Fascinating (if a little hard to stomach) video. This seems to imply that bipedal locomotion is learned, while quadrupeds have the skill hardwired into lower-level structures. (Certainly, having watched my eldest figure it out, it doesn't seem to be an innate ability in humans.)
Just because the structure resides below the brain, doesn't mean that it's simply 'innate'. For example, while deer fawn are famously able to start walking right away, a kitten is actually pretty terrible at walking for many weeks - they still have to learn. Perhaps part of the difference between the 'apparent' skill of quadrapedal and bipedal learning is that a) quadrupedal is inherently easier (its more stable, more forgiving of small deficiencies), b) bipedal 'failure' is a lot more drastic (especially when you have the proportions of a child.. you don't have a lot of leg to catch yourself), and c) we're simply more familiar with bipedal motion, and therefore more critical of flaws on observation.
In addition, there is suggestive evidence that human bipedal motion is CPG driven [1]. I think a reasonable analogy to draw is that at birth a CPG is kind of a lower level controller, all hooked up, but not yet programmed. It takes time and practice to tune and program the controller to generate useful output.
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