News and views Modern or distinct axial bauplan in early hominins? Comments on Haeusler et al. (2011) Scott A. Williams Center for the Study of Human Origins, Department of Anthropology, New York University, 25 Waverly Place, New York, NY 10003, USA article info Article history: Received 6 October 2011 Accepted 18 January 2012 Available online 11 March 2012 Keywords: Diaphragmatic vertebra Australopithecus Homo erectus Modern human variation Bipedalism 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 thoraco- lumbar 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. demon- strated 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 (Haeusler et al., 2002; Toussaint et al., 2003) and Cook et al.’s (1983) prediction of five lumbar vertebrae in the A. afarensis partial skel- eton, A.L. 288-1 (but see Sanders, 1995 regarding incompleteness and uncertainty in predicting lumbar number in the latter specimen). Importantly, Haeusler et al.’s (2011) findings also confirm that, as with Sts 14 and Stw 431, the KNM-WT 15,000 diaphragmatic vertebra, the one that bears flat, posteriorly-facing, and ‘thoracic- like’ prezygapophyses and curved, laterally-directed, and ‘lumbar- like’ postzygapophyses (Fig. 1), is cranially displaced relative to the last rib-bearing vertebra (Fig. 2), a configuration that is not the norm in modern humans or in other extant hominoids (Williams, 2011). While the new material presented in Haeusler et al. (2011) helps resolve the controversial issue of the number of lumbar vertebrae in fossil hominins, I disagree with the authors’ treatment of modern human variation in the placement of the diaphragmatic vertebra and their interpretation of the functional significance of this feature in fossil hominins. Here, I briefly review the sparse and somewhat inconsistent literature on this topic and provide my own data on a large sample (N ¼ 125) of adult modern human vertebral columns and argue that: 1) comparisons among such studies are often hampered by different methodologies and/or lack of a formalized definition of zygapophyseal orientation change, 2) cranial displacement of the diaphragmatic vertebra relative to the last rib-bearing vertebra (see Fig. 2) occurs in modern humans with lower frequency, and common placement of these morphologies at greater frequency, than Haeusler et al. (2011) report, and 3) the consistent presence of six postdiaphragmatic vertebrae (i.e., presacral elements following the diaphragmatic vertebra) in early hominins is unique and likely represents a functional strategy to achieve effective lordosis in the early hominin body plan. After reviewing published data in the literature and analyzing their own sample of subadult modern humans (N ¼ 37), Haeusler et al. (2011: 580) state that cranial displacement “occurs in between 40% and 50% of all skeletons.” However, their summary of the literature is biased towards producing a higher degree of cranial displacement than probably exists biologically. First, Haeusler et al. exclude data from two datasets in their 2011 study that they E-mail address: sawilliams@nyu.edu. Contents lists available at SciVerse ScienceDirect Journal of Human Evolution journal homepage: www.elsevier.com/locate/jhevol 0047-2484/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.jhevol.2012.01.007 Journal of Human Evolution 63 (2012) 552e556