In a recent paper published in the journal Nature, Jeremy DeSilva demonstrates that early hominins did not climb like chimpanzees.
By studying the way chimpanzees climb, DeSilva was able to get a detailed understanding of the role their ankles play. While climbing a tree trunk, a chimpanzee's ankles flex and rotate in ways that would be impossible for a human to replicate.
DeSilva compared the ankle anatomy of chimps and humans, and then compared these to fossil tibia and tali (the tibia is the weight-bearing bone of the lower leg, and the talus is the upper foot bone which, along with the tibia and fibula, forms the ankle joint) from over a dozen hominins from 4.12 to 1.53 million years ago.
What he found is that the anatomy of early hominin ankles shows that they were as poorly adapted as humans to the kind of climbing done by chimps. John Hawks has a really good summary on his website, and brings other recent papers and findings into the discussion-- you should read it.
Here's the rub for me: Was Astralopithecus afarensis a facultative or obligate biped? The thinking up until now has been that they were climbers and facultative walkers. The problem here is that many of the adaptations present in the A. afarensis post-cranial anatomy show that a life in the trees is likely far in their distant past (although clearly it it further in our past-- our body mass relative to arm length/strength is all wrong, and we do not exhibit curved finger bones as the australopithecines did).
For climbing, they no longer have an opposable phallux to allow them to grasp branches with their feet, Their arms are not long enough to wrap around a tree trunk (being closer in proportion to humans than chimps), and DeSilva has shown their ankles are no longer adapted to climbing. Take a look at the figure below, which compares the skeletal anatomies of Homo sapiens (a), H. erectus (c), Pan troglodytes, aka chimps (b), and A. afarensis (f) (from Endurance running and the evolution of Homo).
This leaves us with terrestrial locomotion. Their pelvis, knee, ankle, and big toe are all well-adapted to an upright, striding gait, and they could not have moved about as a quadruped, since their arms are simply not long enough. All of this seems to point to A. afarensis being an obligate biped.
I'm not a biological anthropologist, so don't take my word for it... but it's something to think about.
2009 DeSilva JM. Functional morphology of the ankle and the likelihood of climbing in early hominins. Proc Nat Acad Sci USA 106:6567-6572.
2004 Bramble DM, Lieberman DE Endurance running and the evolution of Homo. Nature 432:345-352.