What happened to the giant bugs?

Imagine a dragonfly with a 70-centimeter wingspan—roughly the size of a carry-on suitcase—flying through swampy forests 300 million years ago. Or a millipede over six feet long. These weren't dinosaurs; they were insects from the Carboniferous period, and for a long time, scientists thought they knew why these giant bugs eventually disappeared. The answer seemed to be oxygen. But new research suggests that story might be wrong.

The oxygen hypothesis

The standard explanation goes like this: during the Carboniferous, Earth was covered in massive swampy forests of tree-sized ferns and club mosses. With all that plant life pumping out oxygen, atmospheric oxygen levels may have reached 30 to 35 percent—significantly higher than today's 21 percent. More oxygen in the air supposedly allowed insects to grow much larger because of how they breathe.