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.
