This episode is with Iñaki Ruiz-Trillo, whose research sheds light on one of the most fundamental transitions in the history of life—the evolution of multicellularity. Iñaki is a group leader at the Institute of Evolutionary Biology in Barcelona, where he investigates how unicellular organisms gave rise to complex multicellular life. Our conversation explores the deep evolutionary forces that shaped this transition, the role of cooperation and cell differentiation, and what studying the ancestors of animals can teach us about life’s complexity. We also discuss broader questions about biological organization, individuality, and the very nature of what it means to be a multicellular being.