Why are human bodies asymmetrical? - Leo Q. Wan
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0:07 - 0:10Symmetry is everywhere in nature,
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0:10 - 0:12and we usually associate it with beauty:
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0:12 - 0:14a perfectly shaped leaf,
-
0:14 - 0:18or a butterfly with intricate patterns
mirrored on each wing. -
0:18 - 0:21But it turns out that asymmetry
is pretty important, too, -
0:21 - 0:23and more common than you might think,
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0:23 - 0:26from crabs with one giant pincer claw
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0:26 - 0:31to snail species whose shells'
always coil in the same direction. -
0:31 - 0:35Some species of beans only climb up
their trellises clockwise, -
0:35 - 0:37others, only counterclockwise,
-
0:37 - 0:41and even though the human body
looks pretty symmetrical on the outside, -
0:41 - 0:44it's a different story on the inside.
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0:44 - 0:47Most of your vital organs
are arranged asymmetrically. -
0:47 - 0:52The heart, stomach, spleen, and pancreas
lie towards the left. -
0:52 - 0:56The gallbladder and most of your liver
are on the right. -
0:56 - 0:58Even your lungs are different.
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0:58 - 1:01The left one has two lobes,
and the right one has three. -
1:01 - 1:05The two sides of your brain look similar,
but function differently. -
1:05 - 1:10Making sure this asymmetry is distributed
the right way is critical. -
1:10 - 1:15If all your internal organs are flipped,
a condition called situs inversus, -
1:15 - 1:17it's often harmless.
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1:17 - 1:19But incomplete reversals can be fatal,
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1:19 - 1:22especially if the heart is involved.
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1:22 - 1:24But where does this asymmetry come from,
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1:24 - 1:29since a brand-new embryo looks identical
on the right and left. -
1:29 - 1:32One theory focuses
on a small pit on the embryo -
1:32 - 1:33called a node.
-
1:33 - 1:36The node is lined with tiny hairs
called cilia, -
1:36 - 1:40while tilt away from the head
and whirl around rapidly, -
1:40 - 1:42all in the same direction.
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1:42 - 1:47This synchronized rotation pushes fluid
from the right side of the embryo -
1:47 - 1:48to the left.
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1:48 - 1:50On the node's left-hand rim,
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1:50 - 1:53other cilia sense this fluid flow
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1:53 - 1:57and activate specific genes
on the embryo's left side. -
1:57 - 2:01These genes direct the cells
to make certain proteins, -
2:01 - 2:02and in just a few hours,
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2:02 - 2:06the right and left sides of the embryo
are chemically different. -
2:06 - 2:08Even though they still look the same,
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2:08 - 2:14these chemical differences are eventually
translated into asymmetric organs. -
2:14 - 2:18Asymmetry shows up in the heart first.
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2:18 - 2:22It begins as a straight tube
along the center of the embryo, -
2:22 - 2:24but when the embryo
is around three weeks old, -
2:24 - 2:27the tube starts to bend into a c-shape
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2:27 - 2:30and rotate towards
the right side of the body. -
2:30 - 2:33It grows different
structures on each side, -
2:33 - 2:36eventually turning into the familiar
asymmetric heart. -
2:36 - 2:41Meanwhile, the other major organs
emerge from a central tube -
2:41 - 2:44and grow towards their ultimate positions.
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2:44 - 2:48But some organisms, like pigs,
don't have those embryonic cilia -
2:48 - 2:51and still have asymmetric internal organs.
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2:51 - 2:55Could all cells be
intrinsically asymmetric? -
2:55 - 2:56Probably.
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2:56 - 3:01Bacterial colonies grow lacy branches
that all curl in the same direction, -
3:01 - 3:04and human cells cultured
inside a ring-shaped boundary -
3:04 - 3:08tend to line up
like the ridges on a cruller. -
3:08 - 3:10If we zoom in even more,
-
3:10 - 3:13we see that many
of cells' basic building blocks, -
3:13 - 3:18like nucleic acids, proteins, and sugars,
are inherently asymmetric. -
3:18 - 3:21Proteins have complex asymmetric shapes,
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3:21 - 3:24and those proteins control
which way cells migrate -
3:24 - 3:27and which way embryonic cilia twirl.
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3:27 - 3:30These biomolecules
have a property called chirality, -
3:30 - 3:35which means that a molecule
and its mirror image aren't identical. -
3:35 - 3:38Like your right and left hands,
they look the same, -
3:38 - 3:42but trying to put your right
in your left glove proves they're not. -
3:42 - 3:48This asymmetry at the molecular level
is reflected in asymmetric cells, -
3:48 - 3:49asymmetric embryos,
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3:49 - 3:52and finally asymmetric organisms.
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3:52 - 3:54So while symmetry may be beautiful,
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3:54 - 3:57asymmetry holds an allure of its own,
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3:57 - 3:59found in its graceful whirls,
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3:59 - 4:01its organized complexity,
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4:01 - 4:03and its striking imperfections.
- Title:
- Why are human bodies asymmetrical? - Leo Q. Wan
- Speaker:
- Leo Q. Wan
- Description:
-
View full lesson: http://ed.ted.com/lessons/why-are-human-bodies-asymmetrical-leo-q-wan
Symmetry is everywhere in nature. And we usually associate it with beauty: a perfectly shaped leaf or a butterfly with intricate patterns mirrored on each wing. But it turns out that asymmetry is pretty important, too — and more common than you might think. Leo Q. Wan takes us into the human body to show how biological asymmetry can be quite beautiful.
Lesson by Leo Q. Wan, animation by Echo Bridge Pictures.
- Video Language:
- English
- Team:
- closed TED
- Project:
- TED-Ed
- Duration:
- 04:19
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