Relative limb strength and locomotion in homo habilis

The Homo habilis OH 62 partial skeleton has played an important, although controversial role in interpretations of early Homo locomotor behavior. Past interpretive problems stemmed from uncertain bone length estimates and comparisons using external bone breadth proportions, which do not clearly distinguish between modern humans and apes. Here, true cross-sectional bone strength measurements of the OH 62 femur and humerus are compared with those of modern humans and chimpanzees, as well as two early H. erectus specimens-KNM-WT 15000 and KNM-ER 1808. The comparative sections include two locations in the femur and two in the humerus in order to encompass the range of possible section positions in the OH 62 specimens. For each combination of section locations, femoral to humeral strength proportions of OH 62 fall below the Obligate terrestrial bipedalism is one of the key characteristics defining modern humans and distinguishing them from other primates. Although evidence for some form of terrestrial bipedal locomotion appears in the hominin fossil record perhaps as far back as 6-7 million years (Ma) ago (Senut et al., 2001; Zollikofer et al., 2005; Richmond and Jungers, 2008), there is also evidence for significant variation in the locomotor repertoires of early hominins and the extent to which they retained (or evolved) adaptations for arboreal locomotion (Johanson et al., 1987; Leakey et al., 1989, 1998; Hartwig-Scherer and Martin, 1991; Heinrich et al., 1993; Spoor et al., 1994; McHenry and Berger, 1998; Asfaw et al., 1999; Stern, 2000; Ward et al., 2001; Harcourt-Smith and Aiello, 2004; Green et al., 2007; Haeusler and McHenry, 2007). However, interpretations have been hampered by a paucity of fossil specimens with associated crania and postcrania (and thus, uncertainty regarding the taxonomic attribution of available postcrania), large error ranges in reconstructions of incomplete specimens, and disagreements regarding the functional significance of particular morphological traits (e.g., Korey, 1990; Stern, 2000; Ward et al., 2001; Richmond et al., 2002; Haeusler and McHenry, 2004; Reno et al., 2005; Wood and Constantino, 2007). The OH 62 partial skeleton has figured prominently in such debates. It is the only definitive Homo habilis (sensu stricto) (Wood, 1992) specimen with securely associated and relatively well-preserved cranial and postcranial material, the latter including both forelimb and hind limb elements. OH 62 was discovered in 1986 in Olduvai Gorge, Tanzania, and is dated to about 1.8 Ma (Johanson et al., 1987). It consists of portions of the skull, the proximal half of the left femoral shaft and neck, a small proximal portion of the right tibia, almost all of the right humeral shaft, part of the right radial shaft, and much of the right ulna. On the basis of simi 95% confidence interval of modern humans, and for most comparisons, within the 95% confidence interval of chimpanzees. In contrast, the two H. erectus specimens both fall within or even above the modern human distributions. This indicates that load distribution between the limbs, and by implication, locomotor behavior, was significantly different in H. habilis from that of H. erectus and modern humans. When considered with other postcranial evidence, the most likely interpretation is that H. habilis, although bipedal when terrestrial, still engaged in frequent arboreal behavior, while H. erectus was a completely committed terrestrial biped. This adds to the evidence that H. habilis (sensu stricto) and H. erectus represent ecologically distinct, parallel lineages during the early Pleistocene.