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Key Words postcranium, craniodental morphology, encephalization
Abstract Significant changes occurred in human evolution between 2.5 and 1.8 million years ago. Stone tools first appeared, brains expanded, bodies enlarged, sexual dimorphism in body size decreased, limb proportions changed, cheek teeth reduced in size, and crania began to share more unique features with later Homo. Although the two earliest species of Homo, H. habilis and H. rudolfensis, retained many primitive features in common with australopithecine species, they both shared key unique features with later species of Homo. Two of the most conspicuous shared derived characters were the sizes of the brain and masticatory apparatus relative to body weight. Despite the shared derived characters of H. habilis and H. rudolfensis, one unexpected complication in the transition from australopithecine to Homo was that the postcranial anatomy of H. habilis retained many australopithecine characteristics. H. rudolfensis, however, seems to have had a more human-like body plan, similar to later species of Homo. H. rudolfensis may therefore represent a link between Australopithecus and Homo.
INTRODUCTION
When Kamoya Kimeu discovered the early Homo skeleton KNM-WT 15000 in 1984, something that had been vaguely understood before snapped into sharper focus: The evolutionary transition from australopithecine to Homo involved not only an expansion of the brain and a reduction of the cheek teeth, but a change in walking and climbing behavior (Walker & Leakey 1993a). Arguments remain about which early Homo species gave rise to later Homo, but the origin of the genus is becoming of greater interest.
New discoveries and new analyses of Homo include three major monographs (Tobias 1991, Walker & Leakey 1993a, Wood 1991). Most paleoanthropologists (e.g. Groves 1989; Tobias 1991; Wood 1991, 1992; Skelton & McHenry 1992; Walker & Leakey 1993a; McHenry 1994c; Strait et al 1997; Asfaw et al 1999a; Klein 1999; Wolpoff 1999; Wood & Collard 1999), but not all (e.g. Oxnard 1975), agree that Homo evolved from Australopithecus, but there is less consensus on which species of Australopithecus is the most likely ancestor and which fossils are the earliest members of Homo.
The search for the immediate ancestor of Homo among known species of Australopithecus may be fruitless because all the possible candidates have unique specializations (i.e. autapomorphies). It is more useful to search for the...