Even though we often refer to this time in history as the Age of Mammals, we should probably be calling it the Age of Insects.

Because, just looking at the numbers, there are way more of them than there are of us.

Humans alone number more than 7 billion at this point, which is ... a lot.

But insects?

Try 10 quintillion!

We may like to think we're in charge because we make the rules and, well, we're bigger than they are.

But insects, and other arthropods, weren't always so small.

About 315 million years ago, they were not only abundant.

They were ... enormous.

To meet the biggest invertebrates to ever crawl across the Earth, we have to go back to the Carboniferous Period, from 298 million to 358 million years ago.

That's when you'd find the likes of Meganeura.

It was a griffinfly, a giant relative of today's dragonflies, that had a wingspan of about 70 centimeters.

That's about the size of a pigeon -- more than three times larger than the biggest living dragonfly.

Meager by comparison was Stephanotypus, another griffinfly that was still some 40 centimeters across, about as big as a robin.

And this greatness in size wasn't limited to insects!

You see out-sized arthropods all over the world during this period... like Arthropleura.

You know those cute little millipedes you find curled up under rotting logs in the woods?

Now imagine one of those about two meters long and a half meter wide, shuffling like a living carpet over the undergrowth.

It was probably the largest arthropod that ever walked on land.

So.

What allowed these invertebrates to get so big?

The answer ... is oxygen.

Take a deep breath.

Right now, the amount of oxygen in atmosphere is about 21 percent.

But back in the Carboniferous, it was nearly 35 percent!

That's because the Carboniferous was a time of incredible, runaway plant growth.

Huge forests full of ferns, mosses, and some of the earliest vascular plants had taken over much of the planet.

They sucked in carbon dioxide and pumped out oxygen in enormous amounts.

Now, you might be thinking: Earth has lots of trees now.

So what's the difference?

Well, today, that big log you find in the woods with all of those bugs under it?

That log is being decomposed by bacteria, among other things, that take in oxygen, and release CO2.

But in the Carboniferous, those wood-eating bacteria didn't exist yet.

So Earth's giant, primordial forests were taking in lots of carbon dioxide and pumping out lots of oxygen.

That's what plants do.

But since the trees weren't decomposing, the CO2 wasn't being released back into the atmosphere.

The result was an all-time high in the world's levels of atmospheric oxygen.

And that's what made giant arthropods possible.

Because, arthropods don't breathe the way we do.

They have a system of external openings called spiracles, that lead to a branching network of tubes called tracheae, that diffuse oxygen through their bodies.

And this puts a limit on their body size.

Arthropods can only get so big before they can no longer draw enough oxygen from the air.

But in the Carboniferous, the abundance of oxygen in the atmosphere made it easier for arthropods to get the O2 that they needed, which allowed them to reach record-breaking sizes.

In fact, paleontologists have managed to make this happen today in the lab, by experimenting with modern insects.

By raising dragonflies, beetles, and other insects in controlled, oxygen-rich enclosures, scientists at Arizona State found that successive generations of arthropods can grow faster and larger.

But, of course, it's possible to get too much of a good thing.

So, some scientists have proposed another theory -- that arthropods got huge not because they could, but because they had to.

Lots of oxygen might have been a beneficial for grown-up arthropods, but it also could've posed a threat to their larvae.

Young invertebrates can't control their intake of air like adults can, and too much oxygen can be deadly.

So researchers at Michigan State have suggested that ancient arthropods began producing bigger larvae, so they'd take in less oxygen relative to their body size.

And those bigger larvae resulted in bigger adults.

But, you know enough about natural history at this point to know that even the biggest creatures don't stay on top forever.

About 275 million years ago, during the Permian Period, the world changed, yet again.

The levels of atmospheric oxygen started to plummet -- why, we're not sure.

Ancient climate shifts might've had something to do with it.

But as oxygen levels fell, the interiors of the world's continents got warmer.

This shrunk the big swamps that were acting as natural carbon sinks.

So, swamps weren't pumping out as much oxygen as they used to, and, on top of that, decomposers finally appeared that were able to start breaking down all of the dead wood.

As these microbes took in oxygen and released carbon dioxide, global levels of O2 dropped even more.

And with less oxygen available, it became increasingly hard for the giant arthropods to survive.

By about 305 million years ago, the two-meter-long Arthropleura could no longer be found on the forest floor.

By 299 million years ago, the last of the Meganeura had flapped its wings.

The arthropods that followed never got quite as spine-tinglingly large as their ancestors were.

But, of course, everything turned out fine for them!

Today, we're totally outnumbered, both in biomass and in diversity, by insects, arachnids, and other land-based arthropods.

But if there ever was a time that was a true Age of Insects, it was probably the Carboniferous Period, when arthropods of all kinds were living large.