How our Brains Evolved to See 7 Million Different Colors
It is estimated that the average human can distinguish up to seven million different colors.
It took multiple mutations in visual pigment genetics, spread over millions of years.
And all of this was just for humans to evolve from a mammal with a muted view of the world into a great ape able to see all the colors of the rainbow.
Scientists recently completed a detailed analysis of the evolution of human color vision which involved more than two decades of painstaking research. The final piece of the puzzle: understanding the process for how humans switched from ultraviolet (UV) vision to violet vision, or the ability to see blue light.
“We have now traced all of the evolutionary pathways, going back 90 million years, that led to human color vision,” said biologist and lead author Shozo Yokoyama. “We’ve clarified these molecular pathways at the chemical level, the genetic level and the functional level.”
The Secrets of Adaptive Evolution
“Why do two colors, put one next to the other, sing? Can one really explain this? no. Just as one can never learn how to paint.” ~Pablo Picasso
Over the years, Yokoyama and his collaborators have uncovered secrets of the adaptive evolution of vision in humans and other vertebrates by studying ancestral molecules.
The drawn out process includes first estimating and synthesizing ancestral proteins and pigments of a species. After that, experiments are conducted on them. The method combines microbiology with biophysics, quantum chemistry, theoretical computation, and genetic engineering.
There are five opsin genes classes which encode visual pigments for dim-light and color vision. Odds and ends of the opsin genes change. Also, vision adapts as the environment of a species changes.
“Gorillas and chimpanzees have human color vision,” Yokoyama says. “Or perhaps we should say that humans have gorilla and chimpanzee vision.”
About 90 million years ago, our mammalian primitive ancestors were nocturnal and had UV-sensitive and red-sensitive color. This gave them a bi-chromatic view of the world.
As early as roughly 30 million years ago, our ancestors had evolved four classes of opsin genes, giving them the ability to see the full-color spectrum of visible light, except for UV.
Losing our UV Vision
Researchers in the study zeroed in on the seven genetic mutations involved in losing UV vision and achieving the current function of a blue-sensitive pigment. They traced this progression from 90-to-30 million years ago.
The researchers established 5,040 potential pathways for the amino acid changes required to bring about the genetic changes.
“We did experiments for every one of these 5,040 possibilities,” Yokoyama says. “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.”
What this means is that just as an animal’s external environment drives natural selection, so do changes in the animal’s molecular environment.
Modern humans cannot perceive UV light directly (unless you are Kevin Spacey in the film K-Pax) because the lens of the eye blocks most light in the wavelength range of 300–400 nm; shorter wavelengths are blocked by the cornea.
But still, the photoreceptors of the retina are sensitive to near UV light and people lacking a lens (a condition known as aphakia) perceive near UV light as whitish blue or whitish-violet, probably because all three types of cones are roughly equally sensitive to UV light, but blue cones a bit more.
Trichromacy or trichromaticism is the condition of possessing three independent channels for conveying color information, derived from the three different cone types
The ability of humans and some other animals to see different colors is mediated by interactions among three types of color-sensing cone cells.
Each of the three types of cones in the retina of the eye contains a different type of photosensitive pigment, which is composed of a transmembrane protein called opsin and a light-sensitive molecule called 11-cis retinal.
Each different pigment is especially sensitive to a certain wavelength of light (that is, the pigment is most likely to produce a cellular response when it is hit by a photon with the specific wavelength to which that pigment is most sensitive).
The three types of cones are L, M, and S, which have pigments that respond best to light of long (especially 560 nm), medium (530 nm), and short (420 nm) wavelengths respectively.