Transition to Bipedalism?

I'm an anthropology student interested in primate and human paleontology, and non-human primate behaviour and ecology. But, I don't know much about it yet. I've only finished my first year anthropology and biology courses. But I've noticed that the great apes today both have the ability to walk some-what up right if they so chose, but also walk on all fours. Chimps regularly stand up and walk, but move about on all fours to move quickly (probably, I think, because they aren't quite at home on just two feet, so more physically strenuous activities like running need to be done on all fours). I'm wondering if bipedalism came about more gradually, the first bipedal apes being more like modern great apes, having both quadrupedal and bipedal behaviours. I have a hard time understanding how bipedalism could have come about quickly, unless it came about by mutation. But my professors have discussed how it was something selected for, perhaps for the purposes of being able to carry objects, or for the purpose of being able to converse energy, as it requires less to walk on two feet than four, or perhaps for less exposure to sun on the hot African savanna. I have this gut feeling that it came about as a slower transition, the first bipedal apes also having the ability to walk on all fours. And as selection became stronger, for whatever reasons the environment imposed on African apes (~3.6-4mya?) quadrupedalism was selected against in what would become the human line. I want to have an understanding of why it is our line diverged in motor behaviour from quadruped to biped.