You'll recall (hopefully) Ardi, the Ardipithecus ramidus, an ancient human ancestor that's recently gotten a whole lot of media attention. Excellently pseudo-named blogger Zinjanthropus (actually a mild-mannered biological anthropology grad student) is doing a series of posts that take a close-up look at some of the biological quirks that make Ardi such a surprise.
The first post is on Ardi's hands...
The extant African apes are knuckle-walkers, they have stiff, inflexible hands and wrists that allow them to support their body weight in sort of a weird position. Because they also have to climb trees for food and protection, their hands are very long and powerful. Humans, on the other hand, have pretty mobile hands and wrists which allows us what we call a 'power grip.' We are very good graspers, and this has allowed us to become the dexterous tool-wielders that we are. Because of our close genetic similarity to chimps, and the close morphological similarity between chimps and gorillas, it has been argued that certain features of the Australopithecine wrist- and even the human wrist- were 'hold overs' from the period of time when we, too were knuckle-walkers who required a stiff wrist and hand.
However, Ardi's hand more closely approximates the human hand than the knuckle-walker hand."