How did life make the leap from solitary cells to creatures with bodies, organs, and lifespans? The Many-Celled World tells the story of one of evolution's greatest inventions: the moment a crowd of cells became a new kind of individual. Written for curious general readers who love big biological questions, it reveals why multicellularity changed the possibilities of life - and why its legacy still shapes our health, our aging, and our vulnerability to cancer. Beginning roughly 720 million years ago, amid the upheavals of Snowball Earth, the book traces how cells first stuck together, learned to cooperate, and divided labor between reproduction and work. Along the way, readers encounter the elegant volvocine algae, a living staircase from single cells to simple bodies, and discover how glue-like matrices, chemical signals, and developmental programs turned clusters into organisms. The same logic, the book shows, also explains what happens when cellular cooperation breaks down. Blending evolutionary biology, geology, and modern biomedicine, The Many-Celled World offers a vivid, accessible account of how bodies are built without a master planner. It is at once a deep history of life, a fresh way of thinking about animal complexity, and a surprising guide to the hidden social rules at work inside every living body.