Why Isn't Our Hair Naturally Blue?

Human hair comes in a variety of colors; including brown, blonde, and black. Why doesn't it grow blue or green?

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Which came first: color, or the ability to see them? Eyes evolved on Earth around 600 million years ago. Before that, things may have been a bit less colorful. Different colors in animals come from a variety of evolutionary tools. Yellows, oranges, and reds come from carotenoids in our diet. Cardinals' red comes from insects and berries, and flamingos absorb carotenoids from algae and tiny animals to become pink (without it, they're just white). And when we eat red, yellow and orange foods, animals can use those pigments to create color, but not for blue.

The color blue isn't as easily absorbed and secreted like other color pigments in nature. Instead, animals evolved physical structures to reflect light in just the right way. By studying hundreds of different species of birds, scientists from the Argonne National Laboratory found that keratin molecules inside feather cells actually change the way light is refracted so that only blue wavelengths of light are reflected. This is known as "Structural Color" as opposed to "Pigmented Color". Human skin can manipulate pigmentation to create different hues of browns, reds, and yellows, but not blue or green shades. Snakes and frogs can create blue pigments but not green ones. They appear green by adding a yellow pigment to a blue structural color.

Learn More:

Why does the ocean appear blue? Is it because it reflects the color of the sky? (Scientific American)
"'The ocean looks blue because red, orange and yellow (long wavelength light) are absorbed more strongly by water than is blue (short wavelength light). So when white light from the sun enters the ocean, it is mostly the blue that gets returned. Same reason the sky is blue.'"

True Blue Stands Out in an Earthy Crowd (NY Times)
"For the French Fauvist painter and color gourmand Raoul Dufy, blue was the only color with enough strength of character to remain blue 'in all its tones.'"

Evolution of the Eye (Scientific American)
"The human eye is an exquisitely complicated organ. It acts like a camera to collect and focus light and convert it into an electrical signal that the brain translates into images."