- Hey, Smart People.

Joe here.

There's been life on this planet for at least 3.7 billion years.

But for most of that time, life was really, really, really boring.

It was all just simple little cells squirming around in water.

No cute furry things.

No weird bugs.

No trees.

(alarm buzzes) Just microscopic blobs blobbing.

And then, around half a billion years ago, after about 3 billion years of boring, something amazing happened.

Life exploded into all kinds of interestingness.

Worms.

Trilobites.

A five-eyed sea-creature called Opabinia.

And countless other complex lifeforms.

I mean, out of nowhere, life on Earth began a new chapter.

It's hard to put time scales like these into perspective.

If the history of life were a single day, and life began at midnight, life didn't get interesting until around 10:30 PM.

And that explosion of interestingness took place in less than an hour on our history-of-life clock.

But why didn't life just stay boring?

What changed?

When we typically think of natural selection, the process by which organisms evolve, change, and adapt, we think of this gradual thing that happens over huge time periods.

But that's not what happened here.

The ancestors of all life's complex creatures showed up, the variety of life on Earth exploded like that.

And that's because, right before that explosion, a new kind of chemistry entered the equation.

A chemistry that, in excess, could spell certain death, but in the right doses was about to set the stage for life as we know it today.

Understanding why this happened, and what this new chemistry enabled, might even tell us something important about what life might look like elsewhere.

(gentle music) When life first showed up on Earth, the air was most likely made of nitrogen, plus some CO2, water vapor, and small amounts of other gases.

There was hardly any oxygen.

So if you went back in time 3.7 billion years, you'd immediately suffocate and die.

So early life was anaerobic: It didn't breathe oxygen.

Instead, it got all its energy from minerals dissolved into the ocean.

Cells just sat there while seawater passed through their membranes, absorbing dissolved compounds.

Those compounds interacted with other molecules in the cells and set off chain reactions.

Electrons hopping from one molecule to the other, molecules joining together or split apart, and in the end, creating a famous molecule known as ATP.

ATP exists in every single cell in every single living thing, it's life's universal way of storing energy cells need to function.

But this lazy anaerobic way to get energy only produced enough ATP to take care of basic housekeeping.

Early boring lifeforms didn't have spare energy they could use to grow bigger.

Now, since these early cells were basically nutrient traps, it might seem like they should work like fishing nets, the bigger the net, the bigger the catch.

But for cells, it actually doesn't work out that way.

As cells get bigger, it's true that they have more surface area to let in nutrients, but they also have much, much more volume.

And that volume grows faster than surface area.

So a bigger single cell can't catch enough stuff to feed all its volume.

This meant that early anaerobic lifeforms were pretty much doomed to stay small and boring.

Even today, organisms that don't use oxygen to make energy are almost all just single, microscopic cells.

But, around 2.4 billion years ago, something happened that completely changed the course of life.

Some of these simple cells learned a new trick.

A whole new way of making ATP, and lots of it.

They started doing photosynthesis.

Eating light and air to power your cells is pretty cool.

And it creates a waste product, oxygen gas.

Over the next few hundred million years, early photosynthetic cells just churned this stuff out.

Oxygen levels went from almost nothing to about 10% the amount of oxygen in our air today, and the planet drastically changed, in a semi-deadly way.

According to one leading theory, oxygen reacted with greenhouse gasses, changing the makeup of the atmosphere so that it stopped trapping as much heat.

That would explain why Earth's temperature plunged 1.9 billion years ago, so much that it froze over from the poles to the tropics.

Oxygen started out as catastrophic.

But Earth's oxygen-rich atmosphere is also the reason that we're around today.

By using oxygen, your cells metabolize food completely differently than those early lifeforms.

You get way more ATP for every bit of organic matter that you consume.

It's about 10 times more efficient than those original anaerobic ways of creating ATP.

And life needed all of that energy to grow and get more interesting.

So this period, called the Great Oxygenation, often gets the credit for paving the way for complex life: As the story goes, it created air that could be breathed, and that air enabled life to get complex, and the rest was history.

But that's not the full story.

Chances are, we would never have gotten this incredible array of lifeforms living on every inch of this planet if it weren't for a couple freak accidents, and a crucial give-and-take between biology and some rocks.

So remember when Earth froze over after oxygen showed up?

Well, volcanoes pumped some greenhouse gases back into the atmosphere, and things gradually warmed up enough to thaw.

And then something happened between two microscopic cells that, well, it changed everything: At least 1.7 billion years ago, one cell just gulped up another one.

The swallowed cell started living inside the other one.

It evolved into what we call the mitochondrion- - [Announcer] The powerhouse of the cell!

(thunder crackles) - An organelle whose main job is to create ATP for its host cell.

Up until this point, all cells were prokaryotes: simple bags of stuff without any division of labor going on inside.

But suddenly, in this leap of evolution, that changed.

Some biologists think evolution basically would have stalled if this hadn't happened, because every single animal and plant on Earth is made of eukaryotic cells, complex cells whose ancestors were born in that moment when one cell swallowed another and took it hostage and squeezed all the energy out of it.

Meanwhile, the general blueprint of single-celled organisms has hardly changed in 3 billion years.

Prokaryotes didn't gradually morph into eukaryotes through a bunch of gradual evolutionary steps.

(alarm buzzes) This one evolutionary leap, basically a freak accident, changed the entire course of life's history.

And this kind of freak event happened twice.

Around 1.25 billion years ago, a eukaryotic cell cannibalized another cell.

