We're all about the science today. Slashdot linked up this Discover article summarizing the findings of a scientist who claims to have discovered the precise gene that gives humans big brains. Turns out there are precisely fifteen mutations on this gene that differentiate it from what you find in chimpanzees. How it works is, of course, not clear.
Again, it's important to understand these mutations didn't happen in a vacuum. Our ancestors happened to have access to a protien-rich resource that lent itself well to a tool-using tree climber (scavenging leopard kills and anything else the heyenas couldn't get to). The dietary change was probably at least as important as the genetic one, as without the former the latter was useless. Likewise, without the latter the former was merely the behavior of jibbering chimp-things with an (oftentimes terminal) adrenaline addiction.
Update: Don't miss this older National Geographic article detailing developments about the very earliest primates. It's beginning to look like they may have walked (well, scurried and squeaked) with the dinosaurs.