Unless you have some type of color vision deficiency, you probably take the ability to see in color for granted. But have you ever stopped to wonder how and why humans and animals evolved to see so many colors?
The truth is that it took countless genetic mutations over millions of years for humans (and many species of animals) to be able to see the rainbow of colors we enjoy today. Here’s how it all happened.
A Quick Intro to Color Vision
To really understand how color vision evolved, you first need to get a sense of how humans and animals see in color.
When light moves through the eye and reaches the retina, it hits cells called photoreceptors. There are two types of photoreceptors: rods and cones.
Rod cells sense light and darkness, and cones sense color. Animals with more rod cells can see better in the dark than animals with fewer rod cells.
There is no singular type of cone cell that can pick up every single wavelength of light. In many cases, the more types of cone cells an animal has, the more colors it can see:
- Monochromats have one type of cone cell and cannot see color
- Dichromats have two types of cone cells and have limited color vision
- Trichromats have three types of cone cells and can see every color on the visible spectrum of light
- Tetrachromats have four types of cone cells, one of which usually interprets ultraviolet light
- Pentachromats have five types of cone cells, theoretically capable of seeing up to 10 billion colors
There’s no real limit to how many cone cells an animal can have. However, researchers have found that the number of colors an animal can see doesn’t always match up to the number of cone types it has.
For example, mantis shrimp have up to 16 different types of photoreceptors. For many years, researchers believed the crustaceans’ huge number of cone cells meant they could see more colors than any animal alive. But in 2014, new research indicated that mantis shrimp were worse at differentiating between colors than humans are.
Human Color Vision vs. Animal Color Vision
It took millions of years for humans and animals to develop the ability to see in color. However, human color vision and animal color vision are not the same. Here’s how our color vision stacks up against that of animals.
Humans are trichromats. That means we have three types of cones: one for blue, one for green, and one for red. We can’t see infrared light or ultraviolet light, but we can see the wavelengths in between.
However, some humans have a condition called tetrachromacy, meaning they have four cone cells instead of the usual three. There’s some evidence to suggest that tetrachromatic people can see the purplish glow of ultraviolet light.
We also have enough rod cells in our eyes to see at night, although our night vision isn’t exceptionally good — we primarily see shades of light and shadow.
Cats, Dogs, Horses, and Other Mammals
Like most mammals, our furry friends can’t see as many colors as humans can. Cats, dogs, horses, and most other mammals are dichromats — they have photoreceptors for blue and yellow, but not for red.
However, cats, dogs, and horses do have one visual advantage: they have more rods than we do, so they can see much better at night!
Most reptiles are tetrachromats, meaning they have four different types of cone cells: one for red, one for green, one for blue, and one for UV light. Scientists believe that their ability to see UV light may help them select good spots to bask and identify food.
However, not all reptiles are alike in this regard. Most geckos are nocturnal, so they have evolved to see color exceptionally well at night. In fact, their eyes are about 350 times more sensitive to color at night than ours! Geckos don’t have rod cells, so over time, their cone cells evolved to behave somewhat like rod cells.
Snakes are typically dichromats, but some species have an interesting and unusual adaptation as well: the ability to detect infrared radiation. These snakes don’t technically “see” infrared light. But when they’re hunting at night and come near a warm body, a highly specialized “pit organ” senses heat.
Snakes can combine the sensory input from the pit organ with their visual input, creating a thermal image of prey. This system works a lot like an infrared camera.
Like most reptiles, birds are tetrachromats with red, green, blue, and ultraviolet cones. They can see more colors than we can, and thanks to the large number of rods and cones in their eyes, their vision is much more detailed.
Birds are also much better at differentiating shades of color than we are. Why? They have a small amount of “filtering oil” in their color receptors. That oil filters light as it enters the eye — the tiny drops of oil act as microlenses that make it easier for birds to tell the difference between colors that look identical to us.
Insects and Arachnids
Most spiders are dichromats. But some, including jumping spiders, are trichromats just like people.
