As reported in the Sept. 1 issue of Science, a team of researchers led by Dr. Wen-Hsiung Li said that trichromatic vision, or the ability to see three colors, evolved independently in New World monkeys and in the common ancestor of Old World monkeys, apes and humans.
Females have two sets of X-linked genes.
Males, however, have only one set, so if one is destroyed, color blindness occurs, which accounts for the higher prevalence of colorblindness in males. In contrast, New World monkeys, which include South American monkeys, possess only one X- linked gene and one autosomal color photopigment gene, which means the male only sees two colors. In some species of the New World monkeys, two variants similar to the human red and green pigments, respectively, arose in the X chromosome. One possiblity is that the two genes were originally identical, but during the evolutionary process, they changed into the red and green pigment genes we have now. The other possibility is that the two genes were the red and green pigment variants so that the recombination immediately produced two X-linked genes already very similar to the red and green pigment genes we now have.