When you think about it, color vision is kind of strange. Our eyes -- unless you have a genetic or physical inability to do so -- are able to sort out the frequencies of light, and each range in the visible light spectrum looks different to us. But why do we have the ability to distinguish between, for example, light with a wavelength of 570 nanometers (which looks yellow) and that with a wavelength of 470 nanometers (which looks blue)? It's a small shift in wavelength, but triggers a completely different response in our eyes and brain -- so it must be important, right?
Color perception in the natural world seems to serve a fairly small number of functions. There's sexual signaling -- the (often) brighter colors of male birds, for example, is most likely a cue for females signaling fitness (and thus good genes, worthy of producing young with). It can be a sign that food is ready to eat, such as fruits changing from the blend-with-the-foliage shades of green to something more eye-catching. It can also be a danger signal, as with the brilliant warning colorations of coral snakes, the foul-tasting bright orange and black monarch butterfly, and Central and South America's dart poison frogs.
So our ability to sense colors, an ability shared with many other mammals, birds, reptiles, amphibians, fish, and some arthropods, seems to have evolved as a way of distinguishing things that need to stand out from the background, for purposes of reproduction or survival. There's a reason, for example, that stop signs are red; our dim-light vision is poorest in the red region of the spectrum, so when car headlights catch a bright red stop sign at night, it immediately grabs our attention. (The flipside of this phenomenon is why snow under moonlight looks blue. It's not that snow preferentially reflects blue light; it's simply that our eyes are better at picking up the blue region of the spectrum in low light levels, so it's almost as if our eyes are subtracting the red frequencies from the white light reflected from snowbanks, resulting in it appearing blue.)
What this means, of course, is that pigment production has to have evolved in tandem with color perception. There are undoubtedly exceptions, where colorful chemicals have evolved for other purposes, and their hues are accidental byproducts of their molecular structure; but otherwise, the evolution of bright pigments must have coevolved with the ability to perceive them. The brilliantly-colored organic compounds produced in the petals of many flowers, for example, are generally for the purpose of attracting pollinators, and the reds, oranges, and yellows of ripe fruit attract animals to consume the fruits and then disperse the seeds.
What's curious about this, and why the topic comes up today, are the findings of a study out of the University of Arizona that appeared in the journal Biological Review last week. It showed that based on genetic studies of distantly-related animal groups, color vision evolved a very long time ago -- on the order of five hundred million years ago, so the middle of the Cambrian Period -- while the first fruits didn't show up for another 150 million years, and the first flowers 150 million years after that.
So the earliest production of functional color (and the ability to perceive it) almost certainly was driven by sexual signaling and warnings. Then, once animals were able to see in color, it became an evolutionary driver in plants to ride the coattails of that capacity in order to facilitate cross-pollination and seed dispersal.
And once that back-and-forth coevolutionary relationship was in place, it was off to the races. Give it another couple hundred million years, and we have the rainbow hues of the natural world today.
One thing I still find hard to explain -- from an evolutionary standpoint, at least -- is why we find brightly-colored things beautiful. Having our attention caught by a bright red apple, or the wild stripes and spots of the venomous lionfish -- sure, those make sense. But why is it almost universal to find a daffodil or a wild rose beautiful?
Ah, well, maybe it's just one of those accidental things that is a consequence of other, more vital, evolutionarily-derived traits. Whatever it is, we can certainly still enjoy it, and not let our wondering why it occurs interfere with our appreciation.
But it's still kind of cool that the ability that allows us to have that experience goes back at least five hundred million years.
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