Reading the papers on Ardipithecus ramidus which just came out in Science one of the take-home points that jumps out at me is that extant apes may be very misleading analogs to extinct hominins. Here is Owen Lovejoy:
In retrospect, clues to this vast divide between the evolutionary trajectories of African apes and hominids have always been present. Apes are largely inept at walking upright. They exhibit reproductive behavior and anatomy profoundly unlike those of humans. African ape males have retained (or evolved, see below) a massive SCC and exhibit little or no direct investment in their offspring (their reproductive strategies rely primarily on varying forms of male-to-male agonism). Although they excel at some cognitive tasks, they perform at levels qualitatively similar to those of some extraordinary birds...and mammals...The great apes are an isolated, uniquely specialized relict species surviving today only by their occupation of forest refugia...Even their gut structure ...
