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  • [♪ INTRO]

  • 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]

[♪ INTRO]

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