Your body is a temple,

but it’s also a museum of natural history.

Look closely and you’ll see parts that aren’t there because you need them

but because your animal ancestors did.

No longer serving their previous function

but not costly enough to have disappeared.

These remnants of our deep history

only make sense within the framework of evolution by natural selection.

With your arm on a flat surface,

push your thumb against your pinky

and tip your hand slightly up.

If you see a raised band in the middle of the wrist,

you’ve got a vestigial muscle in your forearm.

That tendon you see connects to the palmaris longus,

a muscle that around 10-15% of people

are missing on one or both of their arms.

It doesn’t make them any weaker though.

There’s no difference in grip strength.

In fact, it’s one of the first tendons that surgeons will take out

so they can use it in reconstructive and cosmetic surgeries.

You can find the palmaris longus across mammal species,

but it’s most developed among those that use their forelimbs to move around.

In primates, that means the muscle is longer in lemurs and monkeys

and shorter in chimps, gorillas, and other apes

that don’t do a lot of scrambling through trees.

It’s not the only leftover muscle that we've got.

Look at the three that are attached to our outer ear.

We can’t get much movement out of these muscles,

especially compared to some of our mammal relatives

who use them to locate the sources of sounds.

Presumably this would have been quite helpful for early nocturnal mammals.

In humans, you can still detect the remnants of this adaptation with electrodes.

In one study researchers recorded a spike of activity

in the ear muscle cells in response to a sudden sound.

Not enough to move the ear, but detectable.

And you can probably guess the location of the sound based on the results

- it came from a speaker placed to the left of the study subjects.

So this is their left ear subconsciously trying (and failing)

to pivot toward the sound.

You can see another futile effort by our vestigial body parts

when you get goosebumps.

When we’re cold, tiny muscles attached to our body hairs contract,

pulling the hair upright

which causes the surrounding skin to form a bump.

For our furry mammal relatives,

the raised hair increases the amount of space for insulation,

helping them stay warm.

Birds can do this too.

You’ve probably seen a puffy pigeon on a cold day.

Adrenaline is one of the hormones involved

in the body’s response to cold temperatures,

and it’s also part of the fight or flight response.

So it helps some animals appear larger when they’re threatened.

And it may be why surprising and emotional turns in music

can give some people goosebumps.

And then there’s our tail.

At the end of our spine are a set of fused vertebrae.

Some people have 3, some have 5.

We call it the tailbone.

It now serves as an anchor for some pelvic muscles

but it’s also what’s left of our ancestors’ tails.

Every one of us actually had a tail at one point.

When the basic body plan is being laid out

at around 4 weeks of gestation,

humans embryos closely resemble embryos of other vertebrates.

And that includes a tail

with 10-12 developing vertebrae.

In many other animals it continues to develop into a proper tail.

But in humans and other apes,

the cells in the tail are programmed to die

a few weeks after they appear.

Very rarely though,

a mutation allows the ancestral blueprint to prevail

and a human baby will be born with a true vestigial tail.

The most adorable vestigial behavior is the palmar grasp reflex,

where infants up until they’re about 6 months old

have this incredible grasp on whatever you put in their hand.

There’s a similar reflex for their feet.

I wanted to show you this great piece of footage from the 1930s

where they demonstrated this behavior.

These babies are only 1 month old

and you can see that their inner monkey

can support their entire weight.