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- Hello, my name's Lindsay Turnbull

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and I teach biology at
the University of Oxford.

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In this episode,

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I want to explain how the animals leapt

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onto the Earth stage 541 million years ago

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and took the world by storm.

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(birds chirping)

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(frog croaking)

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(birds chirping)

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If we squeeze the entire
four and a half billion years

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of our planet's history
into a single calendar year,

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then this episode is really
all about the 18th of November

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because that is the day when
the animals really spring

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onto our planet in this great profusion.

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Now, all animals are made
from eukaryotic cells

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and they themselves arose
on around 7th of August,

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but they spent about a billion years

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not creating larger
beings, just living alone.

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And in fact, that period

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is sometimes called The
Boring Billion by geologists.

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But once they started to gang up

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and make multicellular beings,

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then boy, did they get going quickly.

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It's like they went into
some kind of creative frenzy,

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creating animals like this
incredible Anomalocaris,

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which terrorised marine ecosystems

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around 500 million years ago.

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Now, Anomalocaris is an animal,

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and although you may never
have seen this animal before,

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you probably don't have too
much difficulty recognising

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that it is an animal.

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So let's just stop and think
about what an animal is.

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If I asked you to name a few animals,

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what might pop into your head?

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Maybe something like this, or this, or this?

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Sure, all those things are animals,

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but actually they all belong
to the same group of animals,

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the same one that we belong to

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and zoologists recognise at least 
30 different animal groups.

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They're called phyla.

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We talked about the animal phyla,

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and a single phyla is called a phylum.

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So what about this, or this, or this?

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Well, these are also all animals,

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they just belong to different phyla.

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So what exactly is it that
makes an animal an animal?

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So what makes an animal an animal?

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Well, the first and most important thing

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is that all animals are descended

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from a single common ancestor,

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and that ancestor didn't give rise

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to any other type of organism.

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In other words, the animals
can claim this ancestor

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as exclusively theirs.

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And what did it look like?

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Well, it probably looked a bit like this.

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So it's just a type of eukaryotic
cell, rather unremarkable.

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It has a long twirling
flagellum that it spins around

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to reel in bacteria,

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which it then engulfs
rather like the amoeba,

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so it can extend little blobby
extensions of the cytoplasm

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and engulf these bacteria whole.

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Why do we think this was
the ancestor of the animals?

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Well, because today, there
are still cells around

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that look a bit like this,
and they do something odd.

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So when there are
certain kinds of bacteria

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around in the water,

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as this cell divides, the
daughter cells stay together

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and they form a small colony,

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a little like a miniature organism.

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But it's not a true animal

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because those cells can
also break apart again

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and go their own way

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and a real animal can't do that.

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But one of the simplest kind of animals

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that we know about is called a sponge.

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So there is a phylum of
animals called the sponges.

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You may not think they're
animals, but they are.

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And if we slice through the sponge,

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we see it just has these thin walls

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and the inner wall is formed from cells

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that look very much like these cells.

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So they also have these long
flagella that they twirl around

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and they pull water through the sponge

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and the sponge cells eat all
the bacteria in the water.

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Now, there are other things about sponges

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that make us sure that
they are also animals.

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All animals eat other cells,

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none of them can photosynthesize

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or do anything like that,

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so they rely on eating other cells

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to get the food they need.

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Sponges also produce egg and sperm cells.

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So when a new sponge forms,

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it's formed from the fusion
of an egg and a sperm cell,

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which then starts to divide
to form a multicellular body.

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And that multicellular body is made up

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of several different cell types.

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So in fact, we could almost say

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that all animals that have ever lived

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are little more than glorified sponges.

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Now it is a bit hard to believe
that a sponge is an animal.

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They really look so different
from all the other animals,

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and we think they're one of
the groups that evolved first.

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And there's something unusual
about the way that they feed.

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Sponges just eat individual cells.

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They can't tackle bigger prey,

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but all the other animal groups feed

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on multicellular things.

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And if you're gonna do that,
then you need a new innovation.

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You basically need a body cavity

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into which you can put that
thing that you've caught

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and then you can pour
digestive enzymes onto it

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and break it down into the molecules

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that you want to build your own body.

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Now, the next two groups
of animals to evolve

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both have that innovation.

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They're both different
kinds of jelly animal.

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The first is called the cnidarians.

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So those are corals,
jellyfish and sea anemones.

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And they are fearsome predators,

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and they have a special adaptation.

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They have a kind of cell

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that contains a little venomous harpoon

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that they can fire out to
sting their prey to death,

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and then they bring it into
the body cavity and digest it.

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The second group of jelly animals

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are very beautiful animals
called comb jellies.

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They're not as successful
today as the cnidarians.

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There aren't so many species.

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They don't fire little venomous harpoons.

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They fire little glue covered nets

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that also can trap their victims,

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but they're far too
weak to capture a human,

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so you don't need to worry
about them when you go swimming.

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But the jelly animals,

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as well as being able to
capture larger kinds of prey,

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also evolved a couple
of other key innovations

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shared by all other animals,

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and that is muscles to
allow bigger movements

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and also nerve cells to
coordinate those movements

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so they don't just wobble
like a...well, like a jelly.

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So let's look at how muscles work.

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Remember, animals are made
from eukaryotic cells,

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and one of the key features
of eukaryotic cells

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is that they have this cytoskeleton.

