Color Vision in Chimpanzees

CHIMPANZEES, like humans, can distinguish among colors that other mammals cannot see. What observers see in a Kandinsky reflects the properties of the paints, the nature of the illumination, and the color vision system of the viewers.
Geoffrey Clements Corbis (painting); Bob Elsdale Corbis (chimps)

"Key Concepts

• The color vision of humans and some other primates differs from that of nonprimate mammals.
• It is called trichromacy, because it depends on three types of light- activated pigments in the retina of the eye.
• Analyses of the genes for those pigments give clues to how trichromacy evolved from the color vision of nonprimate mammals, which have only two kinds of photo pigments.
• The authors created trichromatic mice by inserting a human pigment gene into the mouse genome. The experiment revealed unexpected plasticity in the mammalian brain.

Reviews Scientific American reviews: Lucy's Legacy2009 Issue April

To our eyes, the world is arrayed in a seemingly infinite splendor of hues, from the sunny orange of a marigold flower to the gunmetal gray of an automobile chassis, from the buoyant blue of a midwinter sky to the sparkling green of an emerald. It is remarkable, then, that for most human beings any color can be reproduced by mixing together just three fixed wavelengths of light at certain intensities. This property of human vision, called trichromacy, arises because the retina the layer of nerve cells in the eye that captures light and transmits visual information to the brain uses only three types of light-absorbing pigments for color vision. One consequence of trichromacy is that computer and television displays can mix red, green and blue pixels to generate what we perceive as a full spectrum of color.

Although trichromacy is common among primates, it is not universal in the animal kingdom. Almost all nonprimate mammals are dichromats, with color vision based on just two kinds of visual pigments. A few nocturnal mammals have only one pigment. Some birds, fish and reptiles have four visual pigments and can detect ultraviolet light invisible to humans. It seems, then, that primate trichromacy is unusual. How did it evolve? Building on decades of study, recent investigations into the genetics, molecular biology and neurophysiology of primate color vision have yielded some unexpected answers as well as surprising findings about the flexibility of the primate brain.

Pigments and Their Past
The spectral sensitivities of the three visual pigments responsible for human color vision were first measured more than 50 years ago and are now known with great precision. Each absorbs light from a particular region of the spectrum and is characterized by the wavelength it absorbs most efficiently. The short-wavelength (S) pigment absorbs light maximally at wavelengths of about 430 nanometers (a nanometer is one billionth of a meter), the medium-wavelength (M) pigment maximally absorbs light at approximately 530 nanometers, and the long-wavelength (L) pigment absorbs light maximally at 560 nanometers. (For context, wavelengths of 470, 520 and 580 nanometers correspond to hues that the typical human perceives as blue, green and yellow, respectively.)

These pigments, each consisting of a protein complexed with a light-absorbing compound derived from vitamin A, sit in the membranes of cone cells: photoreceptive nerve cells in the retina named for their tapering shape. When a pigment absorbs light, it triggers a cascade of molecular events that leads to the excitation of the cone cell. This excitation, in turn, activates other retinal neurons that ultimately convey a signal along the optic nerve to the brain.

Although the absorption spectra of the cone pigments have long been known, it was not until the 1980s that one of us (Nathans) identified the genes for the human pigments and, from the DNA sequences of those genes, determined the sequence of amino acids that constitutes each pigment protein. The gene sequences revealed that the M and L pigments are almost identical. Subsequent experiments showed that the difference in spectral sensitivity between them derives from substitutions in just three of the 364 amino acids from which each is built.

Experiments also showed that the M- and L-pigment genes sit next to each other on the X chromosome, one of the two sex chromosomes. (Men have one X and one Y, whereas women have two Xs.) This location came as no surprise, because a common anomaly in human color perception, red-green color blindness, had long been known to occur more often in men than in women and to be inherited in a pattern indicating that the responsible genes reside on the X chromosome. The S-pigment gene, in contrast, is located on chromosome 7, and its sequence shows that the encoded S pigment is related only distantly to the M and L pigments.

By the mid-1990s comparisons of these three pigment genes with those of other animals had provided substantial information about their history. Almost all vertebrates have genes with sequences that are very similar to that of the human S pigment, implying that some version of a shorter-wavelength pigment is an ancient element of color vision. Relatives of the two longer-wavelength pigments (M and L) are also widespread among vertebrates and likely to be quite ancient. But among mammals, the presence of both M- and L-like pigments has been seen only in a subset of primate species a sign that this feature probably evolved more recently."

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