Color, Perception, and Evolution

A while back, I was looking at a small tomato plant out on my porch. A tiny tomato had been growing. For a while, it had been as green as the stem. But it had turned bright red, and rather quickly – transitioning all the way over to the opposite side of the color wheel. It was shouting “eat me!” (not in the rude way).  And doing it as plainly as a traffic light distinguishes stop and go.

I thought, how oddly convenient… Why does the color change so drastically? It’s like it knows that I want to eat it, and it is blatantly letting me know that it’s ready!

Does the tomato also want me to eat it? I guessed that it could be a strategy for spreading its seeds – either through me or some animal spitting the seeds out somewhere.  Or… pooping them out.  And maybe somewhere far away. That’s how evolution works.

Then I thought about what color is… It is really just a perception that we have as a human beings, and not an “absolute reality”. Color is simply an interpretation of the wavelength of light, which is a completely linear thing that the human eye and mind have mapped into kind of a circular color wheel, with “opposite” colors like red and green.

This linear spectrum, just a range of numbers:

Gets mapped to this:

So, did human vision evolve to be able to detect changes in fruits and vegetables like this? Did it evolve to make ~685 nm (red) and ~530 nm (green) so obviously visibly distinct from each other, for the purpose of enabling the vision-bearer to easily find edible fruits and vegetables out in the forest?

I looked some things up online.

It turns out that the answer is that animals and fruit-bearing plants actually co-evolved for this to happen. The idea of fruits and vegetables being eaten by animals is beneficial for both. I thought that this was super-cool. Yes, plants evolved to make the ripeness of their fruits and vegetables obvious to primates and other animals. And, what I thought was maybe more interesting, is that we humans (as primates) have evolved to see red as red and green as green likely because of this exact situation with the tomato plant on my porch.

(Well, I guess not this exact situation. Primate evolution didn’t happen on my porch. But, you know what I mean.)

There are, of course, other theories about why fruits and vegetables are so conveniently-edible. Kirk Cameron’s friend makes some good points in this video about bananas:

…but I’m going to go with Wikipedia on this one. Co-evolution.

You may have heard at some point that dogs only see in “black and white” or grayscale. Well, this is actually not true at all.  Dogs can in fact distinguish color, in addition to being able to perceive brightness.  But, while humans see three primary colors (red, green, and blue), dogs can only see two: yellow and blue.  Humans are called “trichromats”, and dogs are called “dichromats”.

If color is just a “perception thing”, then how can we know what dogs see, versus what humans can see? Well, you may remember learning about “rods and cones” in our retinas. Cones are the cells responsible for detecting color. Science-folk have dissected eyeballs, and found that humans have three different types of cones, and dogs only have two.

A dog’s color spectrum looks like this, compared to a human’s:

Human’s view
Human colorline
Dog’s view
Dog colorline

To a dog, red and green look pretty similar.  So, a dog would not be able to see the ripeness of the tomato on my porch as easily as you would.  Primates rule, dogs drool.

This is kind of like color-blindness in humans. I had a friend in college who was color blind. In our electronics lab, I always asked him to pick out the resistors, because I’m a jerk.

What I think is cooler, is that there are animals that can actually see more colors than humans. Some fish, birds, reptiles, and insects can see a fourth color, in addition to red, green, and blue. The fourth color sits in the ultraviolet part of the spectrum. What does this fourth color look like?

I have no idea!

(Note that the answer is not “ultra violet”. That’s not a color.)

But, there are some cases of humans that may know what it looks like.  Look it up… It’s called “tetrachromacy”.

Of course, none of us really has any idea what colors look like to other people. Is how I perceive a 685 nm wavelength the same as how you perceive a 685 nm wavelength? No idea. Weird. But that’s a whole other topic that I’ll leave that for someone else’s blog.


Next time you are in the forest, foraging for fruits and berries, think about it. The fact that they stick out from the brush is not a coincidence!


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