The air we breathe is 21 percent oxygen, an amount higher than on any other known world. While we may take our air for granted, Earth was not always an oxygenated planet. How did it come to be this way?

Donald Canfield covers this vast history, emphasizing its relationship to the evolution of life and the evolving chemistry of Earth.

He guides readers through the various lines of scientific evidence, considers some of the wrong turns and dead ends along the way, and highlights the scientists and researchers who have made key discoveries in the field.