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These are the dynamic
filaments inside the cell,

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and muscle cells have a
very special arrangement

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of those filaments.

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They have them in parallel rows

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and two sets of filaments.

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So if you put your hands
up, stick the thumbs up,

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and have your fingers aligned like this

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to represent the two
types of filaments.

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So one hand is one kind,

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and your other hand is the other kind.

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And that your thumbs
here represent the ends

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of the muscle cell.

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Now, when the muscle cell contracts,

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it needs to get shorter,

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and that happens because
the filaments slide

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between each other.

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And if you notice, your thumbs
have moved closer together

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so the cell has contracted.

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The problem for the muscle cell

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as it's actually stuck like that now,

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so it can't pull itself back again.

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So muscles always have to come in pairs.

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So there would be a second muscle

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that would work in opposition to this one

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that would pull the filaments apart again

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and re-extend the cell.

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So we always find muscles
working in opposite pairs.

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So for example, in the
body of this little hydra,

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which is a type of sea anemone,

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you see there are muscles in the body wall

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and there are two sets of muscles.

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There are the ones that run top to bottom.

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And when those contract and get shorter,

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then the animal becomes short and squat.

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But there has to be a
second set of muscles

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or it gets stuck like that forever.

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And those go in rings around the body,

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and when those contract,

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they squeeze the body in and
it becomes long and thin again.

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And so by having these
two sets of muscles,

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this hydra can be as long and thin

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or as short and fat as it likes.

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All of the remaining animal phyla

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are what we call bilaterians.

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And that means they have
a single plane of symmetry

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that cuts down the middle of the body,

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giving them near identical
left and right sides,

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but meaning that the back
is different to the front

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and crucially, the head
is different to the tail.

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And this means that around the head,

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a whole bunch of sense
organs have evolved,

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things like eyes, ears, and whiskers

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to allow the animal to
sense the environment well

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and to move purposefully forwards.

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And they often have additional
appendages to aid movement.

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So what are some of
these bilaterian phyla?

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Well, we don't have time
to look at them all,

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but there are three important ones.

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One is a group of worms
called the annelid worms.

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That's the biggest worm phyla.

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These worms have segmented bodies,

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so an earthworm is a very familiar example

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of an annelid worm.

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And by using the segments

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and contracting the segments
in coordinated ways,

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a worm can move purposefully
through the soil.

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The problem is though

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that if you are moving across a surface,

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dragging a heavy body around
is not very efficient.

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It would be much better to sprout legs.

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And that brings us to the second,

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the most successful phyla of all time probably,

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and they're called the arthropods.

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That's things like insects,
spiders and crustaceans.

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They have a hard exoskeleton

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and they have legs, and some
of them even have wings,

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And they're hugely successful.

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The crustaceans dominate the oceans

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and the insects dominate the land.

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Insects have an extraordinary
array of mouth parts

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so they can eat just about anything.

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So there are far more
species of insect on land

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than just about any other phylum.

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The last phylum that's very important

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is called the molluscs.

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They're also a very diverse group.

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There are snails, which you're
probably quite familiar with,

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but there are also bivalves,

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things like oysters and
mussels that people eat.

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And the third type of mollusc
is called a cephalopod,

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and those are octopuses and squid.

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And humans are really
fascinated with those

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because they're really
very intelligent animals.

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In fact, the closest
thing we have on Earth

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to a true alien-intelligence.

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So let's bring back Anomalocaris,

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that predator from the marine environment

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from 500 million years ago
and have a better look at it.

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We can see that it does have
a single plane of symmetry

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running along its body.

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It's got near identical
left and right sides,

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but the back and the front are different,

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and certainly the head and
the tail end are different.

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And at the head end, we can
see powerful sense organs,

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large compound eyes,

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and these downward curving appendages

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with which it rootled
around in the sand or mud,

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hoping to kick up some prey

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that it could hoover up with that mouth.

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We can also see that it was an arthropod.

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It has a hard exoskeleton,

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a segmented body from
which appendages sprout,

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similar to other arthropods today,

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but also of course, very different.

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We don't have anything exactly like this,

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but we can see the similarities
with modern arthropods.

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And in fact, there are incredible
fossil beds from this time

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in which we can see all kinds
of modern phyla represented.

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There are cnidarians.

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There are arthropods.

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There are annelids.

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There are molluscs.

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So it seems that many modern animal phyla

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simply sprung into being

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in a very, very short
space of geological time.

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Why did that happen? How did that happen?

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Well, we don't really know.

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It's still a very
exciting area of research.

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What we do know is in
amongst all those animals,

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there were tiny little things

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that looked a little bit
like a primitive fish,

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and they came from our
phylum, the chordates.

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And over the next tens
of millions of years,

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they would super-size

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and become animals that
would dominate the oceans,

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but also go on to conquer the land.

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And we're gonna look at
those in the next episode.

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Well, I do hope you enjoyed that episode

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and you found it useful.

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And if you did, then please
consider sharing the link

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with friends and colleagues.

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There's a lot more information

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and detail in the book,
of course, about animals,

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including a lot more
about other animal phyla

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and also some speculation

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about what did cause
that incredible explosion

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of animal life 540 million years ago.

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There's a link to where you
can find the book below.

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Otherwise, join me next time as we explore

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that phyla, the chordates,
to which both you and I belong.

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(birds chirping)