For centuries the notion that the adult brain could not make new neurons stood as a central tenet of neurobiology. Even Santiago Ramón y Cajal—the Barcelona-based histologist who essentially invented modern neuroscience at the end of the 19th century—declared such neural renewal impossible. After decades of careful observation and painstaking illustration of the microscopic architecture of nerve cells and their connections, Ramón y Cajal concluded that in the adult brain, “the nerve paths are something fixed, ended, and immutable; everything may die, nothing may be regenerated.”