What if I told you that, more than two billion years ago, some tiny living thing started to live inside another living thing ... and never left?

And now, the descendants of both of those things are in you?

Well, brace yourselves, because I gotta tell you about one of the biggest breakthroughs in the history of life.

It began as a chance encounter between two microorganisms, but it ended up making all complex life on Earth possible.

And the evidence we have of this momentous event isn't some fossil found in rock.

It's found in our very own cells.

It may not surprise you to hear that all living things on Earth are made of ... cells - microscopic structures whose innards are enclosed by a membrane.

And scientists classify all of life into two main groups, depending on what kind of cells it has.

You can either be a prokaryote or a eukaryote.

Prokaryotes were the first kinds of living things to form on Earth.

They first appeared perhaps as much as 4 billion years ago, and they're still around today, in abundance.

They're single-celled organisms like bacteria and archaea.

And their cells are generally very simple - small, with ring-shaped DNA that floats freely around the cell.

But you, my friend, are a eukaryote.

Your cells are bigger, and your DNA is kept inside a nucleus, which is wrapped up in its own membrane.

You're also more physically complex than a prokaryote.

I mean, I haven't met you, but I'm assuming you are.

And all complex forms of life on our planet - - plants, animals, fungi, and anything else that you've ever seen, either with your naked eye or a cheap microscope -- are eukaryotes.

And they form a single lineage on the Tree of Life that arose more than two billion years ago, toward the start of the Proterozoic Eon.

Needless to say, the appearance of this lineage completely revolutionized the nature of life on Earth.

For the first time, life could become larger than a single cell.

This made possible the enormous diversity of living things that we see today: you, me, yeast, bumblebees, blue whales, all of it.

So, understandably, scientists have puzzled for hundreds of years over what made this revolution possible.

And around the middle of the 20th century, an explanation started to come together that remains the prevailing theory to this day.

It hypothesizes that all eukaryotes are the result of an ancient, random act of endosymbiosis.

The thinking goes, in the incredibly distant past, the thinking goes, some bacteria started living within another larger bacterium.

This relationship became symbiotic, where each organism benefited equally from the presence of the other.

Over time, the "guests" and the "host" in this arrangement became inseparable.

They became one living thing.

And the evidence to support this theory can be found in your own cells.

Because you're a eukaryote, you have stuff inside your cells that prokaryotes don't have.

These structures do specific things, just like your organs do, so they're called organelles - - little organs.

So cute.

And there are two kinds of organelles that are unlike any others.

First, eukaryotes almost always have mitochondria in their cells.

You probably remember these from biology class or the internet as "the powerhouses of the cell" because they create an energy-storing molecule called adenosine triphosphate, or ATP.

The second kind are in plant cells, the chloroplasts, which use chlorophyll to convert sunlight into sugar.

What's weird about these two organelles - - mitochondria and chloroplasts -- is that, in a lot of ways, they resemble living things.

Specifically, bacteria.

For example, early in the 20th century, biologists discovered that new mitochondria can only form when existing mitochondria split into two.

And the same is true for chloroplasts.

So, cells can't make new ones.

The organelles themselves have to, basically, reproduce.

Plus, these organelles are bound in membranes, like bacteria are.

And some researchers noticed that mitochondria and chloroplasts just look a lot like bacteria.

Specifically, mitochondria closely resemble infectious bacteria called Rickettsia in their size and shape.

Likewise, chloroplasts kind of resemble cyanobacteria.

They both use photosynthesis, and like the bacteria, chloroplasts have an internal membrane, which they use to hold their chlorophyll.

All these similarities were pointed out by early supporters of endosymbiosis more than a hundred years ago, but most scientists remained unconvinced.

The theory was revitalized in the '60s and '70s by American evolutionary biologist Lynn Margulis and by the 1980s, most of the scientific community had come around because there's one more strikingly weird thing about these organelles.

Genetic research showed both mitochondria and chloroplasts contain their own DNA that's completely separate from the DNA in the nucleus of the cell itself!

Then in the late 1990s, scientists examined the genomes of mitochondria and chloroplasts, founding that mitochondrial genes are very similar to the genes in bacteria, like Rickettsia.

And chloroplast genes turned out to be most like those in cyanobacteria.

So, this combination of anatomical and genetic similarity really seems to have no other reasonable explanation: Mitochondria and chloroplasts must be the descendents of formerly free-living bacteria that somehow got ... stuck ... in our cells.

But, ... how did they get there?

Well, no one's sure.

It may have been that, 2 billion years ago, a host cell was trying to eat a guest cell, but the eaten cell stayed in tact.

Or maybe the guests took up residence as parasites.

Or it could have been some sort of symbiosis that benefitted both hosts and guests from the start; like the gut bacteria living inside all of us.

We just don't know.

What we do know is that the longer this relationship lasted, the more dependent on each other.

the guests and hosts became.

Over time, the ancestors of mitochondria stopped behaving like independent organisms and became ATP factories.

Likewise, chloroplasts became little, solar-powered sugar factories.

They were no longer separate living things.

They were part of the cell.

OK, so two microbes that bumped into each other managed to change the course of history.

That's weird enough.

But what's even more amazing is that this seems to have happened at least three times in Earth's history!

The first time it happened was when some prokaryote got stuck in another cell and eventually became mitochondria, marking the origin of the eukaryotes.

This probably happened at least 2.4 billion years ago, because the oldest known fossils of eukaryotes -- masses of fungal cells found in South Africa in 2017 -- are that old.

But then this phenomenon happened a second time, when cyanobacteria set up shop inside some eurkaryotic cell, and eventually became chloroplasts, which in turn made plant life possible.

This happened at least 1.6 billion years ago - - the age of the earliest known photosynthetic, plant-like things: red algae.

But maybe the most strange occurrence happened only recently -- by which I mean some 450 million years ago -- when yet another lineage of single-celled eukaryotes swallowed up cells of red algae.

This event marked the origin of the group that includes brown algae, like kelp.

And because of it, kelp cells contain the ancestral remains of a red algae cell, with chloroplasts inside of that!

So, endosymbiosis -- and the origin of eukaryotes that it caused -- is a big deal in the story of life on Earth.

Now don't get me wrong!

Prokaryotes are awesome!

They're still the most numerous and diverse organisms on Earth, and they've managed to exploit just about every environment on the planet.

But only eukaryotes have special organelles that provide them with energy.

And these little engines have allowed us to grow bigger and more complex, forming the visible, multicellular life that we know today.

Usually, we look to the fossil record to understand the story of our origins.

But maybe the most important story of all is fossilized within our own cells, and the cells of everyone you've ever met, and every living thing you've ever seen.