Why Can Humans See In Color?
We can live without it, but color vision must give us some evolutionary advantage
Cypresses In Starry Night
From a rainbow after a storm to Van Gogh’s Starry Night, there’s no denying that the world looks better in color. But for color vision to have evolved and perpetuated in humans, it must have provided some sort of advantage. What might that advantage have been? And might color vision continue to improve and evolve in the future?
On a molecular level, humans can see in color because of our cones, special light-absorbing cells that sit on the outer layer of the retina. Each eye contains between six and seven million of them. A person with normal color vision has three different types of cones, each of which absorbs a different wavelength of light that generally correspond to red, green, and blue. That’s called trichromacy (three color channels) and, together, they enable a person to see all the colors in the visual spectrum; the most common type of color vision deficiency, dichromacy, is caused by a genetic mutation that prevents a person from producing one of those cones. When light of a certain wavelength passes through the cornea and hits a cone, the cells relay the information through a series of neurons until it reaches the optic nerve, which then sends it to the brain where it’s processed.
Other animals developed this ability to observe color long before humans even existed. Using genetic analysis, researchers have traced these cone receptors back to early vertebrates, possibly as far as 540 million years ago. There’s evidence that vertebrates have evolved color vision several times. And because of how evolution and natural selection operate, animals wouldn’t have expended so many resources if it didn’t help them survive or reproduce better. So, clearly, color vision must serve some purpose, making individuals more evolutionarily fit than their visually limited counterparts.
There’s not just one answer for why humans can see in color, says Kimberly Jameson, a project scientist at the Institute for the Mathematical Behavioral Sciences at the University of California, Irvine. So it helps to look at the function of color—and the ability to see it—in animals.
Light absorbed by cones
The most straightforward theory is good vision can help individuals distinguish objects when the intensity of light coming from them is the same, says Dale Purves, a neurobiologist at the Duke Institute for Brain Science. Humans and primates alike would probably have been able to survive better if they could distinguish panthers from the dark green forest, or golden lions from the yellow-green grasses of the savannah.
Another theory says animals developed color to attract mates, Jameson says. The male half of many species, from blue-jowled mandrills to elaborately plumed peacocks and orange-bellied guppies, use colorful, elaborate displays to tempt the opposite sex. The more vibrant the color, it seems, the fitter the male, which makes him more attractive to females who want to have fitter offspring, Jameson says. Humans don’t have this facet of sexual selection per se, but we might have inherited the ability to see colors from ancestors that did and found another use for it.
Other researchers suggest that primates’ ability to see color helped them pick up on social cues that would aid their reproductive success. Some primates don’t have much hair on their faces, so it was visible when their faces became red. That might be helpful for males to see a female blush, for example, which might indicate that she’s a receptive sexual partner. Or males could see when a competitor’s face reddened out of anger or frustration, enabling them to avoid injury that might inhibit his successful reproduction in the future. Their human descendants might also use color vision to understand other members of our own species.
But the most convincing theory (and the closest thing we have to a scientific consensus) is that color vision helped our ancestors better feed themselves. There are lots of examples of this in the animal world. Bees can see ultraviolet light because flowers co-evolved with them to have markings near the parts that need pollination, providing food for the bees. Primates that could tell the difference between light dark leaves might have been able to pick the most tender, delicious ones to eat.
A third cone would help humans identify the ripest fruit, which has more carbohydrates and would give our ancestors more energy. The ability to visually distinguish between an unripe green strawberry and a mature, red one might make the difference between starvation and nourishment, between death and life.
If seeing color is so useful, why can’t our eyes make sense of wavelengths beyond our visual spectrum? Detecting light of shorter wavelengths, as do bees and cats, would let us see more purplish hues; picking up longer wavelengths in the infrared, as snakes do, would help us see better at night.
The simple answer is, we haven’t needed to. “It depends on what your niche is and what you need to get along in the world,” Purves says. “We don’t need to see ultraviolet in flowers, so we don’t.” While it might be helpful to observe predators in color at night, humans are mostly moving around during the day, so we aren’t putting ourselves into too many risky situations after dark. If we do, our ears are pretty helpful to hear predators or other threats; if we really need to see, our black-and-white visual receptors called rods can see just enough at night, especially if the moon is out.
There’s nothing magical about tricromacy, either. Up until now, that’s what has worked best for humans, but dichromacy can be found in about 8 percent of men and 0.5 percent of women. Most animals are dichromatic, and in humans it’s not a big deficiency, Purves says: “Sure, you can’t be an airline pilot or a radio signal engineer, but usually people don’t know they’re color blind until someone else tells them.” Some say it might not present any disadvantage at all—if you can see the lion you’re running away from, Jameson adds, who cares if you can pick up all the colors in the Monet?
There are also a few tetrochromats in the world, women with four types of cones who can see a thousand times more colors than the average person. But that doesn’t provide enough of an advantage to humans to be worth the energy to sustain it. “Once present it would tend to stick around, since there is not much evolutionary pressure to drop it,” Purves says.
But, of course, evolution doesn’t just end, and what humans need to survive is shifting. No one knows how evolution will progress—it’s not engineered, as Purves notes—but Purves says it’s possible that humans could “devolve” to see fewer colors, while Jameson thinks that tetrachromacy might become more common. “It would be nice we were at the zenith of our evolutionary trajectory,” Jameson says. “If there’s some weird gamma ray that hits you in the eye, you’re going evolve.”
These sorts of evolutionary changes take so long that, chances are, humans probably wouldn’t notice the shift towards seeing more or fewer colors. But we have millennia of those changes to thank for our ability to appreciate a beautiful sunset, or the dozens of shades of green in a forest. No matter how we evolved to get here, there’s no question that our ability to see color has taken on a large role in how we perceive the world. “Humans are interesting because they appreciate their color vision experience, incorporate it in their lives as an enriching aesthetic,” Jameson says. “You evolve some capacity and it ends up being used for something totally different.”