Bipedalism, besides our ability to talk about random stuff, our way of locomotion probably is the easiest way to distinguish ourselves from other animals. The question whether or not a potential hominid was bipedal is always one the most discussed questions, every time a new fossil is discovered. Of course, this question is oftentimes hard to answer, if the only remains you have available are a distorted cranium and some teeth.
But the question whether or not the fossil in question walked on two legs or not isn’t really that important. What’s more important is the question, if it was an obligate Biped.
There are two ways of bipedal locomotion, facultative and habitual (or obligate) bipedalism.
Animals which are facultative bipeds are able to stand and walk on two legs for a limited amount of time, but it’s not their general mode of locomotion. Apes are facultative bipeds, as are almost all primates in general.
Humans on the other hand are obligate Bipeds, as are Birds (at least when they walk on the ground), some Dinosaurs were obligate Bipeds too. Obligate Bipedalism simply means that the organism in question generally walks on two legs instead of four.
As already mentioned, all extant Apes are facultative Bipeds. This means that it’s quite likely that the ability for facultative bipedalism was the ancestral condition of all Apes.
Let’s imagine I’ve found some fossilized bones and I’m pretty sure that these bones belonged at least to some kind of Ape (who knows it might even be a hominid). Our main hypothesis is”All Apes are at least facultative bipeds.” Which kind of question should I try to answer when I start to work at these bones?
Since the only way in Science to learn something new is by falsification, the questions we should ask are either:
Was this fossil not a facultative biped? If this would be the case, we had to modify our initial hypothesis.
The other question would be, if this fossil was a habitual biped. Because then we had to discuss the question whether or not it might be a hominid.
On the other hand, which question is of no interest whatsoever, because it teaches us nothing new?
Right, the question whether or not the fossil in question was a facultative biped. Why? Because this is already our initial hypothesis, we don’t need to answer this question.
Now let’s take a look at a recent paper which had to deal with this kind of scenario:
Haile Selassie and Colleagues (2012) recently published a paper in Nature where they described some oddly looking foot bones, here’s what they had to say on the topic of bipedalism:
„...when on the ground it was at least facultatively bipedal”
(Haile-Selassie et al. 2012; p. 568)
If anyone is looking for a great representation of what's wrong with modern day science, you can stop right now. This is the kind of scientific paper you get, if you're forced to publish as much as possible, even if you don't have much more to say besides: "Well it looks different then the other stuff we've found."
Haile-Selassie, Y., Saylor, B., Deino, A., Levin, N., Alene, M., Latimer, B. (2012). A new hominin foot from Ethiopia shows multiple Pliocene bipedal adaptations Nature, 483 (7391), 565-569