As a professor of pediatrics and cellular and molecular medicine at University of California San Diego School of Medicine, Alysson R. Muotri, Ph.D., has long studied how the brain develops and what goes wrong in neurological disorders. For almost as long, he has also been curious about the evolution of the human brain—what changed that makes us so different from preceding Neanderthals and Denisovans, our closest evolutionary relatives, now extinct?