0:00:07.003,0:00:09.797
Symmetry is everywhere in nature,

0:00:09.797,0:00:12.184
and we usually associate it with beauty:

0:00:12.184,0:00:13.831
a perfectly shaped leaf,

0:00:13.831,0:00:17.571
or a butterfly with intricate patterns[br]mirrored on each wing.

0:00:17.571,0:00:21.197
But it turns out that asymmetry[br]is pretty important, too,

0:00:21.197,0:00:23.130
and more common than you might think,

0:00:23.130,0:00:25.707
from crabs with one giant pincer claw

0:00:25.707,0:00:30.629
to snail species whose shells'[br]always coil in the same direction.

0:00:30.629,0:00:34.944
Some species of beans only climb up[br]their trellises clockwise,

0:00:34.944,0:00:37.200
others, only counterclockwise,

0:00:37.200,0:00:41.062
and even though the human body[br]looks pretty symmetrical on the outside,

0:00:41.062,0:00:43.735
it's a different story on the inside.

0:00:43.735,0:00:47.404
Most of your vital organs [br]are arranged asymmetrically.

0:00:47.404,0:00:51.939
The heart, stomach, spleen, and pancreas[br]lie towards the left.

0:00:51.939,0:00:55.676
The gallbladder and most of your liver[br]are on the right.

0:00:55.676,0:00:57.568
Even your lungs are different.

0:00:57.568,0:01:00.534
The left one has two lobes,[br]and the right one has three.

0:01:00.534,0:01:05.301
The two sides of your brain look similar,[br]but function differently.

0:01:05.301,0:01:10.159
Making sure this asymmetry is distributed[br]the right way is critical.

0:01:10.159,0:01:15.152
If all your internal organs are flipped,[br]a condition called situs inversus,

0:01:15.152,0:01:16.692
it's often harmless.

0:01:16.692,0:01:19.038
But incomplete reversals can be fatal,

0:01:19.038,0:01:22.099
especially if the heart is involved.

0:01:22.099,0:01:24.030
But where does this asymmetry come from,

0:01:24.030,0:01:28.564
since a brand-new embryo looks identical[br]on the right and left.

0:01:28.564,0:01:31.751
One theory focuses [br]on a small pit on the embryo

0:01:31.751,0:01:33.045
called a node.

0:01:33.045,0:01:36.000
The node is lined with tiny hairs[br]called cilia,

0:01:36.000,0:01:40.189
while tilt away from the head[br]and whirl around rapidly,

0:01:40.189,0:01:42.437
all in the same direction.

0:01:42.437,0:01:46.751
This synchronized rotation pushes fluid[br]from the right side of the embryo

0:01:46.751,0:01:48.301
to the left.

0:01:48.301,0:01:50.201
On the node's left-hand rim,

0:01:50.201,0:01:52.610
other cilia sense this fluid flow

0:01:52.610,0:01:56.958
and activate specific genes[br]on the embryo's left side.

0:01:56.958,0:02:00.724
These genes direct the cells[br]to make certain proteins,

0:02:00.724,0:02:02.079
and in just a few hours,

0:02:02.079,0:02:06.337
the right and left sides of the embryo[br]are chemically different.

0:02:06.337,0:02:08.126
Even though they still look the same,

0:02:08.126,0:02:14.053
these chemical differences are eventually[br]translated into asymmetric organs.

0:02:14.053,0:02:17.812
Asymmetry shows up in the heart first.

0:02:17.812,0:02:21.645
It begins as a straight tube[br]along the center of the embryo,

0:02:21.645,0:02:24.215
but when the embryo [br]is around three weeks old,

0:02:24.215,0:02:27.180
the tube starts to bend into a c-shape

0:02:27.180,0:02:30.115
and rotate towards [br]the right side of the body.

0:02:30.115,0:02:32.942
It grows different [br]structures on each side,

0:02:32.942,0:02:36.459
eventually turning into the familiar[br]asymmetric heart.

0:02:36.459,0:02:40.829
Meanwhile, the other major organs[br]emerge from a central tube

0:02:40.829,0:02:43.664
and grow towards their ultimate positions.

0:02:43.664,0:02:48.299
But some organisms, like pigs,[br]don't have those embryonic cilia

0:02:48.299,0:02:51.288
and still have asymmetric internal organs.

0:02:51.288,0:02:54.570
Could all cells be [br]intrinsically asymmetric?

0:02:54.570,0:02:55.702
Probably.

0:02:55.702,0:03:00.950
Bacterial colonies grow lacy branches[br]that all curl in the same direction,

0:03:00.950,0:03:04.423
and human cells cultured [br]inside a ring-shaped boundary

0:03:04.423,0:03:08.032
tend to line up [br]like the ridges on a cruller.

0:03:08.032,0:03:09.867
If we zoom in even more,

0:03:09.867,0:03:12.562
we see that many [br]of cells' basic building blocks,

0:03:12.562,0:03:17.905
like nucleic acids, proteins, and sugars,[br]are inherently asymmetric.

0:03:17.905,0:03:20.676
Proteins have complex asymmetric shapes,

0:03:20.676,0:03:23.860
and those proteins control [br]which way cells migrate

0:03:23.860,0:03:26.726
and which way embryonic cilia twirl.

0:03:26.726,0:03:30.401
These biomolecules [br]have a property called chirality,

0:03:30.401,0:03:34.652
which means that a molecule [br]and its mirror image aren't identical.

0:03:34.652,0:03:37.652
Like your right and left hands, [br]they look the same,

0:03:37.652,0:03:42.342
but trying to put your right[br]in your left glove proves they're not.

0:03:42.342,0:03:47.621
This asymmetry at the molecular level[br]is reflected in asymmetric cells,

0:03:47.621,0:03:49.073
asymmetric embryos,

0:03:49.073,0:03:51.929
and finally asymmetric organisms.

0:03:51.929,0:03:53.926
So while symmetry may be beautiful,

0:03:53.926,0:03:57.085
asymmetry holds an allure of its own,

0:03:57.085,0:03:58.728
found in its graceful whirls,

0:03:58.728,0:04:00.656
its organized complexity,

0:04:00.656,0:04:02.647
and its striking imperfections.