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 differs substantially from that of humans.... Many key human specializations are related to our reproductive physiology and anatomy; human reproduction is as extraordinary as our dentition, locomotion, and encephalization...Although it remains possible that such uniqueness emerged only during the Pleistocene, this is less likely in light of Ardipithecus, which shows very early evidence of a major social transformation...Moreover, it is the modern African apes that are most derived in many characters, whereas those which are specialized in human evolution (SCC elimination, bipedality) are now known to have been present near the origin of our clade. Our massive brains are obviously a Pleistocene development, but they are also probably sequelae to other major shifts now more fully recorded in the earlier fossil record. It is therefore possible, even likely, that many physiologies and soft tissue features that do not fossilize may have also evolved early in hominid evolution. If so, why were these characters exaptive to our advanced cognition and singular demographic success?
In other words, the common ancestor of chimpanzees & humans (which Ardipithecus ramidus may be in the line of) might have resembled humans in gross morphology, and perhaps social structure & behavior, more than chimpanzees. This is not totally counterintuitive, chimpanzees are relatively specialized frugivores. By contrast Homo erectus had already spread out across Eurasia and Africa, indicating a level of generality in adaptation and lifestyle. If Lovejoy and his fellow travelers are correct, books such as Demonic Males: Apes and the Origins of Human Violence are far less plausible by the analogies they draw, and Franz De Waal's body of work should be viewed with much more skepticism in terms of the generalizations that he, as a primatologist focusing on great apes, can make about the human condition. A similar problem has cropped up in using modern hunter-gatherer populations as analogs with ancient hunter-gatherer populations. Naturally the groups which managed to remain hunter-gatherers down to the present time are not likely to be a random and representative sample of pre-modern hunter-gatherers (e.g., they tend to inhabit extremely marginal territory). Compare the peoples of the Kalahari with social structures of the Pacific Northwest tribes. Citation: Reexamining Human Origins in Light of Ardipithecus ramidus, C. Owen Lovejoy (2 October 2009) Science 326 (5949), 74. [DOI: 10.1126/science.1175834]