From Eurekalert: "The Paleozoic period, about 300 million years ago, was a time of huge and abundant plant life and rather large insects -- dragonflies had two-and-a-half-foot wing spans, for example. The air's oxygen content was 35% during this period, compared to the 21% we breathe now, Kaiser said. Researchers have speculated that the higher oxygen concentration allowed insects to grow much bigger."
Wow! 35% oxygen? Imagine the lift to your step! Imagine the forest fires!
Guess it's not atomic radiation after all. Link
Update 10/25 9:08 am: Just heard them cover this on NPR's "Living On Earth."