Answer: Keratin

Although you may not have known its name, keratin is a substance that you’re certainly familiar with. The fibrous protein is the most abundant component in the structure of human nails (as well as the claws, hooves, and horns of numerous animals) and helps to form a hard and protective covering over the ends of our fingers and toes.

Some evolutionary biologists hypothesize that the broad fingernails found on humans evolved to support our broader fingers while potentially aiding in locomotion (and as a possible side effect from the loss of claws). Compared to our primate cousins, humans have fat fingers, as it were.

The term keratin is derived from the Greek keras/kerat-, meaning, appropriately enough, “horn”.