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In the Light of Evolution Volume IV: The Human Condition
ern Homo grade (Wood and Collard, 1999). The taxon H. habilis Leakey, Tobias, and Napier 1964 was introduced for fossils recovered from Olduvai Gorge, Tanzania. The rest of the H. habilis hypodigm consists of other fossils found at Olduvai Gorge and of fossils from Ethiopia (Omo Shungura and Hadar) and Kenya (Koobi Fora and perhaps Chemeron). Some have claimed that there is also evidence of H. habilis in southern Africa at Sterkfontein, Swartkrans, and Drimolen. The H. habilis hypodigm consists of mostly cranial and dental evidence; only a few postcranial bones can be confidently assigned to that taxon (see below). The endocranial volume of H. habilis ranges from ca. 500 cm3 to ca. 700 cm3, but most commentators opt for an upper limit closer to 600 cm3. All of the crania are wider at the base than across the vault, but the face is broadest in its upper part. The only postcranial fossils that can be assigned to H. habilis with confidence are the postcranial bones associated with the type specimen, OH 7, and the associated skeleton, OH 62; isolated postcranial bones from Olduvai Gorge assigned to H. habilis (e.g., OH 10) could also belong to P. boisei (see below). If OH 62 is representative of H. habilis the skeletal evidence suggests that its limb proportions and locomotion (Ruff, 2009b) and carpal bones (Tocheri et al., 2007) were archaic hominin-like, and the curvature and well-developed muscle markings on the phalanges of OH 7 indicate that H. habilis was capable of powerful grasping. The inference that H. habilis used spoken language was based on links between endocranial morphology and language comprehension and production that are no longer supported by comparative evidence.
Some researchers suggest the transitional hominin grade contains a second taxon, H. rudolfensis (Alexeev 1986) sensu Wood 1992 (Wood, 1991), but not all researchers are convinced the scale and nature of the variation within early Homo justifies the recognition of two taxa (Tobias, 1991; Suwa et al., 1996). Its temporal range would be ca. 2.4–1.6 Ma, and aside from the lectotype KNM-ER1470 from Koobi Fora, Kenya, the members of the proposed hypodigm include other fossils recovered from Koobi Fora and those from Chemeron, Kenya, and Uraha, Malawi. Compared with H. habilis the absolute size of the brain case in H rudolfensis is greater, and its face is widest in its midpart whereas the face of H. habilis is widest superiorly. Despite the absolute size of the H. rudolfensis brain (ca. 725 cm3), when it is related to estimates of body mass based on orbit size the brain is not substantially larger than those of the archaic hominins. The distinctive face of H. rudolfensis is combined with a robust mandibular corpus and mandibular postcanine teeth with larger, broader crowns and more complex premolar root systems than those of H. habilis. At present, no postcranial remains can be reliably linked with H. rudolfensis. The size of the mandible and postcanine teeth suggests that its diet made similar mechanical demands as those of the archaic hominins (see below).