About 550 million years ago, there was literally an explosion of life forms, as all the major animal groups suddenly and dramatically appeared. Although several books have been written about this surprising event, known as the Cambrian explosion, none has explained why it occurred. Indeed, none was able to. Here, for the first time, Oxford zoologist Andrew Parker reveals his theory of this great flourishing of life. Parker's controversial but increasingly accepted "Light Switch Theory" holds that it was the development of vision in primitive animals that caused the explosion. Drawing on evidence not just from biology, but also from geology, physics, chemistry, history, and art, In the Blink of an Eye is the fascinating account of a young scientist's intellectual journey, and a celebration of the scientific method.