Some insects are also trichromats, although their color vision is a bit different from ours. The cone cells in human eyes primarily focus on red, green, and blue light. The cones in insects’ eyes primarily focus on blue, green, and ultraviolet light.
Scientists haven’t tested the color vision of all types of fish. However, the research that has been conducted seems to indicate that some fish are tetrachromats: their eyes have photoreceptors for red, green, blue, and ultraviolet light.
You might wonder how scientists can test the color vision of a fish. For instance, this is how researchers would test if a goldfish is capable of making associations with colors. In one experiment, they would put a goldfish in a tank with two separate windows. One window is gray, and the other is red. When the fish swims to the red side, the researcher rewards it with food.
During the experiment, the researcher would vary the brightness of each window to make sure the fish isn’t only responding to brightness (and not responding to color). When they repeat the experiment with many color combinations, the fish continues to choose the red window.
How Did Human Color Vision Evolve?
Very ancient human ancestors likely had grayish vision that wasn’t nearly as sharp as the vision we have today. Over millions of years, our color vision began to develop. By about 30 million years ago, our ancestors could see the full spectrum of what we now refer to as “visible light.” But why do humans have three types of photoreceptors instead of two?
The most common hypothesis suggests that early humans evolved to have red, green, and blue cone cells to help them better see ripe fruit against green forest backgrounds.
That belief also has been supported by more recent research. Scientists were able to study rhesus macaque monkeys, as some of them are dichromats and others are trichromats. They found that the trichromatic monkeys were consistently better at locating fruit than the dichromatic monkeys.
It’s likely that trichromatic humans similarly had an easier time finding food. That made them more likely to survive and pass on their genes. Over time, all humans (except for those with color blindness) evolved to be trichromats.
While color vision’s ability to help humans find food likely played a part in how our vision evolved, more recent research suggests another possibility: that our eyesight evolved, at least in part, to detect flushes of red in other people’s faces.
Color is an important part of social signaling for many animals, and humans are no different. A rush of blood to another person’s face can be a sign of embarrassment, anger, or lust — all important emotions for a social species to be able to detect!
What About Animal Color Vision?
Humans started seeing in color a long time ago, but color vision existed long before that in other species. Scientists found the first evidence of color vision in the fossil of a fish, Acanthodes bridgei. That fossil dates back to 300 million years ago.
Because the eyes of the fossil were remarkably well-preserved, scientists were able to take a closer look. They found both rod and cone cells that suggested the fish could see in color.
Birds, fish, and reptiles evolved along a different evolutionary “branch” than mammals did, so most species have evolved to see both ultraviolet light and the spectrum of visible light that we see. Scientists believe that perceiving UV light helps these animals find mates and detect signals from other members of the species.
For example, in some bird species, males and females look identical (at least to us). However, research has suggested that the feathers of these birds reflect UV light differently, letting members of the species easily tell males and females apart.
UV light also may make it easier for animals to hunt and forage. In some cases, plants, flowers, and even prey animals reflect UV light in a way that makes them easier to spot.
So why do so many mammals only have two photoreceptors? It’s very likely that the common ancestors of birds, mammals, reptiles, and fish had four types of cone cells. Complex color vision helped the birds, fish, and reptiles function optimally, so these animals largely retained their tetrachromacy.
However, with mammals, it was a different story. Scientists generally believe that the earliest mammals were nocturnal. For an animal active at night, complex color vision isn’t a necessity, but being able to see in the dark is.
Over time, these mammals evolved to have more complex collections of rod cells that gave them impressive night vision. Because there wasn’t an evolutionary need to maintain their complex color vision, they lost two types of cone cells over millions of years.
Is Color Vision Still Evolving?
Evolution is a process that’s never really finished. The changes from generation to generation (and even century to century) are small enough that we aren’t likely to notice them. But just like any other species, the human species is constantly evolving to fit an ever-changing environment. How will our color vision (and that of our animal friends) evolve in the future? Only time will tell.