Ugh. First, deciding whether the ancestor of humans and apes was an ape or a human seems silly. Does he mean to imply that we modern humans could interbreed with the common ancestor of both (and therefore are conspecific), but modern chimps could not? Yikes.
An upright ancestor, ok. That ain’t a human by a longshot. If his title was, “A bipedal ancestor for the apes?” then I wouldn’t be twirling my finger by my temple.
The most parsimonious explanation, to me, seems to be that his spine character is entirely unrelated to bipedalism, and is simply a hominoid symplesiomorphy. …If this is the case it would have the unfortunate effect of making the guy’s PhD thesis irrelevant.
That said, there is no question that the origin of bipedalism and knuckle-walking are greatly problematic. There are no knuckle-walking fossils. Why?