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HOW COME? What's behind tough fingernails

Where do our nails come from? It seems like they are never-ending, writes reader Maria Andres. How come cutting your fingernails doesn't hurt? asks a student in Woodside, NY.

y do people get hiccups? asks Salma Khan, of Woodside.HOW COME?Fingernails and toenails are fun to decorate, but they're also a kind of armor, protecting our soft, sensitive finger and toe tips like built-in shields. Meanwhile, the hard nails can help us pick up a coin, scratch an itch, or scrape off a gluey price tag.

Fingernails and toenails are made of a tough protein called keratin, which is also the main ingredient in animal hooves and horns. Trim a nail, and it just keeps on coming. Why? Each nail grows (continuously) from the area under the white half-moon at the base. (The older we get, the more our half-moons tend to disappear. But most adults can still find them on their thumbs.)

The half moon (lunula) is the visible part of our nail-plate factory, aka the matrix. It's the matrix's job to make new keratin. And as keratin is added to the bottom of the nail, it pushes the nail up.

Nail growth speeds up in summer and slows down in winter. But each finger and toe follows its own growth timetable, too: The nails on longer digits, like your index finger, actually grow faster than the nails on your pinkie fingers and toes.

When it's time for a trim, we don't feel any pain. Unlike in our skin, which is a living body organ, there are no nerve endings in the piled-up layers of protein forming a nail plate.

Human, chimpanzee and other primate fingernails (as well as horse, hippo and other hooves) evolved from the curving, pointy claws of ancestor animals. Until recently, no one was sure when the first nail-wearing primates roamed Earth. But in 2011, University of Florida scientists published a study of a primate that lived in what's now Wyoming more than 55 million years ago. Scientists say that "Teilhardina brandti" looked like a modern mouse lemur, with a body length of only 6 inches.

The surprise was found at the end of the primate's tiny fossilized finger and toe bones: equally teensy nails. According to the researchers, the fossils of this long-extinct tree dweller are the oldest evidence of true, modern-primate fingernails like our own.

They are also the tiniest true fingernails found so far, leading researchers to believe our own nails evolved first in small primate ancestors. Ridged finger and toe pads topped with nail plates, scientists say, are especially useful for navigating smaller tree branches.

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