You’ve probably seen them in dioramas at natural history museums: giant insects with wingspans up to 71 cm (more than 2.25 feet) that lived about 300 million years ago. But how did these high flyers do it? After all, you’ve just learned why it’s hard for big bugs to get enough oxygen with their tube-based respiration systems…

Scientists are investigating an intriguing answer to this question: there was simply more oxygen in the air at that time. The Earth’s atmosphere has not always been the same. Studying the chemical composition of deposits buried in ancient rocks along with other lines of evidence has led scientists to estimate that 300 million years ago, the Earth’s atmosphere contained 35% oxygen — compared with a measly 21% today!

These high oxygen levels would have made a big difference to animals that depended on diffusion for their oxygen. Click on the button below to see how different oxygen levels in the atmosphere affect diffusion and the cells receiving oxygen.

The more oxygen outside the insect, the more oxygen gets inside the insect. That means that, with more oxygen in the atmosphere, insects could have evolved larger body sizes without suffocating their cells — and if giant dragonflies are any indication, they certainly took advantage of this opportunity!