Genes that code for light-sensitive proteins called opsins had already evolved 550 million years ago. Early animals had four types of light-sensitive molecules, also called visual pigments, that responded at most to four colors: red, green, blue, and ultraviolet. Early mammals, however, were nocturnal and had no need for elaborate color vision. “Apparently most of them lost two of the four gene families,” says John Dowling, the Gund Research Professor of Neurosciences at Harvard University. “Dogs, cats, horses, and most other mammals today still have only two pigments, one for short wavelengths of light and one for long wavelengths.”
About 35 million years ago another visual pigment evolved in primates, so that today humans and many, but not all, primates are trichromatic. The gene for the visual pigment sensitive to longer-wavelength light duplicated and allowed for a third pigment sensitive to yet longer wavelengths of light to evolve. The three visual pigments found in humans are maximally sensitive to blue, green, and red-yellow light, allowing humans the ability to see millions of colors.
Elizabeth Dougherty is a science writer based in Massachusetts.