Reexamining human origins in light of Ardipithecus ramidus.

Referential models based on extant African apes have dominated reconstructions of early human evolution since Darwin's time. These models visualize fundamental human behaviors as intensifications of behaviors observed in living chimpanzees and/or gorillas (for instance, upright feeding, male dominance displays, tool use, culture, hunting, and warfare). Ardipithecus essentially falsifies such models, because extant apes are highly derived relative to our last common ancestors. Moreover, uniquely derived hominid characters, especially those of locomotion and canine reduction, appear to have emerged shortly after the hominid/chimpanzee divergence. Hence, Ardipithecus provides a new window through which to view our clade's earliest evolution and its ecological context. Early hominids and extant apes are remarkably divergent in many cardinal characters. We can no longer rely on homologies with African apes for accounts of our origins and must turn instead to general evolutionary theory. A proposed adaptive suite for the emergence of Ardipithecus from the last common ancestor that we shared with chimpanzees accounts for these principal ape/human differences, as well as the marked demographic success and cognitive efflorescence of later Plio-Pleistocene hominids.