We learn to tell right from left around age five, but our brains never quite get the memo.

Your brain is split down the middle,

with the right side controlling the left half of your body and vice versa.

It might sound weird, but this reversal is very common among vertebrates.

So common, in fact, that biologists have looked back half a billion years

to try and figure out why it happens.

And a few of them have suggested something pretty bizarre:

that some ancient vertebrate ancestor was born with its brain on backwards.

Or, at least on sideways.

We've known about this left-right switch in our brains for at least a couple thousand years,

and it's pretty strange.

I mean, each side of your body generally only talks to the opposite side of your brain.

That seems kind of silly.

But, hey, that seems to be the way things are, at least, for many species.

It's known as decussation.

And it's true for just about every vertebrate.

The exact amount of flip-flopping varies between species,

but the trend is so widespread that scientists think it must have evolved very early on,

maybe in some of vertebrates' earliest common ancestors.

As for why, there have been various explanations over the last hundred years or so.

In the late 1800s, for example, one physician thought our brains had to be swapped-around

for our visual field to line up correctly.

And his explanation became pretty popular,

despite being wrong about how our vision actually works.

These days, some researchers think decussation instead lets us pack nerves into our skulls

more efficiently.

Or maybe it helps us defend ourselves from attacks somehow.

But some scientists have gone a different route with all this.

They've suggested that maybe decussation didn't stick around because it

gives vertebrates an advantage.

Maybe an early ancestor was just born this way, and we've been stuck like this ever since.

There are two main versions of this idea.

The first was proposed by one neuroscientist in 2013,

and they said some pre-vertebrate ancestor was born with its head on backward.

This would have been before there was too much of a difference between the front and

back of an animal's body,

so the mutation wouldn't have been the disaster you might think.

But it would explain our brains' left-right flip-flopping,

and it could also illuminate some odd parallels between vertebrate and invertebrate bodies.

For example, many invertebrates have a string of nerves like our spinal cord that runs down

their front rather than their back.

And the vision centers of their brains are also toward the front,

not the back, where ours are.

So at one point, this researcher proposed, our bodies just got reversed.

Theoretically, they suggest we should be able to find some evidence for this,

maybe in our DNA or in some piece of anatomy.

But as of right now, there's no smoking gun.

If that seems like an outlandish idea, never fear:

In 2012, a different team of biologists proposed something that might be a little bit more palatable.

Instead of being on backward, our heads are just on sideways.

This team said that, instead of a single, simple, 180-degree turn,

decussation happened because an early vertebrate

had some mutation that made its head twist 90 degrees,

putting it sideways compared to the rest of the body.

Then, to compensate, its body twisted 90 degrees, too, but in the opposite direction.

So again, they ended up with a head that was on backward.

The wild thing is, this two-twist group was actually able to see their idea play out by

observing zebrafish embryos.

Over the course of development, the fish's heads twisted one way,

their bodies twisted the other, and the nerves were crossed in the process.

Which is a bit bizarre!

And while they haven't seen this happen in human embryos,

possibly because it's just too hard right now,

there's evidence that all of us bear the marks of doing the developmental twist.

Like, besides the whole brain thing, children's faces tend to be rotated by a couple degrees

from being perfectly symmetrical.

Seriously.

The next time you're around a baby,

just stare really hard at their face and wait for your mind to be blown.

That asymmetry does get smaller as we age, but it never goes away completely.

And neither do its effects, at least, according to some papers.

Like, in one, a researcher observed that when people kiss,

they tend to tilt their heads to the right.

And when they hug, they tend to turn their bodies to the left.

In each case, the researcher proposed that we're naturally accounting for our natural asymmetry.

And the directions we turn are different because, during development,

our faces rotate to the right while our bodies rotate to the left.

Of course, this is just a general average.

People tilt different amounts in different directions for different reasons.

But the trend was clear.

Ultimately, we probably can't ever know for sure why decussation has stuck around

for a few hundred million years.

But evidence is piling up that a common ancestor of fish and humans and lampreys

and almost every vertebrate in-between was a little bit twisted.

Thanks for watching this episode of SciShow!

Every time we make content like this about weird, little-known hypotheses,

we feel especially thankful for our patrons on Patreon.

Thanks to you, we're able to reach out to scientists, dig through literature,

and give our episodes the attention we think they deserve.

So to everyone who supports the show: thank you!

If you want to learn more about supporting SciShow,

you can head over to patreon.com/scishow for all the details.

[♪ OUTRO]