The science-writer explores the theory that 700 million years ago, the earth was a vast, whited-out iceball. Paul Hoffman, a brilliant and irascible Harvard scientist, claims that a climactic cataclysm 700 million years ago triggered the great Cambrian Explosion, the hitherto unexplained moment in geological time when a profusion of complex life forms first sprang from the primordial soup. Walker tells his story.