Pedagoguery

We often take color for granted, but the genetic and physiological underpinnings of color vision are really quite extraordinary. In addition, color vision is quite different in different kinds of animals, and primates among mammals are unique in our ability to perceive color.

Color vision arises in the cone cells in the retina. However, each cone cell carries a pigment which is sensitive to a certain range of colors. A single cone cell does not allow us to see color, it is only with the brain comparing the input from different cone cells with different pigments that color arises. The visual cortex, which is actually at the back of the brain, is where we see color because this is where the differing intensities of signal from different types of cone cells are compared. If, for example, the visual cortex detects a higher level of signal from the cone cells that detect longer frequencies of light, it signals that the color red is being seen.

Most vertebrates have four types of cone cells, providing a visual range from red to near ultraviolet. Birds and reptiles in particular have quite acute color vision. Mammals are the exception. During the Mesozoic era, when dinosaurs dominated the landscape, mammals were primarily nocturnal. Cone cells require relatively high levels of light to function, so color is not particularly important to nocturnal animals. And, in evolution, when a trait does not help a species survive, it tends to get lost. That is what happened with mammal color vision – two of the pigments were lost.

It is a commonly held belief that most mammals see in black and white. Like many commonly held beliefs, it is also not quite true. Most mammals can see color, it is just that they don't see the range of colors that birds and primates can. Of the two pigments that most mammals have, one of them is called the S-pigment because it is primarily sensitive to short-wavelength light. The gene that encodes this pigment is located on chromosome 7. The second pigment is called M-pigment, and it has its highest sensitivity in the green part of the spectrum. Its gene is encoded on the X chromosome. It is the lack of a long wavelength pigment that explains some of the coloration of some mammals. For instance, zebras seem quite flashy to our eyes, but to the eyes of a lion, who can't see longer wavelengths, the black and white patterns do a better job of breaking up the outline of the animal, particularly when they are in groups. Another example is the bright orange-yellow coloration of a tiger. In the eyes of its prey, the tiger appears much more muted, and it blends in much more to the surrounding foliage.

Clearly, somewhere along the line, however, humans picked up a third pigment. The location of the L-pigment provides a clue to its origin. It is located alongside the M-pigment on the X chromosome. At some point in the early primate past, a mutation in the M-pigment occurred, shifting its sensitivity to the longer wavelengths. At some later point, a female was produced that had one version of the pigment on one of her X chromosomes and the other on the other X chromosome. There is a phenomenon that exists whereby genes are swapped between paired chromosomes, and both pigments ended up on the same chromosome in later generations.

How did the mutation survive? Well, it conferred a competitive advantage. The ability to discern red allowed those primitive primates to better identify ripe fruit, which is frequently red or orange, which contrasts nicely with the surrounding foliage.

This also explains why the most common type of color blindness – red/greed color blindness, mostly affects men. If a man happens to inherit an X chromosome with only one pigment, he only has the two, whereas with a woman, her other X chromosome is likely to have both pigments.

Next time, how planets lose their atmospheres.