This episode is supported by The Great Courses Plus 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. Thanks to the The Great Courses Plus for sponsoring this episode. The Great Courses Plus is a digital learning service that allows you to learn about a range of topics from educators including Ivy league professors and other educators from around the world. Go to TheGreatCoursesPlus.com/Eons and get access to a library of different video lectures about science, math, history, literature, or even how to cook, play chess, or become a photographer. New subjects, lectures, and professors are added every month, like the Introduction to Paleontology series taught by Professor Stuart Sutherland. You can learn about everything from Earth’s shifting crust to Taxonomy and more! With The Great Courses Plus, you can watch as many different lectures as you want – anytime, anywhere without any tests or exams. Help support the series and start your free one month trial by clicking the link below or going to TheGreatCoursesPlus.com/Eons What do you want to know about the story of
life on Earth? Let us know in the comments. And don’t forget to go to youtube.com/eons
and subscribe! If you think dragonflies are fearsome, wait till you see their babies. Our friends at Deep Look filmed them shooting out their super-fast mouthpart to catch a meal. Check it out here.
I always loved that age. I was captivated by it as a kid when I saw a depiction of it on TV.