With your arm on a flat surface, push your thumb against your pinkie, and tip your hand slightly up.

If you see a raised band in the middle of your wrist, you 've got a vestigial muscle in your forearm.

That tendon you see connects to the palmaris longus,

a muscle that around ten to fifteen percent 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 that 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 these 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.