4.3 Eukaryotic Evolution and the Phylogeny of All Life - The Transition from the Prokaryotic to the Eukaryotic Cell and the Endosymbiont Theory - Martin V. Sørensen

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From the course by University of Copenhagen

Origins - Formation of the Universe, Solar System, Earth and Life

362 ratings

University of Copenhagen

Origins - Formation of the Universe, Solar System, Earth and Life

362 ratings

The Origins course tracks the origin of all things – from the Big Bang to the origin of the Solar System and the Earth. The course follows the evolution of life on our planet through deep geological time to present life forms.

From the lesson

Transition from Microbial to Macrobial Life: Snowball Earth and the Ediacara Biota / Eukaryotic Evolution and the Phylogeny of All Life

In this module, we take a closer look at how the physical and biological conditions that made the Cambrian Explosion possible arose. In the first lectures Svend Stouge will tell you about the dramatic consequences of climate changes seen toward the end of the Precambrian. Geological evidence supports the idea that the Earth was completely covered in ice during periods that we, for obvious reasons, refer to as Snowball Earth. In the remaining lectures Martin Sørensen will tell you about one of the most significant building blocks of life on Earth – the cell – and how the early bacterial cells evolved and became capable of forming the huge variety of life that we see today. Martin Sørensen will also show how different evolutionary trends of cells resulted in six major organism groups, of which several gave rise to multicellular life.

4.1 Transition from microbial to macrobial life: Snowball Earth and the Ediacara Biota - Part 1 - Svend Stouge19:43

4.2 Transition from microbial to macrobial life: Snowball Earth and the Ediacara Biota - Part 2 - Svend Stouge13:47

4.3 Eukaryotic Evolution and the Phylogeny of All Life - The Transition from the Prokaryotic to the Eukaryotic Cell and the Endosymbiont Theory - Martin V. Sørensen11:48

4.4 Eukaryotic Evolution and the Phylogeny of All Life - The Eukaryotic Tree of Life and the Six Super Kingdoms: Archaeplastida - Martin V. Sørensen7:26

4.5 Eukaryotic Evolution and the Phylogeny of All Life - The Six Super Kingdoms: Excavata and Rhizaria - Martin V. Sørensen7:03

4.6 Eukaryotic Evolution and the Phylogeny of All Life - The Six Super Kingdoms: Chromalveolata - Martin V. Sørensen5:56

4.7 Eukaryotic Evolution and the Phylogeny of All Life - The Six Super Kingdoms: Amoebozoa and Opisthokonta - Martin V. Sørensen 7:06

Meet the Instructors

Henning Haack
Associate Professor
Natural History Museum
James Connelly
Professor
Natural History Museum of Denmark
Vivi Vajda
Professor
Department of Geology, Lund University
Jon Fjeldså
Professor, Curator
Thomas Gilbert
Professor
Michael Houmark-Nielsen
Associate Professor
Natural History Museum of Denmark
Bent Erik Kramer Lindow
Curator
Natural History Museum of Denmark
Gilles Guy Roger Cuny
Associate Professor
Natural History Museum of Denmark
Ole Seberg
Associate Professor
Natural History Museum of Denmark
Gitte Petersen
Associate Professor
Natural History Museum of Denmark
Tais Wittchen Dahl
Assistant Professor of Geobiology
Arne Thorshøj Nielsen
Associate Professor
Natural History Museum of Copenhagen
Martin Vinther Sørensen
Associate Professor, Curator
Natural History Museum of Denmark
Svend Stouge
Associate Professor
Natural History Museum of Denmark
Danny Eibye Jacobsen
Associate Professor, Curator
Natural History Museum of Denmark
Jan Audun Rasmussen
Associate Professor
Natural History Museum of Denmark
Emily Catherine Pope
Assistant professor
Natural History Museum of Denmark

[MUSIC]

My name is Martin Sorensen.

I'm a zoologist and

I'm a specialist in microscopic organisms and in evolutionary biology.

Today we're going to talk about one of the most significant events in

the evolution of life.

