Significant changes occur in human evolution between 2.5 and 1.8 mya. Stone tools first appear, brains expand, bodies enlarge, sexual dimorphism in body size decreases, limb proportions change, cheek-teeth reduce, and crania begin to share more unique features with later Homo. Although the two earliest species of Homo, H. habilis and H. rudolfensis, retain many primitive features in common with australopithecine species, they both share key unique features with later species of Homo. Two of the most conspicuous shared derived characters are the sizes of the brain and masticatory apparatus relative to body weight. Scaling features to body weight is essential because species of early hominid vary so substantially in overall mass that unscaled characters may cause incorrect assessments of relationship. One unexpected complication in the transition from australopithecine to Homo is the apparent fact that the postcranial anatomy of H. habilis retained many australopithecine characteristics in contrast to the more human-like body-plan of H. rudolfensis and all later species of Homo.

(from the abstract of McHenry, H.M. and Coffing, K. (2000) Australopithecus to Homo: Transformations in Body and Mind. Annual Review of Anthropology 29: in press)