Three hundred million years ago, in Carboniferous times, dragonflies grew as big as seagulls, with wingspans of nearly a meter. Researchers claim they could have flown only if the air had contained more oxygen than today-probably as much as thirty-five percent. Giant spiders, tree-ferns, marine rock formations, and fossil charcoals all tell the same story. High oxygen levels may also explain the global firestorm that contributed to the demise of the dinosaurs after the asteroid impact. The strange and profound effects that oxygen has had on the evolution of life pose a riddle, which this book sets out to answer. Oxygen is a toxic gas. Divers breathing pure oxygen at depth suffer from convulsions and lung injury. Fruit flies raised at twice normal atmospheric levels of oxygen live half as long as their siblings. Reactive forms of oxygen, known as free radicals, are thought to cause aging in people. Yet if atmospheric oxygen reached thirty-five percent in the Carboniferous, why did it promote exuberant growth, instead of rapid aging and death? Oxygen takes the listener on an enthralling journey, as gripping as a thriller, as it unravels the unexpected ways in which oxygen spurred the evolution of life and death.