It's actually the remnant of a third eyelid.

Known as the “plica semilunaris,”

it's much more prominent in birds and a few mammals,

and functions like a windshield wiper to keep dust and debris out of their eyes.

But in humans, it doesn't work.

It's vestigial, meaning it no longer serves its original purpose.

There are several other vestigial structures like the plica semilunaris

in the human body.

Most of these became vestigial long before homo sapiens existed,

quietly riding along from one of our ancestor species to the next.

But why have they stuck around for so long?

To answer this question, it helps to understand natural selection.

Natural selection simply means that traits

which help an organism survive and reproduce in a given environment

are more likely to make it to the next generation.

As the environment changes, traits that were once useful can become harmful.

Those traits are often selected against,

meaning they gradually disappear from the population.

But if a trait isn't actively harmful, it might not get selected against,

and stick around even though it isn't useful.

Take the tailbone.

Evolutionary biologists think that as the climate got drier

and grasslands popped up,

our tail-bearing ancestors left the trees and started walking on land.

The tails that had helped them in the trees

began to disrupt their ability to walk on land.

So individuals with mutations that reduced the length of their tails

became more successful at life on land,

surviving long enough to pass their short tails on to the next generation.

The change was likely gradual over millions of years until,

about 20 million years ago,

our ancestors' external tails disappeared altogether.

Today, we know human embryos have tails that dissolve as the embryo develops.

But the stubby tailbone sticks around,

probably because it doesn't cause any harm—

in fact, it serves a more minor function

as the anchor point for certain other muscles.

Up to 85% of people have a vestigial muscle called the “palmaris longus.”

To see if you do,

put your hand down on a flat surface and touch your pinkie to your thumb.

If you see a little band pop up in the middle of your wrist,

that's the tendon that attaches to this now-defunct muscle.

In this case, the fact that not everyone has it has helped us trace its function.

Vestigial traits can persist when there's no incentive to lose them—

but since there's also no incentive to keep them,

random mutations will sometimes still eliminate them

from part of the population.

Looking at our primate relatives,

we can see that the palmaris longus is sometimes absent

in those that spend more time on the land,

but always present in those that spend more time in trees.

So we think it used to help us swing from branch to branch,

and became unnecessary when we moved down to land.

The appendix, meanwhile, may once have been part of the intestinal system

our ancestors used for digesting plant materials.

As their diets changed, those parts of the intestinal system began to shrink.

Unlike other vestigial structures, though, the appendix isn't always harmless—

it can become dangerously inflamed.

For most of human history, a burst appendix could be a death sentence.

So why did it stick around?

It's possible that it was very slowly on its way out,

or that mutations simply hadn't arisen to make it smaller.

Or maybe it has other benefits—

for example, it might still be a reservoir of bacteria that helps us break down food.

But the fact is, we're not really sure why the appendix persists.

Evolution is an imperfect process.

Human beings are the result of millions of years of trial, error, and random chance—

and we're full of evolutionary relics to remind us of that.