A leading developmental biologist argues that cells, not DNA, hold the key to understanding life’s past and present. What defines who we are? For decades, the answer has seemed obvious: our genes, the “blueprint of life." It’s not our genes that define who we are, but our cells. While genes are important, nothing in our DNA explains why the heart is on the left side of the body, how many fingers we have, or even how our cells manage to reproduce. Drawing on new research from his own lab and others, Martinez Arias reveals that we are composed of a thrillingly intricate, constantly moving symphony of cells.