Or feet rather. Apparently our feet diverged from the other primates about 2 million years ago. Most primates have flexible feet, that allow them to grip trees, branches and the like, even though they don't climb trees so very often.
Out own feet are a little different. The rigid nature of them allows us to gain leverage against the ground and propel ourselves foreword, keep balance and move faster. Primates do not have this advantage, hence the ambling gait.
Its still not known why the divereged, but my bet would be that it had something to do with leaving the jungle canopy and heading out on a walkabout.