What defines who we are? Until now, the biological answer has been our genes. Leading biologist Alfonso Martinez Arias breaks with popular tradition to make a bold argument: what defines us is our cells. Drawing on groundbreaking research, he reveals that we are composed of a thrillingly complex, constantly rearranging symphony of cells that know how to count, feel, and give form to our bodies. While DNA is important, nothing in your genes explains why your heart is on the left, why you have five fingers and not ten, why genetically identical twins have different sets of fingerprints, or why it's possible for a mother to apparently share no DNA with the children she gave birth to! At the heart of it all is a powerful new conception of the essence of life. Our identities are shaped by the interconnections between cells, working cooperatively, creating something greater than its parts - the unbroken lineage that connects us to the fertilized egg from which we developed and back through the billions of years of our planet's history, to the very first cell of all life on Earth. A sweeping revision of both the present and the history of life, The Master Builder puts forward a new paradigm for understanding biology, transforming our approach to where we come from, what shapes us, and where we are going - as individuals, a species, and the community of life itself.