Namely, the transition from prokaryotic life to eukaryotic life, or

I could say the transition from bacterial life to all the varieties of

life that we see around us today.

Subsequently we'll talk about the major lineages of the eukaryotic organisms and

see how we today are able to group life into six super kingdoms.

In previous lectures, you've heard about how life began and

how it developed in the earliest bacterial life.

You also heard how all bacteria belong to the prokaryotes,

which are rather simple cells that basically are characterized by

cell membranes with some free-floating DNA inside.

A simple design doesn't mean that it's a bad design, though.

Today, we can basically find bacterial life everywhere, so

it's hard to claim that a prokaryotic design was not successful.

But it has its limitations though.

Because the prokaryotic cells are so simple as they are,

it is unable to differentiate and form more complex organisms.

Also interaction with other cells, and

hence becoming a multicellular organism, is not an option for the prokaryotic cell.

For this, we need a much more complex design and

that's what we are going to find in the eukaryotic cell.

Let's just recap and

take a look at the prokaryotic cell that you already learned about.

What we see here is a rather simple cell design.

We have the plasma membrane with the capsule on the outside and

eventually a flagellum also.

Inside we find the cytoplasm and free-floating in the cytoplasm,

we have the ribosomes for protein synthesis and the DNA.

If we compare this with the eukaryotic cell,

we'll realize that this type of cell is much more complex.

We can still find the DNA, but

in the eukaryotic cell it is contained in a nucleus.

And together with the nucleus, we see several other units inside the cell.

Together these units are called organelles.

For instance, we see the Golgi apparatus, where proteins can be stored, and

we see mitochondria that are responsible for the aerobic metabolism.

We also see all these folded membranes here

that form an almost labyrinth-like mesh.

That's a transport system called the endoplasmatic reticulum, or

simply the ER, which ensures that different molecules like RNA or

amino acids can be transported inside the cell.

If you look closer at the endoplasmatic reticulum, you'll see these little dots.

That's the ribosomes.

They receive copies of DNA from the nucleus, the copies come in shape of RNA,

and use this as a template to synthesize proteins.

You can also take a look at another kind of eukaryotic cell.

We see several similarities between the two cells, such as the nuclei,

and the mitochondria, but we also find differences.

For instance, we have these green organelles here.

They are called chloroplasts, and they are used to make photosynthesis or,

in other words, to turn sunlight into energy.

So, this also tells us that the eukaryotic cell to the left would be from

the plant or an algae.

Whereas the one to the right, for instance, could be from an animal.

So how did this transition from prokaryotic to eukaryotic cell take place?

Actually, we have a pretty good idea about this and the transition can be

explained by membrane movements and by the endosymbiont theory.

At the top of this figure, we see a prokaryotic cell.

There's basically only the membrane and the free-floating DNA inside it.

What we imagine, then, is that this cell membrane could make some infoldings.

This would, for instance, make sense if the cell wanted to

increase the ratio between its surface area relative to its volume.

We can then imagine also that these infoldings from the outer membrane

can get pinched off so that we in this way get membranes inside the cell.

These membranes could, for instance, fold and make up the system of tubes and

in this way we could have the endoplasmatic reticulum,

this network I talked about that eukaryotic cells use for

intracellular transport.

Or the membranes could wrap around the DNA and

in this way we would have formed a nucleus.

In this way we would have a very simple eukaryotic cell.

This process didn't happen overnight of course.

The membrane foldings and

movements probably developed through numerous generations of cells.

But at some point, this new design was finally established.

But it doesn't stop here.

After the formation of the nucleus and the endoplasmatic reticulum, it seems like

some other prokaryotic organisms has entered our newborn eukaryotic cell.

It could be as prey for

instance, because the eukaryotic cell simply wanted to eat the other prokaryote.

But it could also be because this prokaryotic cell was a parasite or

a commensal that wanted to take advantage of living inside another cell.

