Frank Sinatra crooned about blue skies, but it turns out Ol' Blue Eyes was really Ol' Brown Eyes. It turns out all human eyes are brown—or at least various shades of it.
It all comes down to melanin and how much of the pigment we have in our eyes, reports CNN. Blue eyes have the least amount of melanin, which explains why brown-eyed newborns can appear to have blue eyes as their melanin (melanocyte cells) forms, darkening the iris.
"Everyone has melanin in the iris of their eye ... there's really only (this) one type of pigment," says Dr. Gary Heitling. Melanin levels also determine our skin and hair color.
While dark brown in color, melanin absorbs different amounts of light. The more light absorbed, the less light is reflected out, and the browner the iris appears.
Reflected light is what we see when we gaze into someone's eyes. Blue reflects more light at a shorter wavelength of the visible color spectrum. If you think your sweetheart's hazel eyes appear amber in the sunset, you're not wrong.
Eye color can indeed change depending on the light, and hazel and green eyes are in the middle of the spectrum. "It's an interaction between the amount of melanin and the architecture of the iris itself," Heitling says.
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This architecture is so unique, he adds, that it can act like a fingerprint. Migration from hotter to cooler climes by our ancestors could explain eye color evolution.
(More melanin protects against UV radiation.) Another theory is that a genetic mutation turned off melanin production in blue eyes. But don't bet on two blue-eyed parents having a blue-eyed child; eye color is influenced by several genes.
(One study found blue-eyed people are more likely to be alcoholics.)
This article originally appeared on Newser: All Human Eyes Are Brown