We often call Earth the ‘blue  planet,’ because of its vast   oceans of water—which are actually pretty rare.

But our preoccupation with water  misses something important:   Earth isn’t the only watery planet in the known  universe, but it is the only fiery planet.

Now, you might be saying ‘wait a second.

What   about all the lightning and volcanoes in  our solar system, and oh yeah, the sun?’ Turns out, none of those things are actually fire.

The sun is a huge ball of mostly hydrogen   in the gas and plasma states of  matter undergoing nuclear fusion.

And while volcanoes spew molten rock,   magma isn’t on fire, either.

And lightning  is an electrical discharge—also not fire.

Only Earth has fire.

So what, exactly, is fire?

And why is it unique to Earth?

Well, to get to fire, it took billions  of years of photosynthesis – which means   fire can’t exist without life.

And fire and  life have been shaping each other ever since.

Today, the main oxygen gas in the atmosphere is  dioxygen, or two oxygen molecules bonded together,   which we usually call O2.

That’s  what we breathe every second.

But before Earth’s early  photosynthesizers hit the scene,   we didn’t have a lot of O2 in the atmosphere.

Instead, the air was a spicy stew of  methane, water vapor, and carbon dioxide.

Between the initial evolution of life  and the emergence of fire, a billion   years of cyanobacteria photosynthesizing in the  ocean had to forge a huge amount of dioxygen,   that would eventually make the planet not only  habitable for animal life, but also for fire.

The blip on the planet’s timeline when  this transformation happened is called   “The Great Oxygenation Event,” and  it’s the reason we’re here today.

You might remember it from our episode  ‘That Time Oxygen Almost Killed Everything.’ At first, all the excess O2 was  absorbed by iron in the earth’s crust.

That’s why a billion years of  photosynthesis had to happen—it   had to overcome this sink before it could  build up a reservoir in the atmosphere.

We know this because, as dioxygen was  created by the first photosynthesizers,   it reacted with iron in the earth’s crust  to form iron oxide—better known as rust—and   sank to the bottom of the ocean.

There, it formed a geologic record.

Eons later, we can see these records – called  banded iron formations – in places like Western   Australia and right here in Montana, where they’ve  been pushed up to the surface of the Earth.

Eventually, after almost 2 billion  years of photosynthesis, a nice,   thick layer of oxygen formed in the atmosphere.

But, everything still lived in water.

There  were no forests, or any plants at all, on land.

Now, fire is the product of combustion,   which is a pretty specific chemical reaction.

It needs two ingredients: fuel and dioxygen.

When things get hot enough, combustion combines   these two ingredients and releases  water vapor and carbon dioxide.

When combustion happens, the flames you see  are gasses emitting light, and we call it fire.

So fire needs fuel - and not  just anything can be fuel.

This is because combustion breaks apart  the atoms in molecules of fuel and   re-attaches them to the oxygen atoms,  creating new molecules as byproducts.

And this process is very efficient when  it has a specific type of fuel molecule:   one with lots of bonds between  carbon and hydrogen — exactly   like the most common molecule in  the bodies of plants, cellulose.

Now scientists still aren’t sure about what kind   of plant life first took hold on  land and when exactly it happened.

And that’s because the oldest  fossil evidence of life on land,   which dates to the Cambrian period  about 500 million years ago,   consists of microscopic spores that  are hard to make inferences from.

But by 430 million years ago there is  a strong fossil record of life on land.

There were tiny, tubular filaments called  nematophytes, microscopic spheres called   pacytheca, and the comparatively gargantuan  Prototaxites, which grew to about 9 meters tall.

It’s unclear whether they photosynthesized,   and they didn’t have leaves, roots,  or seeds.

But they did catch on fire.

We know this because we’ve found  fossil charcoal that’s that old,   in a very deep hole dug by the British Geological  Survey back in 1978 near a small town in Wales.

The researchers who discovered the  oldest charcoal think that Prototaxites,   being the tallest thing around,  probably attracted lightning.

And once these early organisms were set ablaze,  they became the world’s first wildfires.

70 million years later, Pangea  started to form and plants spread   and diversified.

They ushered in a golden  age of wildfire—the Carboniferous Period.

Now, the world’s first plant-life didn’t  decompose very much when they died—scientists   think that microbial decomposers who  could eat the plants hadn’t evolved yet.

And without a chemical or biological  process that returned the bodies of   the plants to the biosphere, they were  buried and became part of the geosphere.

