For much of the twentieth century it was assumed that genes alone mediate the transmission of biological information across generations and provide the raw material for natural selection. Yet, it's now clear that genes are not the only basis of heredity. In , evolutionary biologists Russell Bonduriansky and Troy Day explore the latest research showing that what happens during our lifetimes—and even our parents' and grandparents' lifetimes—can influence the features of our descendants. Based on this evidence, Bonduriansky and Day develop an extended concept of heredity that upends ideas about how traits can and cannot be transmitted across generations, opening the door to a new understanding of inheritance, evolution, and even human health.