- Hello, I'm Lindsay Turnbull,
and I teach biology at the University of Oxford.
In this episode, I want to describe
how one group of cells became monsters,
combining bacterial brilliance
with some pretty good tricks of their own.
These cells are called eukaryotes
and they're what you and I are made from.
(bird chirping) (frog croaking)
Once the microscope was invented,
biologists quickly realised
that there appeared to be two different types of cell.
On the one hand, there were the bacteria,
pretty difficult to see, even with a light microscope,
because they're very small
and they just have simple rigid shapes,
like rods or cylinders.
On the other hand, there were the eukaryotes.
These cells were much bigger,
much easier to see under a light microscope,
like these twirling paramecia,
and inside they appeared
to be much more complicated as well.
So if you looked under a higher magnification,
you could see inside them,
and they have this kind of grainy appearance,
and that's because inside eukaryotic cells
there are internal membranes
that make internal structures called organelles,
and we'll look at those a bit later.
So that seemed to make sense to scientists,
two kinds of cells, bacteria and eukaryotes.
Then in about the 1970s,
biologists started to study cells
in a different kind of way,
so they started to sequence their genomes.
That means reading every letter
in the book of instructions that all organisms carry,
and then you can compare those letters
to find out just how similar different organisms are.
And when they did that for lots of different cell types,
they found that there seemed to be a third type of cell,
hiding in plain sight.
Now scientists called this third group of cells archaea.
They weren't new to science, people had found them already,
but originally scientists thought
that they were just another type of bacteria
because they also had simple rigid shapes,
just like bacteria, and they were similar size,
very small cells, hard to see under a microscope.
But what was unusual when looking at the genomes
is it was very clear that they actually were very different,
and now we know that their ribosomes are a bit different,
their membranes are a bit different.
They were called Archaea because most of them
at that time were known from what are thought of
as quite extreme environments,
like these sulphurous vents around a volcano,
and so people had the idea that maybe
these were like the original cells
that populated the early Earth,
which was a a very difficult place to live.
We don't think that anymore.
In fact, bacteria and archaea are equally ancient
and they probably split away from each other
more than 3 billion years ago.
But now, scientists were thinking,
"Okay, so we have three different cell types.
We have bacteria, we have archaea,
and we have the eukaryotes."
Now we'll return to that story in a minute
because it has yet another twist to it,
but if we're going to understand it fully,
we first need to look inside a eukaryotic cell.
So let's take a closer look inside a eukaryotic cell.
Now it is more complicated,
but we can see things that are very familiar.
All cells have to have a genome
and eukaryotic cells are no exception.
What's unusual, though,
is that the eukaryotic cell keeps
its genome enclosed in a membrane.
So when you look at
a eukaryotic cell underneath a microscope,
you can often see this sort of darkish blob
that's called the nucleus, and that's what that is,
it's the genome encased in a membrane.
Now, of course, the genome's no use
if messages can't fly out of it,
so that membrane has to have little holes in it
and those are called pores,
so you've got the nuclear pores
that the messages fly out of.
The rest of the cell is often called the cytoplasm
and that includes other organelles,
which are membrane-bound compartments,
and also the sort of liquid,
and floating around in the liquid part,
there are of course the ribosomes,
all cells have to have those,
and they trap messages
and they make the tools and the machinery
and the spare parts that the cell needs,
exactly the same as what's happening in the bacterial cell,
but there are some structures that are unique to eukaryotes.
So next to the nucleus
there's this sort of labyrinthine arrangement of membranes.
It looks a bit like a maze and it's studded with dots
and those dots are actually ribosomes.
And then close to that,
you've got a series of long sausage-shaped compartments.
So the first thing is the endoplasmic reticulum
and the second thing is the Golgi apparatus,
and those things work together.
The ribosomes feed in some of the proteins
that they're making into the endoplasmic reticulum
and the proteins are finished there.
Some proteins that eukaryotic cells make are very large
and it's difficult to get them to fold into the right shape,
so that takes place inside the warehouse,
and they also distribute these proteins
to different parts of the cell.
Some of them are targeted to different destinations
and they're carried in tiny little membrane-bound vesicles.
Now there are other organelles in the cell
and one of the prominent types of organelle
is also a bit like a sort of sausage-shaped thing,
and they're called mitochondria,
and those are the powerhouses of the eukaryotic cell.
They have two membranes, an outer and an inner one,
and the inner one is covered in these little turbines
that recharge the ATP batteries that the cell needs.
In a bacterial cell,
those are in the bacterial outer membrane.
