We humans like to think of ourselves as pretty different from other animals. Language, philosophy, art, technology - we do things it seems like no other animal is capable of. But what makes us this way? In part one of our investigation, we focus on two features of the brain that seem to be particular to people. We start with Arnold Kriegstein of the University of California, San Francisco, who studies a type of stem cell that does something special during human brain development. We then turn to Kira Poskanzer and Anna Molofsky, also of UCSF, who believe the secret to human-ness might lie with a totally different, often neglected kind of brain cell.