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.”
So, when Joseph Altman, then at the Massachusetts Institute of Technology, published a series of papers in the 1960s showing that new neurons cropped up in the brains of adult guinea pigs, he was largely ignored. This disregard was perhaps not surprising, because from a logical standpoint, adding new neurons into a fully developed brain would be a recipe for disaster. After all, if the brain stores information in specific webs of neural connections, it would seem that randomly inserting inexperienced cells into these preexisting networks could cripple our ability to properly encode and recover information and thus garble our memories.