I've always found the evolution of humans fascinating, reading about how we see the evolution of our bodies from originally living in trees to start walking around on two legs, and then I saw this CBS TV report which makes me wonder if a monkey fell out of the tree and broke one or both of its fore-limbs but still survived, leading to other monkeys starting to play with the survivor and learning what has become our gait.
https://www.youtube.com/watch?v=Ry2QA4nu5a4