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NATURE

Color vision originated earlier than believed

ENN



Female red ruffed lemurs have the potential for full color vision, according to University of Chicago researchers.
Female red ruffed lemurs have the potential for full color vision, according to University of Chicago researchers.  

November 12, 1999
Web posted at: 11:31 a.m. EST (1631 GMT)

Color vision may have originated 20 million years earlier than previously believed, according to scientists at the University of Chicago in Illinois.

"We hypothesize that color vision originated in prosimians rather than in simians, much earlier than commonly believed," said Wen-Hsiung Li, a professor of ecology and evolution at the university.

Scientists have long believed that full color vision was restricted to humans, apes and Old World monkeys, because only these primates were known to possess the three classes of photopigment in the retina which are required to produce color vision.

However, the researchers studied the gene that produces the photopigments in 20 so-called prosimian species a more primitive group than simians, which includes lemurs and lorises and found that some female prosimians should possess the ability to see in full color.

Three protein pigments in the retina called opsins produce full color vision in primates. These pigments absorb light of various wavelengths. Their combined input is processed by the brain to produce full color images.
 MESSAGE BOARD
Evolution


One of the pigments (the short wavelength opsin) is encoded by an autosomal (non-sex) 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 have dichromatic vision at best and that the ability to see in full color originated in simians 35 to 40 million years ago.

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Full color vision gives primates the ability to find ripe fruits. Since prosimians also eat fruit, it follows that should also be able to see in full color.  

However, Li and his colleague Ying Tan discovered a gene variation in several prosimians that codes for either a middle (M) or long (L) wavelength opsin.

This variation, 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 an L on the other to produce three classes of opsin. Thus, these females should possess the ability to see in full color.

"Only female prosimians can see in full color and these must be females with two different variants on the X chromosomes. Male prosimians and female prosimians with the same variant in their two X chromosomes can at best see only two colors," said Li.

Since the research shows that color vision appears to have existed in prosimians 20 million years earlier than thought, the research also challenges a belief that ancestral prosimians were nocturnal.

"Since color vision appears to exist even in ancestral prosimians and since color vision is of no use to a nocturnal animal, it is likely that the ancestral prosimians (or the ancestor of all primates) were diurnal," said Li.

Full color vision gives primates the ability to find ripe fruits. "As prosimians also eat a considerable amount of fruits, diurnal prosimians would enjoy the same advantage as simians," said Li.

Copyright 1999, Environmental News Network, All Rights Reserved


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RELATED SITES:
Research on the origins of primate full color vision was conducted by Wen-Hsiung Li and colleagues at the University of Chicago in Illinois.
An article on this research was published in the Nov. 4 issue of the journal Nature.
To learn more about lemurs, a type of prosimian, visit the Duke University Primate Center.
To learn more about how humans see in color, visit the Make a Splash with Color web site.
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