Plural of keratin; tough, fibrous proteins that form the main structural component of hair, nails, and skin in animals.
From Greek 'keras' (horn, wax) + Latin suffix '-in' for proteins. The spelling 'keratin' was Anglicized in the 19th century after scientists identified this substance as the material making up horns and hooves.
Your fingernails are basically made from the same protein as a rhinoceros horn—nature uses this one tough material across the animal kingdom because it's incredibly strong yet flexible enough to grow continuously.
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