[music]
We. Love. Symmetry.
It's in our art, and architecture...
we even balance equations on it.
Our love of symmetry makes sense, because WE'RE symmetrical.
Two eyes, two ears, two arms, and two legs. Bodies built like paper dolls.
But that outward beauty is superficial. Peel back the curtain, and that symmetry disappears.
Your guts are a mess.
Unless you're from Gallifrey, you have one heart, on the left side of your body, same
as your stomach and spleen, but your liver is on the right. Even organs that look symmetrical
at first seem to tip one way or the other.
The right lung is divided into three lobes while the left lung has only two. And in your
brain, two areas needed for speech and language are found only on one lobe, usually the left.
Animals as distantly related as starfish show similar internal asymmetry. The interesting
thing isn't that our bodies are asymmetrical in the first place, but that we all have the
SAME asymmetry.
Unless you have a case of oppositus.
1 in 20,000 people on Earth have a genetic condition called situs inversus, where the
internal organs are inverted left to right. These people usually show no negative effects
from their reversed innards. Unless their name is Donny Osmond.
In 1972, the singer was checked into the hospital nearly unconscious from a pain in the left
side of his abdomen. Sounds like a classic case of appendicitis, except the pain was
in the wrong place. The doctors operated anyway, and removed his burst appendix with only hours
to spare . . . from the opposite side of his body where it should have been.
Enrique Iglesias and Catherine O'Hara also have situs inversus... maybe one of you do
too. But if people can live happy, healthy lives with inverted organs, why don't half
of us have half of us on either half of us? And if it's doesn't hurt to be asymmetrical
on the inside, why are we so NON-asymmetrical on the outside?
One theory says that throughout evolution, when animals are looking to pass their genes
to the next generation, sometimes it's what's on the outside that counts.
Female barn swallows seem to choose males based on the symmetry of their long forked
tails. Highly symmetrical bodies in animals might signal quality genes, and looking for
balanced bodies might make better offspring.
Of course when humans pair up, we display many kinds of symmetry, and we look for a
lot more than just balanced bodies. I mean, you wouldn't decide to date someone based
on where their heart was, as long as it's with you.
So how does it get this way? All we need in order to define left from right is to determine
which ends will be the head and tail, and which will be back and belly. But that only
sculpts our outer shell. The orientation of your organs may just have to go with the flow.
In the earliest days of an embryo, left and right are symmetrical. Last year researchers
found that hairlike cilia on this special bunch of cells began to beat in unison, pointing
toward what will later become the left side. They wash a current over the outside of the
embryo, and sensing that current leads to the expression of a gene that determines the
side on every one of your organs will live on.
It's strange, because doesn't have to be that way. But for most of us, it just is that way.
Hopefully you've seen the other videos in this series on guts. It's obvious that reversing
the other axes of the body, the back and belly, or the mouth and the anus, would have some
serious evolutionary consequences. But left and right seem to be more interchangeable.
But time and time again in the story of our body we have seen the incredible influence
of seemingly chance events on the course of evolution. The first animal to put its eyes
near its mouth might be the reason you have a brain in your head. A worm doing the backstroke
might be the reason your spine runs down your back.
And a tiny molecular propeller, leaning one way or the other might be the reason that
your heart is in the right . . . or left place.
Stay curious.
This video is part of a three-part series all about guts.
If you wanna know more about why your body
is organized the way that it is,
check out the rest of these videos.