New vertebral and rib material point to modern bauplan of the Nariokotome Homo erectus skeleton

Abstract

The double S shape of the vertebral column is one of the most important evolutionary adaptations to human bipedal locomotion, providing an optimal compromise between stability and mobility. It is commonly believed that a six element long lumbar spine facilitated the critical adoption of lumbar lordosis in early hominins, which contrasts with five lumbars in modern humans and four in chimpanzees and gorillas. This is mainly based on the juvenile Homo erectus skeleton KNM-WT 15000 from Nariokotome, Kenya. Yet, the biomechanical advantage of a long lumbar spine is speculative. Here we present new vertebral and rib fragments of KNM-WT 15000. They demonstrate that the sixth to the last presacral vertebra possesses rib facets and therefore indicate the presence of only five lumbar and twelve thoracic segments, as is characteristic of modern humans. Moreover, they show that no additional element was located between the sixth to the last presacral vertebra and Th11 as suggested in the original description. The transition from thoracic to lumbar type orientation of the facet joints that takes place at Th11 is thus at the same segment as in over 40% of modern humans, suggesting an identical lumbar mobility and capacity for lordosis. Taken together, KNM-WT 15000 had one vertebra less than previously thought irrespective of whether rib-free lumbar vertebrae or vertebrae that bear lumbar-like articular processes are counted. Furthermore, the new rib fragments imply a rearrangement of the ribs that results in a symmetrical rib cage. This challenges previous claims for idiopathic or congenital scoliosis. We conclude that the bauplan of the hominin axial skeleton is more conservative than previously thought.

Revisiting scoliosis in the KNM-WT 15000 Homo erectus skeleton

“…Furthermore, in contrast to Latimer and Ohman's (2001) assertion, extra-spinal asymmetries in the rib cage are absent in the Nariokotome boy skeleton (Haeusler et al, 2011), and those in the clavicles and hipbones are not characteristic of scoliosis.…”

“…Importantly, these alleged rib distortions disappear after a minor rearrangement of the ribs. This rearrangement is supported by recently described new rib fragments of KNM-WT 15000, and it makes his thoracic shape completely symmetric (Haeusler et al, 2011). On the other hand, while asymmetry of the three-dimensional shape of the clavicles is frequent in the normal human population (Abdel Fatah et al, 2012), clavicular length differs but by 0.1 mm in KNM-WT 15000 (Walker and Leakey, 1993b).…”

“…The fact that this is the only significant asymmetry in the midthoracic region strongly suggests that the abnormal difference between left and right pedicle thickness of T7 is due to a measurement error. Moreover, calliper-based measurements of minimal pedicle thickness do not suggest a marked asymmetry at T7 (Haeusler et al, 2011).…”

“…The axial skeleton preserves the last cervical vertebrae, ten thoracic vertebrae and twelve pairs of ribs, all five lumbar vertebrae, and five sacral elements. Our identification of the vertebrae is according to Haeusler et al (2011;see also Haeusler et al, 2002;Haeusler et al, 2012;Williams, 2012).…”

“…There is only one review that affirmed the diagnosis of adolescent idiopathic scoliosis based on the observation of "both thoracic asymmetry and vertebral fracture" (Lovejoy, 2005: 103). Yet vertebral fracture is not known to lead to idiopathic scoliosis (Weinstein et al, 2008), and a recent analysis of the KNM-WT 15000 axial skeleton found no evidence for fractured vertebrae Scoliosis in the Nariokotome boy skeleton reassessed 4 (Haeusler et al, 2011). Moreover, no substantial asymmetry of the rib cage remains after a realignment of the ribs (Haeusler et al, 2011).…”