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- Hello, I'm Lindsay Turnbull,

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and I teach biology at
the University of Oxford.

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In this episode, I want to describe

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how one group of cells became monsters,

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combining bacterial brilliance

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with some pretty good tricks of their own.

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These cells are called eukaryotes

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and they're what you and I are made from.

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(bird chirping)
(frog croaking)

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Once the microscope was invented,

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biologists quickly realised

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that there appeared to be
two different types of cell.

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On the one hand, there were the bacteria,

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pretty difficult to see,
even with a light microscope,

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because they're very small

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and they just have simple rigid shapes,

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like rods or cylinders.

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On the other hand, there
were the eukaryotes.

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These cells were much bigger,

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much easier to see under
a light microscope,

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like these twirling paramecia,

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and inside they appeared

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to be much more complicated as well.

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So if you looked under
a higher magnification,

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you could see inside them,

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and they have this kind
of grainy appearance,

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and that's because inside eukaryotic cells

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there are internal membranes

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that make internal
structures called organelles,

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and we'll look at those a bit later.

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So that seemed to make
sense to scientists,

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two kinds of cells,
bacteria and eukaryotes.

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Then in about the 1970s,

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biologists started to study cells

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in a different kind of way,

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so they started to sequence their genomes.

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That means reading every letter

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in the book of instructions
that all organisms carry,

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and then you can compare those letters

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to find out just how similar
different organisms are.

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And when they did that for
lots of different cell types,

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they found that there seemed
to be a third type of cell,

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hiding in plain sight.

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Now scientists called this
third group of cells archaea.

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They weren't new to science,
people had found them already,

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but originally scientists thought

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that they were just
another type of bacteria

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because they also had simple rigid shapes,

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just like bacteria, and
they were similar size,

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very small cells, hard to
see under a microscope.

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But what was unusual when
looking at the genomes

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is it was very clear that they
actually were very different,

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and now we know that their
ribosomes are a bit different,

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their membranes are a bit different.

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They were called Archaea
because most of them

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at that time were known
from what are thought of

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as quite extreme environments,

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like these sulphurous
vents around a volcano,

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and so people had the idea that maybe

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these were like the original cells

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that populated the early Earth,

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which was a a very
difficult place to live.

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We don't think that anymore.

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In fact, bacteria and
archaea are equally ancient

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and they probably split
away from each other

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more than 3 billion years ago.

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But now, scientists were thinking,

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"Okay, so we have three
different cell types.

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We have bacteria, we have archaea,

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and we have the eukaryotes."

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Now we'll return to that story in a minute

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because it has yet another twist to it,

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but if we're going to understand it fully,

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we first need to look
inside a eukaryotic cell.

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So let's take a closer look
inside a eukaryotic cell.

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Now it is more complicated,

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but we can see things
that are very familiar.

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All cells have to have a genome

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and eukaryotic cells are no exception.

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What's unusual, though,

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is that the eukaryotic cell keeps

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its genome enclosed in a membrane.

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So when you look at

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a eukaryotic cell underneath a microscope,

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you can often see this
sort of darkish blob

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that's called the nucleus,
and that's what that is,

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it's the genome encased in a membrane.

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Now, of course, the genome's no use

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if messages can't fly out of it,

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so that membrane has to
have little holes in it

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and those are called pores,

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so you've got the nuclear pores

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that the messages fly out of.

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The rest of the cell is
often called the cytoplasm

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and that includes other organelles,

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which are membrane-bound compartments,

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and also the sort of liquid,

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and floating around in the liquid part,

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there are of course the ribosomes,

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all cells have to have those,

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and they trap messages

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and they make the tools and the machinery

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and the spare parts that the cell needs,

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exactly the same as what's
happening in the bacterial cell,

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but there are some structures
that are unique to eukaryotes.

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So next to the nucleus

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there's this sort of labyrinthine
arrangement of membranes.

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It looks a bit like a maze
and it's studded with dots

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and those dots are actually ribosomes.

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And then close to that,

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you've got a series of long
sausage-shaped compartments.

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So the first thing is
the endoplasmic reticulum

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and the second thing
is the Golgi apparatus,

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and those things work together.

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The ribosomes feed in some of the proteins

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that they're making into
the endoplasmic reticulum

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and the proteins are finished there.

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Some proteins that eukaryotic
cells make are very large

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and it's difficult to get them
to fold into the right shape,

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so that takes place inside the warehouse,

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and they also distribute these proteins

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to different parts of the cell.

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Some of them are targeted
to different destinations

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and they're carried in tiny
little membrane-bound vesicles.

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Now there are other organelles in the cell

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and one of the prominent
types of organelle

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is also a bit like a sort
of sausage-shaped thing,

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and they're called mitochondria,

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and those are the powerhouses
of the eukaryotic cell.

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They have two membranes,
an outer and an inner one,

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and the inner one is covered
in these little turbines

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that recharge the ATP
batteries that the cell needs.

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In a bacterial cell,

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those are in the bacterial outer membrane.

