Some of the many questions to do with human evolution centre on our way of getting around - just when did bipedalism evolve? Is bipedalism a derived state, found only in our own twig of the primate family tree? Or is it a feature seen in the last common ancestor that we share with our sister species, chimpanzees?
Bipedalism is certainly a feature that evolved fairly early in human history. A tibia attributed to Australopithecus anamensis is around 4 million years old, & indicates that this species was bipedal. Orrorin and Sahelanthropus are older, at around 6-7mya, and may also have walked on 2 legs at least part of the time, although the evidence here is more tentative. In the case of Sahelanthropus, it's based on a virtual reconstruction of the cranium that suggests the foramen magnum was further forwards underneath the skull than is the case in the great apes.
While things like the position of the foramen magnum, the form of the tibia where it articulates in the knee, and the presence of a valgus angle are all indications that an organism is bipedal, we shouldn't forget the ankle joint. In a just-published paper, Jeremy DeSilva has examined the form & function of the ankle in a variety of species and used this to draw some inferences about locomotion in early hominins.
DeSilva looked at what was going on in the foot and ankle of wild chimpanzees that were filmed as they climbed vertically up tree trunks, & then related this to skeletal features of apes' feet. Then he compared the skeletal data with measurements from early hominins.
He found that there's a great deal of flexion in chimps' ankles as they climb vertically: the upper surface of the foot flexes towards the shin by up to 45o. The same thing's been recorded in the other great apes, & seems to be unique to them as dorsiflexion in climbing monkeys is in the range 15-25o. As for humans, our ankles aren't capable of anything like that range of movement: as DeSilva comments, dorsiflexing the human ankle to 45o results in soft-tissue failure and severe injury. A climbing chimp will also 'invert' each foot in turn, turning the sole of the foot inwards by between 15 & 25o to increase contact with the treetrunk. (& I know from personal experience that the human ankle can't take too much of that, either!)
The sort of flexion seen in chimp ankles must place considerable stress on the bones of that joint, which would in turn affect the bones' structure. And you could predict that ankle morphology would be different in humans, with their much decreased range of (normal) movement in the joint. DeSilva examined the relevant bones (tibia & talus) in great apes, humans, and extinct hominins ranging from A. anamensis to Homo erectus, & found that this was indeed the case. The ankle end of a modern human has a fairly square profile, compared to the elongated joint of a chimp. And - the fossil tibiae show the modern human profile, which suggests that they were better adapted to upright walking than to climbing around in trees. Similarly, measurements of the angle between the upper surface of the talus (the bone that articulates with the tibia) and the tibia's vertical axis show that in sapiens, erectus, and A. anamensis & africanus, the tibia is perpendicular to the talus (related to full bipedalism). But in chimps the tibia is oblique to the plane of the ankle. This is significant because at least some early hominin fossils have features (including curved fingers & toes) that have been interpreted as belonging to organisms that did a lot of climbing.
However, the results don't rule out such an arboreal lifestyle, and DeSilva warns that, although these results are interesting, they should still be treated with caution. On the basis of these data, if early hominins were engaging in any substantial amount of arboreal climbing, then they were doing it in a manner kinematically distinct from modern chimpanzees. Caution should thus be used when employing the chimpanzee as a model for understanding locomotion in the early hominins.
DeSilva, J.M. (2009) Functional morphology of the ankle and the likelihood of climbing in early hominins. Proceedings of the National Academy of Sciences 106(16): 6567-6572. doi/10.73/pnas.0900270106