[music playing] In our last video I talked about why and how life became multicellular, one of the most important transitions in our evolutionary history.

But still, one that happened dozens of times.

Today I'm going to tell you about a different event.

One that's so rare, it only happened once in the nearly 4 billion years life has been on Earth.

What I would call the most important moment in history.

If it hadn't happened-- [boom] [wind howling] We got lucky.

[scribbling] [boom] Can you pick out the human cell?

[ticking] [buzzer] [buzzer] [buzzer] [ding] Complex life looks pretty different on the outside.

But whether it's a plant, a protist, or a peacock, at the most basic level, all of Earth's complex life, eukaryotes, are built from essentially the same stuff.

Because eukaryotes share so many things, it's likely that we all have a common ancestor.

You and that oak tree that you used to climb on are cousins, just a few billion times removed.

But for most of the history of life on Earth, there was no such thing as eukaryotes.

Earth was home to only two domains of basic life-- bacteria and archaea.

Known as prokaryotes, these single-celled organisms dominated the Earth.

But in all of life's history, they've never gotten big or complicated like eukaryotes did because they couldn't muster the energy.

Now, our energy currency is ATP, like a token that gives you one play at the cellular arcade.

Now, maybe you use that token in a protein that cuts DNA, or maybe one that breaks down food.

When you run out of tokens-- game over.

But prokaryotes can only manufacture those energy tokens on their surface membrane.

And expanding their surface area to create more energy tokens would exponentially increase their volume, which then needs more energy to sustain.

Say a bacteria wanted to increase its radius by 25 times.

Then it would have 625 times as much membrane to create ATP with.

All right!

But you'd also need to manufacture 625 times more protein and membrane for those new ATP factories.

And in the meantime, the volume of your cell has increased 15,000 times, and you've got to fill that with protein machinery too, so you're going to need 15,000 genomes to keep up with all that demand.

This was a catch-22 that prokaryotes could never overcome on their own, but then something unimaginably improbable happened.

An event so rare, it only happened once in all of life's history.

Now, one day, an archaean cell bumped into a bacterium and slurped it up, and for some reason it didn't digest it.

We don't exactly know why, but for the first time, one organism was living in symbiosis not with another organism but inside it-- endosymbiosis-- and every living thing that you can see with the naked eye is descended from that synthesis.

By cooperating with its new host, this bacterium was able to ditch 99% of its genome, shut down most of its own machinery, and concentrate on one task-- making ATP.

Thus, the first mitochondria was born, and its power level was over 9,000.

[beeping] That first eukaryote was rich!

[music playing] More ATP than it knew what to do with, it was free to grow and specialize and experiment with new crazy abilities.

Later, some of them gulped up a second bacterium that had figured out photosynthesis, and they eventually became plants.

And before too long, eukaryotes morphed into everything from jellyfish to Redwood trees to pangolins and penguins.

Even us.

A single one of your cells can use 10 million ATP tokens every second.

All together, you burn through your body weight in ATP every day.

Now, considering we only have about 60 grams of ATP in our whole body at any one time, there's a lot of recycling going on, and that all happens in the mitochondria.

To recycle all the ATP we need in a day, it takes an incredible number of mitochondria, hundreds or thousands per cell, maybe a quadrillion in our whole body.

If we stretched their crinkled membranes out altogether, your body would hold four football fields worth of ATP-making real estate.

We know the mighty mito started this way because they still contain their own genome-- a circle of DNA like a prokaryote's-- and we can trace one third of our genes to either bacteria or archaea.

Eukaryotes are a genetic mashup.

In a world filled with bacteria and archaea bumping into each other every minute of every day for the past couple billion years, this eukaryotic origin story has only happened once.

You and the oak tree's common ancestor tells us so.

It's all thanks to that single moment when two branches from two divergent trunks on the tree of life joined back into one, freeing life from its energetic constraints, and giving nature the freedom to build endless forms most beautiful.

Have you ever done a science and wished that you had a t-shirt to celebrate the occasion?

Well, [snap] now you do.

This new "It's Okay To Be Smart" shirt is now available on our merch store at dftba.com if you'd like to pick one up for yourself.

You can also find the link down in the description.

And if you find yourself doing a science wearing this shirt, then post a picture on Instagram or tweet us with the hashtag ididascience.

All right.

Stay curious and stylish.