Researchers at the University of Chicago have found evidence that trichromatic or full color vision originated in prosimians, a group of lemurs, Bush Babies and pottos rather than in higher primates, pushing the origin of primate color vision back roughly 20 million years.
Previously it was thought that color vision first evolved in the common ancestor of higher primates about 35-40 million years ago. The new research pushes the origin of color vision in primates back to about 55 million years ago.
Wen-Hsiung Li, Ph.D., professor of ecology & evolution at the University and his postdoctoral fellow Ying Tan, Ph.D., published their findings in the November 4 issue of Nature.
Full color vision in primates is produced by three protein pigments in the retina called opsins. These pigments absorb light of various wavelengths. Their combined input is processed by the brain to produce full color images.
One of the pigments (the short wavelength opsin) is encoded by an autosomal gene and the other two by similar opsin genes (the middle and long wavelength opsins) on the X chromosome.
Primates with dichromatic vision (the ability to see shades of only two colors) have only two opsin genes-one on an autosome and only one on the X chromosome.
Scientists have long believed that the prosimians, such as lemurs and bush babies, have dichromatic vision at best. In fact, some nocturnal prosimians have been shown to have only a single class of color photopigment in their retinas and thus lack color vision entirely, seeing only in black and white. The diurnal prosimians that have been previously investigated produce two classes of opsins and so have dichromatic color vision (similar to classical red/green human color blindness).
Li and Tan discovered a polymorphism-a gene variation in several prosimians that codes for either a middle (M) or long (L) wavelength opsin. This polymorphism, found on the X chromosome, together with the autosomal short (S) wavelength opsin gene should enable females with an M on one X chromosome and a L on the other to produce three classes of opsin. These females should possess the ability to see in full color.
Since the variation is X-linked, trichromatic vision is possible only in females who carry the M variant on one X and the L variant on the other X. Females with an M or L variant on both X chromosomes and males, who have one X and one Y chromosome, can see only dichromatically.
"Nobody had discovered this polymorphism before," notes Li, "although a similar, more recent polymorphism allows the New World monkeys to see in full color vision."
Li and Tan analyzed genes from tissue samples taken from 20 different kinds of prosimians, mostly lemurs. They found the polymorphism in two diurnal species (active during the day) and in one nocturnal species.
Because color vision is useful only in daylight, Li wasn't surprised to find the polymorphism in the diurnal prosimians. The polymorphism in the one nocturnal species adds weight to the theory that nocturnal species originally evolved from prosimians that had been diurnal.
"The nocturnal condition, at least in the species in which we discovered the polymorphism, is recent. In nocturnal prosimians that lack the polymorphism, we can assume that they have been nocturnal for far longer and the polymorphism conferring trichromatic vision has been lost," says Li.
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