This time, it was a photosynthesizing bacterium that got gulped up.

Over time, that evolved into a chloroplast, the organelle that does photosynthesis.

Today, every single plant has cells like this, full of chloroplasts and mitochondria.

Forget gradual change.

Without sudden, huge leaps in evolution, plants and animals just don't exist.

But what's weird is, even though, 1.25 billion years ago, life already had the building blocks it needed to create all sorts of complex plants and animals, we didn't get this immediate burst of interestingness at all.

In fact, some biologists call this time period, from 1.8 billion years ago to 800 million years ago, the "Boring Billion."

Evolution was happening, but really slowly.

Because even though photosynthesizing bacteria were pumping out a bunch of oxygen, it wasn't enough to support big, breathing animals.

The problem was, the amount of oxygen bacteria could make likely depended on something else: phosphorus.

No one gives phosphorus any love.

Cells use phosphorus to make membranes, proteins, DNA.

Basically, no phosphorus, no cells.

And at the time, Earth had lots of phosphorus, but it was mostly locked up in the crust, not dissolved in the ocean, where the bacteria were.

Prokaryotes were starved for P, so their population likely stalled for hundreds of millions of years.

But over time, a series of ice ages created glaciers that scraped up the ground, freeing up some of the locked-up phosphorus.

Wind and rain wore down the continents, washing a bunch of that phosphorus into the oceans.

Life finally had all the ingredients that it needed to get interesting: It had eukaryotic cells, which would become the building blocks of complex organisms.

It had photosynthesizing cells pumping out oxygen.

And it had enough phosphorus to sustain all of this.

So, after around 3 billion years of life on Earth, evolution wasn't on hold anymore.

Natural selection could do its thing, and life exploded into zillions of different forms in just hundreds of millions of years.

I know that sounds like a long time, but as major evolutionary changes go, it's basically overnight.

So how does life get complex?

Well, first, cells start living together, and they split up tasks: some digesting the food, others providing structure, some just for mating.

Eventually, some of these cell communes evolved into simple animals.

And over time, animals kept getting bigger and weirder.

Some were shaped like ribbons and tubes.

They stopped staying in one place, and started taking advantage of those big stores of ATP to seek out other organisms for fuel.

Harvesting and hunting brought in more fuel for their bodies, which let them get even bigger.

Organisms also got more complex as they evolved new tactics for finding dinner and avoiding becoming dinner.

They grew heads and tails and armors and spines.

I mean, natural selection went wild.

While none of this would have happened without oxygen in our atmosphere, an oxygen-rich planet comes with a few risks.

I mean, quite possibly the first thing it did to our planet was freeze it into an Earth-sized snowball, which we already talked about.

That was not great.

But on the flip side, oxygen also gave us fire.

Nothing would burn on Earth without oxygen in the air, and the more oxygen there is, the more flammable things are.

But when it comes to oxygen and life, the big catch is that oxygen is toxic.

We all breathe air that's around 21% oxygen.

But if people are exposed to higher concentrations of oxygen, they can actually be poisoned.

'Cause the thing about oxygen is that it's super reactive.

To put it chemistry specific, it has two unpaired electrons, which really want to form chemical bonds.

So oxygen yanks electrons off other molecules whenever it gets the chance.

This is known as oxidizing, and it's how we get things like rust.

If oxygen steals electrons from molecules in our bodies, it creates unstable molecules called free radicals that can damage cells.

Our bodies have some natural defenses against this.

Antioxidants are molecules in lots of fruits and vegetables that can easily give up electrons to free radicals and keep them from reacting with cells and doing damage.

But these defenses can get overwhelmed if the oxygen concentration is too high.

And even if antioxidants aren't overwhelmed, some free radicals still slip through, slowly damaging our cells year after year.

Some scientists think we have oxygen to blame for part of aging and age-related diseases.

In a way, oxygen itself both gives life and takes it away.

So, sure, the Great Oxygenation was pretty great.

But oxygen gas was not this elusive elixir of life.

It was a double-edged sword.

And it took much more than a burst of oxygen to give us life as we know it.

The story of life on earth isn't just a story about biology.

It's a story about geology and chemistry overlapping with biology.

We needed a planet that could support simple lifeforms for billions of years.

We needed some of those lifeforms to fill our air with oxygen.

And we needed some well-timed freak accidents to create the building blocks of complex life.

Then we needed geological conditions to line up just right before life could get interesting at all.

So, complex life might not be the inevitable endpoint of generations of evolution.

You might need more than a planet with just the right ingredients at just the right distance from a star.

It kinda suggests that even if we do one day find another planet with simple lifeforms, that life may never get interesting or intelligent.

So, as we live on our planet covered in incredible plants and animals, breathing this deadly, flammable gas that also somehow gives us life, the fact that we're here at all might be even more remarkable than we thought.

Stay curious.

And as always, thank you for sticking around to the end of the video, and thank you to everyone who supports this show on Patreon.

We literally could not do this without you.

Much like oxygen, you give us life and you keep these videos complex, because summarizing the entire history of complex life into one episode is not an easy task.

It takes us a lot of work to research and put these videos together.

The support of our patrons is essential to keep making videos the way that we know that you love them.

So support at any level will help us keep doing that.

You can find out more information down in the description.

Go ahead and clicky-click the button and we will see you in the next video.

Explanation of demo.

(crew member laughs) Okay.

Thank you.

I just read what it says.

(Joe grunts)