Modern or distinct axial bauplan in early hominins? Comments on

Abstract

The vertebral column plays a central role in the evolution and performance of positional behaviors, including upright posture and bipedal locomotion in the human lineage. The lumbar column, in particular, is associated with locomotor function. As such, its numerical composition has been a major source of contention in the paleoanthropological literature. Ever since Robinson’s (1972) description and interpretation of the nearly complete thoracolumbar vertebral column of Sts 14 (Australopithecus africanus), researchers have, with few exceptions, consistently stated that early hominins possessed six lumbar vertebrae (Benade, 1990; Latimer and Ward, 1993; Shapiro, 1993; Walker and Leakey, 1993; Sanders, 1995, 1998; Tobias, 1998; Pilbeam, 2004; Rosenman, 2008; McCollum et al., 2010). In 2002, Haeusler et al. demonstrated that these reconstructions were incorrect because they were based on aberrant vertebral morphologies, conflation of multiple definitions (costal versus zygapophyseal) of thoracic and lumbar vertebrae, and the fragmentary nature and associated uncertainty of consecutiveness of fossil vertebral elements in general (see also Williams, 2011). In a new study, Haeusler et al. (2011) describe newly identified vertebra and rib fragments associated with the KNM-WT 15000 juvenile Homo erectus skeleton that reinforce their previous contention (Haeusler et al., 2002) that this specimen has five instead of six lumbar vertebrae, a finding consistent with recent reconstructions of the A. africanus specimens Sts 14 and Stw 431