COLOR-VISION OF ANCESTRAL ORGANISMS OF HIGHER PRIMATES

The color vision of mammals is controlled by photosensitive proteins c alled opsins. Most mammals have dichromatic color vision, but hominoid s and Old World (OW) monkeys enjoy trichromatic vision, having the blu e-, green-, and red-sensitive opsin genes. Most New World (NW) monkeys are either dichromatic or trichromatic, depending on the sex and geno type. Trichromacy in higher primates is believed to have evolved to fa cilitate the detection of yellow and red fruits against dappled foliag e, but the process of evolutionary change from dichromacy to trichroma cy is not well understood. Using the parsimony and the newly developed Bayesian methods, we inferred the amino acid sequences of opsins of a ncestral organisms of higher primates. The results suggest that the an cestors of OW and NW monkeys lacked the green gene and that the green gene later evolved from the red gene. The fact that the red/green opsi n gene has survived the long nocturnal stage of mammalian evolution an d that it is under strong purifying selection in organisms that Live i n dark environments suggests that this gene has another important func tion in addition to color vision, probably the control of circadian rh ythms.