Why Is It So Funny When People Fall Down?

One thing that has been at the cornerstone of comedy since the dawn of man is someone falling down.  It was followed very very very closely by a guy getting hit in the nuts.  There is just something about people getting hurt that makes us laugh.  But why?  What makes our brain thing that other people's pain is so damn funny?

Think you are above it?  Watch America's Funniest Home Videos or Tosh.O or Web Soup and try not to laugh.  You can't because you are human (or a reasonable facsimile who is plotting eventual world domination) and humans laugh at bad stuff happening to other people. This week we look and the theory and the science of why.  Are we all just heartless jerks or are we just genetically predisposed to find nut shots funny?