Thursday, September 17, 2009

What Color is Bigfoot's World?

In a recent article, Scientific American reveals how primates are uniquely evolved to see in three colors, known as trichromacy.  Unique not only among mammals, but unique among the entire animal kingdom. To find out why read the excerpt below.  It seems, genetically, it is not such a feat to see in blues and greens (Dichromatic view), but seeing the additional red hues (Trichromatic view) requires a mutation of a gene nowhere near the other two genes. You can read the full Scientific American article here

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.
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. It seems, then, that primate trichromacy is unusual. 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. 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. 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.

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. Most nonprimate mammals have only one longer-wavelength pigment, which is similar to the longer-wavelength primate pigments. The gene for the longer-wavelength mammalian pigment is also located on the X chromosome. Those features raised the possibility, then, that the two longer-wavelength primate pigment genes first arose in the early primate lineage in this way: a longer-wavelength mammalian pigment gene was duplicated on a single X chromosome, after which mutations in either or both copies of the X-linked ancestral gene produced two quite similar pigments with different ranges of spectral sensitivity the M and L pigments.

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