Evolution of Human Color Vision

In recent years, the study of color vision in nonhuman species has expanded greatly, as has our view of the nature of opsin genes and photopigments in a host of different species. This has begun to allow an understanding of the evolution of human color vision. Comparison of opsin gene sequences indicates that visual pigments have an ancient origin and that there is evidence for the presence of two separate cone pigments very early in vertebrate history (perhaps as long as 400 million years ago). This arrangement would have provided the necessary basis for a dichromatic color vision system, although there is currently no way of knowing if that potential was actually realized. The nature of color vision and the physiology that allows for color vision vary significantly among present-day vertebrates. As previously mentioned, humans normally have three classes of cone pigment and trichromatic color vision. The retinas of many fishes, reptiles, and birds contain at least four classes of cone pigment, and there is evidence to support the conclusion that these may well provide a four-variable form of color vision—a tetrachromacy. On the other hand, the retinas of most mammals (e.g., domestic dogs and cats) contain only two types of cone pigment allowing a basic dichromacy. This difference has led to the hypothesis that the potential for a sophisticated form of color vision was lost sometime early in mammalian history. Among mammals, the presence of three types of cone pigment and trichromatic color vision is restricted to primates. From this fact, it can be concluded that our exceptional color vision sense appeared during the evolution of primates.

Comparison of opsin genes and color vision in Old World primates (in particular, Old World monkeys, apes, and humans) and in New World monkeys indicates that the essential change from a basically dichromatic form of color vision to uniform trichromatic color vision occurred shortly after the divergence of Old World and New World primates approximately 30-40 million years ago. The means for accomplishing this was an X chromosome opsin gene duplication that yielded two separate M and L genes. Whether that duplication arose from identical X chromosome genes that eventually diverged in structure or whether it was built from the baseline of a polymorphism at a single gene site is not clear. In any case, the origins of human trichromacy can be traced to changes that first appeared in the retinas of our early primate ancestors. An important consequence of this is that human color vision is very similar in its detail to the color vision enjoyed by all contemporary Old World monkeys and apes.

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