More than half a billion years ago, our world was completely covered by glaciers, a "Snowball Earth" that persisted for millions of years. Incredibly, this unimaginable cold led to the remarkable diversification of life on earth known as the Cambrian explosion. With a geologist's eye and a knack for storytelling, Graham Shields explores when and how such inhospitable conditions enabled animals to evolve, radiate, and diversify into our earliest ancestors.

This journey navigates the wild swings between hot and cold climates, oxygenation and asphyxiation, biological radiations and extinctions, asking how such instability relates to grander forces that brought our planet to its modern state. Shields guides readers through evidence found in the Australian outback, Mongolia, Scotland, and other locales, revealing how geologists can trace glaciation, the atmosphere, oceans, mountain building, and more through the earth's rocks, providing a comprehensive theory of how life evolved and diversified.