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So we have little structures
called mitochondria

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that are recharging ATP batteries.

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They're about the same size
and shape as a bacterium

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and inside a mitochondria, it turns out,

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it also has its own small genome

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and it also has a few
ribosomes of its own.

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So this is really quite a strange thing

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to have inside another cell.

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Well, one day, Lynn Margulis,
a biological scientist,

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was peering at some cells and
she had a bit of a brainwave.

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She thought maybe those mitochondria

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were bacteria once upon a time.

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In other words, a eukaryotic
cell at some time in the past

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has engulfed a bacterial cell

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and then kind of
domesticated it and kept it,

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and that's why it's about the same size

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and shape as a bacteria,

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that's why it has its own genome,

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that's why it has its own ribosomes.

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There wasn't really any other
explanation that made sense.

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But then, hang on a minute,

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what kind of cell did the engulfing?

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In other words, what kind
of cell was the ancestor

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of the eukaryotic cell?

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Well, once again, genome
sequencing came to our rescue

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and it turns out that the
ancestor of the eukaryotic cell

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was an archaeal cell,

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and this meant that really we were back

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to only having two types of cell.

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There were bacterial cells
and there were archaeal cells,

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and eukaryotes are just a
special type of archaeal cell,

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one that has engulfed a bacterium.

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So eukaryotes are this
kind of unlikely mashup

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between bacteria and archaea.

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But it still leaves us with a problem,

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because we just have to understand

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how this event took place.

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How is it that a simple rigid
cell with a rigid shape,

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i.e. an archaeal cell,

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was somehow able to engulf

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a bacterium millions of years ago?

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Well, to understand this
problem a bit better,

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let's have a look at

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this enormous eukaryotic
cell called amoeba,

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and we can see it moving around.

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Now no bacterial or archaeal
cell that we know about

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can move like this, they just
have simple rigid shapes,

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and that's because they
have an outer cell wall,

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but most eukaryotic cells ditch that wall,

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and instead they're supported
by an internal spider web

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of filaments called the cytoskeleton,

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and this allows them to do
this kind of dynamic crawling.

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And an amoeba can engulf
other cells, no problem.

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It can extend two of
those kind of blobby arms

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and engulf it.

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But the problem is,

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the ancestor of the eukaryotic cell

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can't have looked like this,
this is a eukaryotic cell,

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so how could it have engulfed a bacteria?

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Well, the answer to this problem lay

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in a rather surprising place.

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Once scientists realised

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that archaeal cells were our ancestors,

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they became a lot more interested in them

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and they started looking harder for them,

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and one of the places they looked

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were these hydrothermal vents
at the bottom of the ocean,

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and sure enough, there
were archaeal cells there,

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but they were a different
kind of archaeal cell

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that nobody knew anything about.

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Unfortunately, it turned out

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that they were very difficult to culture,

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so scientists couldn't see them properly,

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But a Japanese team worked
really hard at that, and in 2020,

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they showed some pictures
that shocked the world.

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They showed an archaeal
cell that looked like this,

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with extendable spaghetti-like arms.

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Now we don't know whether that cell

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can actually capture a bacteria,

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but it looks like at least

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there's a possibility that it might,

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and so this theory of Lynn Margulis

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that started off as frankly
seeming slightly crackpot,

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is now mainstream science.

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Just about everybody believes

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that this is how eukaryotic
cells got started,

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an ancient archaeal
cell engulfed a bacteria

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and put it to work.

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So one of the most surprising
things about eukaryotic cells

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is that they're able to gang up

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to form multicellular
organisms like you and me.

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How did that start?

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Well, that's a complicated business,

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but we can see

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a very simple multicellular
organism right here.

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This is just made from
a collection of amoebae,

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and it's called a slime mould.

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Amoebae normally live alone
in soil, hunting cells,

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but if food becomes hard to find,

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they start to collect together

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and coalesce to form these structures,

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and people are quite fascinated by them

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as they are pretty amazing.

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But to form a proper multicellular
being, like you or I,

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which is a permanent collection
of cells working together,

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you have to form in a different way.

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You can't just get
together with your mates.

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You have to start as a single cell,

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and you and I are made
from 37 trillion cells,

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but every single one of those
is a product of cell division.

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We all started our lives as a single cell

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and that's what you need
to get the cooperation

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that you must have to get

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a multicellular being working properly.

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Now one really major group
of multicellular beings

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is the animals,

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and we're gonna find out more about them

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in the next episode.

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Well, I hope you enjoyed that
episode and found it useful,

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and if you did,

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do please share the link
with friends and colleagues.

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There's a lot more information
about eukaryotic cells

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in my book, "Biology: The Whole Story,"

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in particular there's a lot
more about eukaryotic genomes

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and how different they are
from bacterial genomes,

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there's some fun stories there.

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Also, if you wanna buy the
book, there is a link below.

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Otherwise, join me for the next episode,

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which is covering chapter 7,
which is all about the animals.

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(bird chirping)