| When objects interact with light in particular ways — by absorbing or reflecting it — we see in color. A sunset's orange hues and the ocean's deep blues inspire artists and dazzle observant admirers. But colors are more than pretty decor; they also play a critical role in life. They attract mates, pollinators and seed-spreaders, and signal danger. And the same color can mean different things to different organisms: A red bird might attract a mate, while a red berry might warn off a hungry human.
For color to communicate meaning, systems to produce it had to evolve, by developing pigments to absorb certain wavelengths of light or structures to reflect them. Organisms also had to produce the machinery to perceive color. When you look out into a forest, you might see lush greenery dappled with yellowish sunlight and pink blooms. But this forest scene would look different if you were a bird or a fly. Color-perception machinery — which include photoreceptors in our eyes that recognize and distinguish light — can differ between species. While humans can't see ultraviolet light, some birds can. While dogs can't see red or green, many humans can. Even within species there's some variation: People who are colorblind have trouble distinguishing some combinations, such as green and red. And many organisms can't see color at all.
Within one planet, many colorful worlds exist. But how did colors evolve in the first place?
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To pinpoint when different kinds of color signals may have evolved, researchers recently reviewed many papers, covering hundreds of millions of years of evolutionary history, to bring together information from the fossil record and phylogenetic trees (diagrams that depict evolutionary relationships between species). Their analysis across the tree of life suggested that color signals likely evolved much later than color vision. It's likely that color vision evolved twice, developing independently in arthropods and fish, between 400 million and 500 million years ago. Then plants started using bright colors to attract pollinators and animals to disperse their seeds, and then animals started using colors to warn off predators and eventually to attract mates.
One of the most common colors that we see in nature is green. However, this isn't a color signal: It's a result of photosynthesis. Most plants absorb almost all the photons in the red and blue light spectra but only 90% of the green photons. The remaining 10% are reflected, making the plants appear green to our eyes. But why did they evolve to do this? According to a model, this makes photosynthetic machinery more stable, suggesting that sometimes evolution favors stability over efficiency.
The majority of colors in nature are produced by pigments that absorb or reflect different wavelengths of light. While many plants can produce these pigments on their own, most animals can't; instead, they acquire pigments from their diet. Some pigments, though, are hard to acquire, so some animals instead rely on nanoscale structures that scatter light in particular ways to create "structural colors." For example, the shell of the blue-rayed limpet has layers of transparent crystals, each of which diffracts and reflects a sliver of the light spectrum. When the layers grow to a precise thickness, around 100 nanometers, the wavelengths in each layer interact with one another, canceling each other out — except for blue. The result is the appearance of a bright blue limpet shell.
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