Three hundred million years ago, dragonflies grew as big as seagulls, with wingspans nearly a yard across. Researchers claim they could have flown only if the air had contained more oxygen than today-probably as much as 35 per cent. But oxygen is a toxic gas. Fruit flies raised at twice the normal level of oxygen live half as long as their siblings. If atmospheric oxygen reached 35 per cent in the Carboniferous, why did oxygen promote exuberant growth,...