Every other Friday on Morning Edition, the Outside/In team answers a question from a listener about the natural world.
We often solicit listener questions on social media, and this one comes from @BethanyJane320 on Instagram. She asks: “Why did our eyes evolve to see many shades of green?”
Rods and cones
Before we consider the evolutionary benefits, we first need to understand how humans perceive color.
You may have heard about rods and cones, maybe in a diagram of the eye on a poster on your eye doctor’s wall.
Rods and cones are photoreceptor cells on our retina. “Photo-” is the greek prefix for light, which means these are cells that are sensitive to light. Rods are useful in low-light situations; cats, for example, have more rods than humans, so they see better in dim light at night.
RGB is everything
Cones send signals to the brain in bright light. The brain then interprets those signals into what we think of as color — but only red, green or blue, the primary colors of light. Humans have evolved to be trichromatic: that is, we build all of the colors we see out of combinations of those three colors.
TVs and computer monitors also rely on those same three colors — red, green and blue — often referred to as “RGB.” And if you have ever used a color wheel in art class, or just mixed paints, you know that combinations of primary colors can make secondary colors. That's where we get oranges, purples and yellows.
Your photoreceptors are doing the same thing. Shades of green are greens mixed with colors from our other two cones. Take green plus a little red, and you get greenish yellow. Add a little blue, and you’ll see more of a shade of turquoise.
Evolutionary benefits
As far as why the ability to see lots of different shades of green might be helpful to humans, you can imagine it might have something to do with the amount of green on our planet.
“One of the earliest hypotheses for why there's a benefit to having red-green color vision is that it allows primates to forage for red conspicuous fruits against a green foliage,” says Adriana Briscoe, an evolutionary biologist at the University of California, Irvine.
That helps us spot all the good stuff: strawberries, raspberries, pomegranates, tomatoes and more! But, as we know, ripe fruit is not always available.
“Sometimes they [our evolutionary ancestors] fall back onto eating foliage, like leaves,” Briscoe says. “And young, reddish leaves or flowers often have a nutritional benefit that is potentially advantageous compared to old tough leaves.”
But what if you’re colorblind?
There is, of course, a little wrinkle: Not all humans have evolved to have three color receptors. Some people have only two photoreceptors: the blue and the green, or the blue and red. And because they only have two photoreceptors, they are dichromatic, or colorblind. Colorblindness is much more common among men than among women, affecting somewhere between 7 and 8% of the total population.
How did they fare without the evolutionary benefit of both red and green photoreceptors? Scientists think those who are dichromatic have other evolutionary advantages.
“It allows them to be better at detecting and capturing, for example, camouflaged insects under low illumination,” Briscoe says. “If you're in a group and one part of your group can detect the ripe fruit against the green leaves and another one can find delicious protein-rich insects that are maybe less conspicuous, then there would be potentially a benefit to everybody in the group from different kinds of perceptual ability.”
In addition to detecting camouflaged insects, a dichromatic person might also be better at detecting movement — like being the first to spot a snake moving through the foliage and sending up the alarm.
Different ways of perceiving color might be an evolutionary advantage that benefits an entire group. As Briscoe explains, we don’t perceive color in isolation, we also perceive “pattern, shape, size, movement, depth, brightness.”
The color green is important, but there are many factors at play in our perception of the world.
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