Life history is full of bizarre things. One of these things is the evolution of abhorrently large insects. Hundreds of millions of years ago, these bugs ranged in size from small robins to large pigeons. But how exactly did they grow so large? And could they ever grow this large again?

To answer this question, we must travel back in time to the carboniferous period, a slice of geologic time that lasted from around 360 to 299 million years ago. During this time, Earth’s atmosphere was strikingly different. Not only did it have less carbon dioxide in the air. It had more oxygen.

The Carboniferous

During the carboniferous period, the Earth was rich with plants. Some of these plants were vascular, rising above their mossy counterparts, while others were low to the ground. Either way, this abundance of plant species afforded the Earth ample oxygen, breathing in carbon dioxide and releasing oxygen as a byproduct. As this oxygen filled the air, it enabled animals on the ground to breathe it in and grow as a response.

The amount of oxygen we experience in today’s atmosphere is around 21 percent. The rest of the elements in Earth’s atmosphere exist as follows: nitrogen: 78 percent; argon: .9 percent; and carbon dioxide, nitrous oxide, methane, and ozone: 10 percent. Water vapor also lingers in the atmosphere, varying in quantity from 0 to .4 percent.

During the carboniferous period, the amount of oxygen in the atmosphere was around 35 percent. With this increased amount of oxygen, insects could grow larger and larger. The same thing has actually been replicated in microcosm in lab experiments: If you enclose certain insects like the dragonfly into an artificially oxygen-rich environment, after a few successive generations they will grow up to 15 percent larger.

Abundant Oxygen

Why was the oxygen composition of the atmosphere different during the carboniferous? The answer lies in the other creatures that existed (and also those that didn’t) at the time.

Today, when plants and animals die, they are sacrificed to the world of fungi. These voracious decomposers will eat and digest most of the things that they encounter, turning them into precious nutrients for themselves and other plants and animals nearby. Back in the carboniferous, however, no such thing happened.

The fungi primarily responsible for plant digestion is the species Amanita muscaria. This particular species, however, didn’t evolve until after the carboniferous period. In its absence, the dead flora species remained stagnant,  keeping their carbon dioxide for themselves. Their carbon-rich bodies would later solidify as coal, the fuel source that we would eventually resurrect and send back into the atmosphere with unprecedented speed.

As you probably could have guessed, it is this relationship that helped build the period’s oxygen-skewed environment: Plants were pumping more oxygen into the air without releasing any carbon. When the plants died, their bodies weren’t broken down to release their carbon dioxide back into the environment. The result is that the atmosphere grew more oxygenic.

The lack of fungal decomposers is why this period was named the carboniferous. Because plants were not broken down by fungi, they remained on the ground to be fossilized. The result, as you probably could have guessed, was an increase in the formation of coal. Most of the coal we use today comes from the plant-rich carboniferous.

Of Insects and Oxygen

So how, exactly, do insects respond to this oxygen? Well, as we already discussed, they grow. And over tens of millions of years, they grew so large as to look like birds. Some of them even evolved beaks.

A few of the most daunting of these ancients bugs was the meganeura. This four-winged, dragonfly-looking beast had a wingspan of around 25 to 28 inches. That’s about the size of a full-grown pigeon. Another regrettable creature was arthropleura, a millipede-like creature that is the largest land-dwelling invertebrate ever discovered. It measured in at around 7.5 feet long and 1.6 feet wide.

When fungi evolved and the amount of oxygen in the Earth’s atmosphere dwindled, these insects began to shrink. And since then, other flying animals have taken their place. The age of the giant insects has receded, and for that we can be thankful.