Here is one biologist's interpretation of the chronology of life during the last six hundred million years of earth history: an extended essay that draws on the author's own data and a wide-ranging literature survey to discuss the nature and dynamics of evolutionary change in organisms and their biological surroundings. Geerat Vermeij demonstrates that escalation—the process by which species adapt to, or are limited by, their enemies as the latter increase in ability to acquire and retain resources—has been a dominant theme in the history of life despite frequent episodes of extinction.