It is no coincidence that human vision evolved sensitivities to match the spectral characteristics of our sunlit world. It is also the case that we humans can’t take the credit for our ability to see in full color. The evolutionary process began around 90 million years ago, when our primitive mammalian ancestors began to shed their nocturnal habits, instigating a series of adaptations to changing circumstances. As it turned out, about 60 million years of evolutionary give-and-take had to pass before our intrepid forbears acquired the genes that would let them glimpse the full light of day.
Much less time—about 20 years—was needed for scientists to figure out all the molecular details of how full-color vision evolved. The final details, which pertain to our ability to see blue light, were worked out in a recent study by a team of scientists based at Emory University. The team, led by Shozo Yokoyama, an Emory biologist, uncovered the evolutionary changes that essentially shifted an ancestral UV-sensitive pigment into the blue.
The story of how the last color of the rainbow came to be visible was told December 18 in PLOS Genetics, in an article entitled, “Epistatic Adaptive Evolution of Human Color Vision.” To uncover the story, the article’s authors conducted mutagenesis experiments of the UV-sensitive pigment in the Boreoeutherian ancestor. These experiments relied on the techniques of microbiology, theoretical computation, biophysics, quantum chemistry, and genetic engineering.
Ultimately, the researchers evaluated 5,040 evolutionary pathways that could have led to the right combination of amino acid changes—that is, changes that could have altered the function of a protein, adapting it to absorb blue light.
“We did experiments for every one of these 5,040 possibilities,” Yokoyama said. “We found that of the seven genetic changes required, each of them individually has no effect. It is only when several of the changes combine in a particular order that the evolutionary pathway can be completed.”
About 80% of the 5,040 pathways the researchers traced would have stopped in the middle. In each of these cases, a protein would have become nonfunctional because mutations in the underlying gene would not have occurred in the right order. Typically, an unfortunate sequence of mutations would have given rise to proteins and pigments that could not have interacted with water as necessary. Water channels, which need to extend through a vision pigment, would have been blocked.
“The remaining 20 percent of the pathways remained possible pathways, but our ancestors used only one,” Yokoyama continued. “We identified that path.”
“Phylogenetic analysis suggests that the blue sensitivity was achieved only gradually and almost exclusively by the seven nonadditively interacting amino acids,” wrote the authors of the PLOS Genetics article. “During the period between 45 and 30 million years ago, [the human pigment that had been UV sensitive] was in the final stage of developing its blue-sensitivity.
“This was the time when two red-sensitive pigments appeared by gene duplication and one of them became green-sensitive. Trichromatic color vision in the human lineage was fully developed by 30 million years ago by interprotein epistasis among the three visual pigments.”
Overall, there are five classes of opsin genes that encode visual pigments for dim-light and color vision. Bits and pieces of the opsin genes change and vision adapts as the environment of a species changes. Such changes, in a particular combination and order, occurred in primitive mammals, which were nocturnal and had visual pigments that were sensitive to UV and red, giving them a bi-chromatic view of the world. Ultimately, these animals evolved the ability to see the full-color spectrum of visible light.
“Gorillas and chimpanzees have human color vision,” Yokoyama added. “Or perhaps we should say that humans have gorilla and chimpanzee vision.”