After millions of years of increased  temperature and pressure, they formed   coal—the same coal that powered the  fires of the industrial revolution.

In the Carboniferous, this surge in  photosynthesis meant a surge in O2 as a byproduct.

Today, air is only about 21 percent O2,   but estimates suggest that back then  it was between 30 and 35 percent!

One possible side-effect of all of this extra O2   is that insects and other  arthropods could get huge.

For example, a dragonfly found in  Britain had a 50 centimeter wingspan   and a scorpion called Pulmonoscorpius  reached almost a meter in length.

But more importantly—fire could get huge.

Though we don’t know for sure,   fire with this much ambient oxygen  was probably widespread and intense.

Flame lengths may have commonly  reached over a hundred meters high,   and large fire whirls as well as true  fire tornadoes may have been common.

Also during the Carboniferous, plants with true   leaves and some true wood became  abundant in the fossil record.

So for the first time, fire could use  leaves to travel from tree to tree.

These plants were the lycophyte trees,   which could grow over 30 meters tall and  had spore-bearing cones on their branches.

There were also giant horsetails,   which had symmetrical whorls of  leaves at repeated intervals.

Yet, by 60 million years later,  in the late Permian period,   many of these plants were gone,  outcompeted by trees that made seeds.

But that didn’t mean fire  couldn’t continue to thrive.

Seeds are better than spores  at living in dry environments,   so for the first time, forests  existed all over the world.

And charcoal in the fossil record shows  that those forests carried fire with them.

By the late Jurassic period, 150 million years  ago, a family of trees you might recognize today   were protecting their seeds in cones and  wafting their needle-leaves in the sun.

These were the pines.

Today, different species in the family of pines  show a range of different adaptations to fire.

Some have thick bark and have  shed their lower branches so   fire can’t climb to their crowns  and burn all of the living leaves.

Others have adapted to be consumed by  fire almost completely so their seeds   can start the next generation  in the nutrient-rich ashes.

And in 2012, researchers used DNA, charcoal  in the fossil record, and measurements of   atmospheric O2 to conclude that extra-thick bark  and branch-shedding evolved at the same time as   the strategy to be totally consumed by fire—about  89 million years ago, in the Cretaceous Period.

The amount of charcoal in the fossil  record at this time suggests that   there was a lot of fire—so the pines  evolved two different ways to cope.

But, while sometimes life and evolution  march along in a rhythmic progression,   other times totally random events  can turn everything on its head.

And after millions of years of slow change,   an asteroid struck the Gulf of  Mexico at the end of the Cretaceous.

The impact triggered tsunami waves,  earthquakes, and other mass destruction.

It’s even likely that, at this time,  the whole world caught on fire.

Tens of thousands of cubic kilometers of hot   debris were thrown up into the air by  the impact and then rained back down.

A layer of charcoal found in the fossil  record covering nearly the whole planet   in the aftermath of the impact suggests that  entire forests must have been scoured away.

But the earth rebuilt over  many more millions of years,   and recently, the most fire-loving  ecosystem yet took root—grasslands.

Grasses themselves had been  around for 100 million years,   but not long after the end of the Cretaceous  they began forming a biome where they dominated.

And by the end of the Miocene epoch, around  5 million years ago, grasslands were one   of the major biomes of the world.

Grasses keep the majority of their   anatomy underground, where the  fire is less likely to reach it.

But the portion of grasslands exposed  to the sun seem to love to burn.

Their above-ground tissue dies each fall,   creating a thick thatch that new  shoots and seedlings can’t penetrate.

Fire clears this away and returns nutrients to  the soil, invigorating each season’s new growth.

And fire kills grasses’  competitors: young shrubs and trees.

Without repeated fires, grasslands  would transform to forests,   and all the millions of species that  live in grasslands would be lost.

Grasslands are special ecosystems with  their own collection of unique animals,   like horses, cheetahs, prairie dogs, and  flocks of millions of migrating birds.

Grasses include the world’s most important  crops, like wheat, maize, and rice.

And some researchers have suggested  that grasslands played a role in   the evolution of our hominin ancestors in Africa.

Without life, fire can’t exist, and in return,   fire has acted as a force to replenish and  diversify life in almost every ecosystem.

Without fire, there would probably  be no grasslands and the forests of   the world would likely have a lot less diversity.

Earth is a water planet, sure, but even more  spectacularly, Earth is the only fire planet.