of photoreceptor cells in the retina that preferentially absorb lights at different wavelengths. These are known as cone cells and each type contains a particular kind of light-absorbing sensor protein. Short-wavelength-sensitive (S) cone cells are most sensitive to blue lights, medium-wavelength-sensitive (M) cone cells are most sensitive to green lights, and long-wavelength-sensitive (L) cones are most sensitive to red lights. When light strikes the retina and activates the cone cells, the brain compares the responses of the S, M, and L photoreceptors, and it is the brains assessment of their relative levels of activation that we perceive as color.

Most mammals, including mice, are dichromats, possessing only S and M cone pigments. As a consequence, they can distinguish only a fraction of the wavelengths that can be distinguished by humans. John Mollon at the University of Cambridge has suggested that the evolution of trichromacy could have permitted primates to discriminate between unripe fruit, which is typically green, and ripe red- and orange-colored fruits. Reciprocally, the colors of ripened fruits may have coevolved with primate trichromacy, since animals that could recognize and eat the ripe fruit would have assisted plants by spreading their seeds.

Nathans, a Howard Hughes Medical Institute researcher at Johns Hopkins, worked out the structure of the human S, M, and L pigments and the genetic basis of human color vision variation beginning in the 1980s. At the same time, Jacobs, at UCSB, deciphered the distinctive genetic mechanism that gives rise to trichromatic color vision in New World (South American) primates. Together, their work has suggested that the type of trichromatic color vision that New World monkeys possess may also be the evolutionary precursor to the form found among Old World (African) primates, including humans.

In the current study, the researchers set out to replicate what most scientists had considered the crucial s
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Contact: Jennifer Michalowski
michalow@hhmi.org
301-215-8576
Howard Hughes Medical Institute
22-Mar-2007

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