How we think complex cells evolved - Adam Jacobson

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Imagine you swallowed a small bird and suddenly gained the ability to fly … or you ate a cobra and were able to spit poisonous venom! Well, throughout the history of life (and specifically during the evolution of complex eukaryotic cells) things like this happened all the time. Adam Jacobson explains endosymbiosis, a type of symbiosis in which one symbiotic organism lives inside another.

Lesson by Adam Jacobson, animation by Camilla Gunborg Pedersen.

What if you could absorb
another organism

and take on its abilities?

Imagine you swallowed a small bird
and suddenly gained the ability to fly.

Or if you engulfed a cobra

and were then able to spit poisonous venom
from your teeth.

Throughout the history of life,

specifically during the evolution
of complex eukaryotic cells,

things like this happened all the time.

One organism absorbed another,

and they united to become a new organism
with the combined abilities of both.

We think that around 2 billion years ago,

the only living organisms on Earth
were prokaryotes,

single-celled organisms
lacking membrane-bound organelles.

Let's look closely at just three of them.

One was a big, simple blob-like cell

with the ability to absorb things
by wrapping its cell membrane around them.

Another was a bacterial cell

that converted solar energy into sugar
molecules through photosynthesis.

A third used oxygen gas to break down
materials like sugar

and release its energy into a form useful
for life activities.

The blob cells would occasionally absorb
the little photosynthetic bacteria.

These bacteria then lived inside the blob
and divided like they always had,

but their existence became linked.

If you stumbled upon
this living arrangement,

you might just think that the whole thing
was one organism,

that the green photosynthetic bacteria
were just a part of the blob

that performed one of its life functions,

just like your heart is a part of you

that performs the function
of pumping your blood.

This process of cells living together
is called endosymbiosis,

one organism living inside another.

But the endosymbiosis didn't stop there.

What would happen
if the other bacteria moved in, too?

Now the cells of this species started
becoming highly complex.

They were big and full
of intricate structures

that we call chloroplasts
and mitochondria.

These structures work together
to harness sunlight,

make sugar,

and break down that sugar using the oxygen

that right around this time started
to appear in the Earth's atmosphere.

Organisms absorbing other organisms

was one way species adapted
to the changing environmental conditions

of their surroundings.

This little story highlights what
biologists call the endosymbiotic theory,

the current best explanation
of how complex cells evolved.

There's a lot of evidence
that supports this theory,

but let's look at three main pieces.

First, the chloroplasts and mitochondria
in our cells multiply the very same way

as those ancient bacteria,

which are still around, by the way.

In fact, if you destroy these structures
in a cell, no new ones will appear.

The cell can't make them.

They can only make more of themselves.

Second piece of evidence.

Chloroplasts and mitochondria both contain
their own DNA and ribosomes.

Their DNA has a circular structure

that is strikingly similar to the DNA
of the ancient bacteria,

and it also contains many similar genes.

The ribosomes, or protein assembly
machines of chloroplasts and mitochondria,

also have the same structure as ribosomes
of ancient bacteria,

but are different from the ribosomes

hanging around
the rest of eukaryotic cell.

Lastly, think about the membranes involved
in the engulfing process.

Chloroplasts and mitochondria
both have two membranes surrounding them,

an inner and outer membrane.

Their inner membrane contains
some particular lipids and proteins

that are not present
in the outer membrane.

Why is that significant?

Because their outer membrane
used to belong to the blob cell.

When they were engulfed
in the endosymbiosis process,

they got wrapped up in that membrane
and kept their own as their inner one.

Surely enough, those same lipids

and proteins are found on the membranes
of the ancient bacteria.

Biologists now use this theory

to explain the origin of the vast
variety of eukaryotic organisms.

Take the green algae that grow on
the walls of swimming pools.

A larger eukaryotic cell with spinning
tail structures, or flagella,

at some point absorbed algae like these
to form what we now call euglena.

Euglena can perform photosynthesis,

break down sugar using oxygen,

and swim around pond water.

And as the theory would predict,

the chloroplasts in these euglena
have three membranes

since they had two before being engulfed.

The absorbing process
of endosymbiotic theory

allowed organisms to combine
powerful abilities

to become better adapted to life on Earth.

The results were species
capable of much more

than when they were separate organisms,

and this was an evolutionary leap

that lead to the microorganisms, plants,

and animals we observe
on the planet today.