Primate bipedalism: who, when, why

For those of you interested in primate bipedality, there's a paper out in PLoS One titled:
Homeotic Evolution in the Mammalia: Diversification of Therian Axial Seriation and the Morphogenetic Basis of Human Origins
by Aaron G. Filler
The paper is pretty complex, and I didn't bother trudging through all the analyses of spine organization, but the news summary of the paper is a convenient shortcut to understanding the possible implications:

"This research pushes back the date for the origins of bipedalism roughly 15 million years, to before the last common ancestor of humans, chimps, gorillas and orangutans, as well as lesser apes such as gibbons. The results match up with recent findings that suggest upright walking might have started before humanity's ancestors even left the trees."