However, instead of getting digested or instead of harming the host cell,

the two cells started to live together in a mutual symbiosis,

which is an intimate relationship where both organisms benefit from it.

And then again, over many generations, of course,

this symbiosis became more and more intimate until the endosymbiotic

prokaryote no longer could be considered an independent organism.

Instead, it had become a part of the eukaryotic cell and

hence a cell organelle.

Now, we know that some prokaryotes actually went through this process.

And some of them resemble the so-called proteobacteria.

Proteobacteria are aerobic bacteria,

which means that they use oxygen when they produce energy.

And in an oxygen-rich environment,

such endosymbiotic bacteria would give the host cell a great advantage.

If we look at the mitochondria in eukaryotic cells,

we see that they have much in common with these proteobacteria.

They are structurally, very similar.

And mitochondria are, in fact, the organelles that are responsible for

our cell metabolism, because they use oxygen to produce energy.

Based on this,

we have pretty good evidence supporting that the mitochondria which are found in

most eukaryotic cells originated from proteobacteria that started out

as endosymbionts and subsequently became integrated cell organelles.

And if we take a look at another quite important organelle,

namely the chloroplasts that we find in photoautotrophic organisms,

those that makes the photosynthesis, we see the same story repeated.

A photoautotrophic, prokaryotic cell,

very similar to a cyanobacteria, started out as an endosymbiont and

was then gradually integrated into the cell and became a chloroplast organelle.

This may, of course, sound like some freaked out theory, but as I said,

the similarity between, on one hand, proteobacteria and mitochondria, and

on the other, cyanobacteria and chloroplasts is striking.

And, we even have more evidence.

I said previously that all DNA in the eukaryotic cell was contained in

the nucleus, but it would be more correct to say almost all DNA.

Actually, we find a little DNA in two other organelles,

which are the mitochondria and the chloroplasts.

And if you sequence this DNA and compare it with DNA from other organisms,

I guess you can figure out where we find the greatest resemblance.

Mitochondrial DNA is very close to proteobacterial DNA,

whereas the chloroplast DNA resembles the DNA of the cyanobacteria.

So in other words, we have pretty good evidence supporting that cell

evolution through endosymbiosis actually took place.

And in fact, we can even show that it wasn't just a single event.

It has happened many times.

Here on this graphic, we see our eukaryotic cell again.

We can see that a cyanobacterium has been engulfed and that it,

over time, has turned into a chloroplast.

If we look closely at it, we can also see that the chloroplast actually has two

membranes, the original membrane from the cyanobacterium and

the membrane from the prokaryotic host cell that originally engulfed it.

However, sometimes we find chloroplasts with three or even four membranes.

Now, how can this happen?

Well, the answer is simple.

It's not only prokaryotes that may become endosymbionts and

subsequently turn into organelles.

The same may happen with a eukaryotic cell.

In the picture to the left,

you see a eukaryotic cell that is about to be engulfed in another eukaryotic cell.

So this eukaryotic cell has already been through one

event of endosymbiotic evolution and now it happens a second time.

On the next picture,

you see how the eukaryotic cell has become an endosymbiont.

And now we can identify four membranes around the chloroplast: the two

original ones, the outer membrane of the eukaryotic cell, and

the membrane from the prokaryotic cell or eukaryotic cell that engulfed it.

When this endosymbiotic eukaryote becomes an organelle,

the third membrane will often disappear, together with its nucleus.

But what we are left with is a chloroplast with three membranes, instead of two.

This kind of chloroplast has been through two events of endosymbiotic evolution.

And accordingly, we call it secondary endosymbiosis.

And it doesn't even end here.

Even a eukaryotic cell that evolved through secondary endosymbiosis may

become an endosymbiont again if a new cell, takes it in.

And again, it may become integrated in the cell and turn into an organelle.

We call this tertiary endosymbiosis, and

we typically find no less than four membranes around the chloroplast then.

This shows that evolution through endosymbiosis happened several times and

as we'll see in a moment, this may also helps us to understand how the tree of

life, looks, and how the first eukaryotic groups evolved.

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