Why do humans have a third eyelid? - Dorsa Amir

You know that little pink thing nestled
in the corner of your eye?

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.