retinal pigment protiens], which are sensitive to blue, green, and red. In fact, most of the primates that evolved in Africa and Asia, including the great apes and chimpanzees, are fully trichromatic. In contrast, most New World primates, such as the tamarins and marmosets of South America, are dichromatic, having just blue-sensitive and green-sensitive opsins. (Travis 235)

Although these two branches of primate vertebrate developed concurrently, the difference in color vision has largely continued to persist to the present. There has been a trade off noticed by researched regarding the human achievement trichromatic vision. Our sense of smell is certainly much less acute than that of other species that are dichromatic or monochromatic (perceiving only black and white). Approximately sixty percent of the mammalian olfactory receptor genes in humans are dormant.

In other simians and Old World monkeys only about thirty percent are dormant. However, in mice and dogs which have no trichromatic vision, only around twenty percent of their olfactory genes do not work. This gives them an eighty percent functional olfactory system as compared to less than half that amount of functionally active olfactory system in humans. (Kliener 12) Furthermore it is clear that most ground-living mammals are colorblind. "These facts support the evidence from other sources which shows that vision is of secondary importance in the life activities of all the mammals except the tree-living primates and man, who evolved from arboreal stock." (Guilford 40) in a recent study researchers, measured the number of non-functional olfactory receptor genes in 18 species of ape and monkey and in people. Primates with trichromatic vision all had a significantly worse sense of smell… READ MORE

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