WEBVTT

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Symmetry is everywhere in nature,

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and we usually associate it with beauty:

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a perfectly shaped leaf,

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or a butterfly with intricate patterns
mirrored on each wing.

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But it turns out that asymmetry
is pretty important, too,

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and more common than you might think,

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from crabs with one giant pincer claw,

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to snail species whose shells'
always coil in the same direction.

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Some species of beans only climb up
their trellises clockwise,

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others, only counterclockwise,

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and even though the human body
looks pretty symmetrical on the outside,

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it's a different story on the inside.

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Most of your vital organs 
are arranged asymmetrically.

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The heart, stomach, spleen, and pancreas
lie towards the left.

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The gallbladder and most of your liver
are on the right.

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Even your lungs are different.

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The left one has two lobes,
and the right one has three.

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The two sides of your brain look similar,
but function differently.

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Making sure this asymmetry is distributed
the right way is critical.

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If all your internal organs are flipped,
a condition called situs inversus,

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it's often harmless.

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But incomplete reversals can be fatal,

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especially if the heart is involved.

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But where does this asymmetry come from,

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since a brand-new embryo looks identical
on the right and left.

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One theory focuses 
on a small pit on the embryo

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called a node.

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The node is lined with tiny hairs
called cilia,

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while tilt away from the head
and whirl around rapidly,

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all in the same direction.

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This synchronized rotation pushes fluid
from the right side of the embryo

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to the left.

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On the node's left-hand rim,

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other cilia sense this fluid flow

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and activate specific genes
on the embryo's left side.

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These genes direct the cells
to make certain proteins,

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and in just a few hours,

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the right and left sides of the embryo
are chemically different.

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Even though they still look the same,

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these chemical differences are eventually
translated into asymmetric organs.

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Asymmetry shows up in the heart first.

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It begins as a straight tube
along the certain of the embryo,

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but when the embryo 
is around three weeks old,

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the tube starts to bend into a c-shape

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and rotate towards 
the right side of the body.

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It grows different 
structures on each side,

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eventually turning into the familiar
asymmetric heart.

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Meanwhile, the other major organs
emerge from a central tube

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and grow towards their ultimate positions.

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But some organisms, like pigs,
don't have those embryonic cilia

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and still have asymmetric internal organs.

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Could all cells be 
intrinsically asymmetric?

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Probably.

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Bacterial colonies grow lacy branches
that all curl in the same direction,

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and human cells cultured 
inside a ring-shaped boundary

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tend to line up 
like the ridges on a cruller.

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If we zoom in even more,

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we see that many 
of cells' basic building blocks,

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like nucleic acids, proteins, and sugars,
are inherently asymmetric.

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Proteins have complex asymmetric shapes,

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and those proteins control 
which way cells migrate

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and which way embryonic cilia twirl.

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These biomolecules 
have a property called chirality,

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which means that a molecule 
and its mirror image aren't identical,

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like your right and left hands, 
they look the same,

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but trying to put your right
in your left glove proves they're not.

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This asymmetry at the molecular level
is reflected in asymmetric cells,

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asymmetric embryos,

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and finally asymmetric organisms.

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So while symmetry may be beautiful,

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asymmetry holds an allure of its own,

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found in its graceful whirls,

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its organized complexity,

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and its striking imperfections.