Why Do Humans Have Fingernails And Toenails? - Science ABC

The Evolutionary Origins Of Nails

Humans are distinct from other animals in many ways—cognitive capacity, hairlessness, and our opposable thumb—but having nails isn’t one of them. Our ancestors, primates, also have finger and toenails that look like ours. In fact, mammals, birds and reptiles have similar features at the ends of their appendages, such as claws and talons.

These nails or claws or talons are made of a protein called keratin. This is the same protein that makes up your hair, the horns on cows and sheep, and the hooves of horses, zebras, bulls, and other ungulates.

Cells in the matrix called keratinocytes produce the protein keratin that go on to form the nail. The actual nail is therefore composed of layers of dead, compacted cells, along with keratin, which makes it strong and rigid, as well as flexible. As the nail grows and pokes out from your skin, the cells it contains actually die, which is why trimming your nails doesn’t hurt. The same is true for hair.

These keratin extensions evolved sometime between 400 million and 300 million years ago, when four-legged water animals, tetrapods, were venturing onto land. I say ‘sometime’ because fossils between those years are rare. This dark shroud over the time between 360 and 335 million years ago is called Romer’s gap, after the palaeontologist Alfred Romer.

The transition from water tetrapods to land tetrapods is a little fuzzy for this reason, which makes the evolutionary origins of claws, talons, and our own nails a bit fuzzy too. Most of the early evidence for claws comes from track marks left behind by our four-legged ancestors or from a clawed frog, the African bush frog that has claws, which is unusual for frogs and amphibians.

However claws emerged, they proved extremely handy (pun intended). It allowed creatures to grasp better, everything from penetrating the bark and branches of trees, clutching food, and digging. The claws, along with the structure of the limbs, gave terrestrial animals an advantage to climbing trees and diversifying to many other niches.

Our own human nails evolved from claws. Many tree-climbing animals, like squirrels, have claws to clutch tree bark. Similarly, ancient primates that lived approximately 50 million years ago also had claws, new research has found. Teilhardina brandti, the oldest known primate, also had claws, called grooming claws. These grooming claws were used to get rid of parasites that live in the hair and skin, hence the name.