So we have little structures called mitochondria
that are recharging ATP batteries.
They're about the same size and shape as a bacterium
and inside a mitochondria, it turns out,
it also has its own small genome
and it also has a few ribosomes of its own.
So this is really quite a strange thing
to have inside another cell.
Well, one day, Lynn Margulis, a biological scientist,
was peering at some cells and she had a bit of a brainwave.
She thought maybe those mitochondria
were bacteria once upon a time.
In other words, a eukaryotic cell at some time in the past
has engulfed a bacterial cell
and then kind of domesticated it and kept it,
and that's why it's about the same size
and shape as a bacteria,
that's why it has its own genome,
that's why it has its own ribosomes.
There wasn't really any other explanation that made sense.
But then, hang on a minute,
what kind of cell did the engulfing?
In other words, what kind of cell was the ancestor
of the eukaryotic cell?
Well, once again, genome sequencing came to our rescue
and it turns out that the ancestor of the eukaryotic cell
was an archaeal cell,
and this meant that really we were back
to only having two types of cell.
There were bacterial cells and there were archaeal cells,
and eukaryotes are just a special type of archaeal cell,
one that has engulfed a bacterium.
So eukaryotes are this kind of unlikely mashup
between bacteria and archaea.
But it still leaves us with a problem,
because we just have to understand
how this event took place.
How is it that a simple rigid cell with a rigid shape,
i.e. an archaeal cell,
was somehow able to engulf
a bacterium millions of years ago?
Well, to understand this problem a bit better,
let's have a look at
this enormous eukaryotic cell called amoeba,
and we can see it moving around.
Now no bacterial or archaeal cell that we know about
can move like this, they just have simple rigid shapes,
and that's because they have an outer cell wall,
but most eukaryotic cells ditch that wall,
and instead they're supported by an internal spider web
of filaments called the cytoskeleton,
and this allows them to do this kind of dynamic crawling.
And an amoeba can engulf other cells, no problem.
It can extend two of those kind of blobby arms
and engulf it.
But the problem is,
the ancestor of the eukaryotic cell
can't have looked like this, this is a eukaryotic cell,
so how could it have engulfed a bacteria?
Well, the answer to this problem lay
in a rather surprising place.
Once scientists realised
that archaeal cells were our ancestors,
they became a lot more interested in them
and they started looking harder for them,
and one of the places they looked
were these hydrothermal vents at the bottom of the ocean,
and sure enough, there were archaeal cells there,
but they were a different kind of archaeal cell
that nobody knew anything about.
Unfortunately, it turned out
that they were very difficult to culture,
so scientists couldn't see them properly,
But a Japanese team worked really hard at that, and in 2020,
they showed some pictures that shocked the world.
They showed an archaeal cell that looked like this,
with extendable spaghetti-like arms.
Now we don't know whether that cell
can actually capture a bacteria,
but it looks like at least
there's a possibility that it might,
and so this theory of Lynn Margulis
that started off as frankly seeming slightly crackpot,
is now mainstream science.
Just about everybody believes
that this is how eukaryotic cells got started,
an ancient archaeal cell engulfed a bacteria
and put it to work.
So one of the most surprising things about eukaryotic cells
is that they're able to gang up
to form multicellular organisms like you and me.
How did that start?
Well, that's a complicated business,
but we can see
a very simple multicellular organism right here.
This is just made from a collection of amoebae,
and it's called a slime mould.
Amoebae normally live alone in soil, hunting cells,
but if food becomes hard to find,
they start to collect together
and coalesce to form these structures,
and people are quite fascinated by them
as they are pretty amazing.
But to form a proper multicellular being, like you or I,
which is a permanent collection of cells working together,
you have to form in a different way.
You can't just get together with your mates.
You have to start as a single cell,
and you and I are made from 37 trillion cells,
but every single one of those is a product of cell division.
We all started our lives as a single cell
and that's what you need to get the cooperation
that you must have to get
a multicellular being working properly.
Now one really major group of multicellular beings
is the animals,
and we're gonna find out more about them
in the next episode.
Well, I hope you enjoyed that episode and found it useful,
and if you did,
do please share the link with friends and colleagues.
There's a lot more information about eukaryotic cells
in my book, "Biology: The Whole Story,"
in particular there's a lot more about eukaryotic genomes
and how different they are from bacterial genomes,
there's some fun stories there.
Also, if you wanna buy the book, there is a link below.
Otherwise, join me for the next episode,
which is covering chapter 7, which is all about the animals.
(bird chirping)