Inferring lumbar lordosis in Neandertals and other hominins

Abstract

Lumbar lordosis is a key adaptation to bipedal locomotion in the human lineage. Dorsoventral spinal curvatures enable the body's center of mass to be positioned above the hip, knee, and ankle joints, and minimize the muscular effort required for postural control and locomotion. Previous studies have suggested that Neandertals had less lordotic (ventrally convex) lumbar columns than modern humans, which contributed to historical perceptions of postural and locomotor differences between the two groups. Quantifying lower back curvature in extinct hominins is entirely reliant upon bony correlates of overall lordosis, since the latter is significantly influenced by soft tissue structures (e.g. intervertebral discs). Here, we investigate sexual dimorphism, ancestry, and lifestyle effects on lumbar vertebral body wedging and inferior articular facet angulation, two features previously shown to be significantly correlated with overall lordosis in living individuals, in a large sample of modern humans and Neandertals. Our results demonstrate significant differences between postindustrial cadaveric remains and archaeological samples of people that lived preindustrial lifestyles. We suggest these differences are related to activity and other aspects of lifestyle rather than innate population (ancestry) differences. Neandertal bony correlates of lumbar lordosis are significantly different from all human samples except preindustrial males. Therefore, although Neandertals demonstrate more bony kyphotic wedging than most modern humans, we cast doubt on proposed locomotor and postural differences between the two lineages based on inferred lumbar lordosis (or lack thereof), and we recommend future research compare fossils to modern humans from varied populations and not just recent, postindustrial samples.

Fig. 1.

Measurements taken on lumbar vertebrae and three types of WAs. (A) Depictions of the three measurements collected for this study: superior vertebral body dorsoventral length (SBL), ventral vertebral body superior–inferior height (VBH), dorsal vertebral body superior–inferior height (DBH). (B) Ventral (kyphotic) wedging (positive WAs), neutral wedging (WA ∼0°), and dorsal (lordotic) wedging (negative WAs).

Fig. 2.

Sum of lumbar vertebral body WAs (∑WA) in modern humans and fossil hominins. The sex-specific postindustrial and preindustrial samples differ significantly, with postindustrial samples demonstrating more dorsal (lordotic) wedging than preindustrial samples. The female preindustrial and male postindustrial samples do not differ significantly. The small male Neandertal sample differs significantly from all but the male preindustrial sample. The shaded areas represent the 95% CIs of the means.

Fig. 3.

Sum of lumbar inferior APs (∑AP) in modern humans and fossil hominins. The sex-specific preindustrial and postindustrial samples differ significantly, with postindustrial samples demonstrating higher angles (more dorsal projection of the inferior articular processes) than preindustrial samples. Sexes within samples do not differ significantly, nor do female preindustrial and male postindustrial samples. The small male Neandertal sample differs significantly from all but the male preindustrial sample. The shaded areas represent the 95% CIs of the means.

Fig. 4.

Neandertal vertebral body WA plotted with the male preindustrial modern human sample (black circles with 95% PIs of the means shown with shading). Kebara 2 (red hexagons), Shanidar 3 (blue squares), and La Chapelle-aux-Saints 1 (yellow stars) are not distinct in magnitude or pattern of WAs than male preindustrial modern humans. The dashed lines outline a zone of relatively neutral wedging (−1° to 1°). La Chapelle-aux-Saints 1 does not preserve a second lumbar vertebra; therefore, a line is used to connect the L1 and L3 data points.

Fig. 5.

Comparison of lower lumbar vertebrae of nonambulatory and normal walking adolescent modern humans. The nonambulatory individual (solid tracing) attained a similar vertebral height as the normal walking individual (dashed tracing) but did not gain a similar degree of dorsoventral expansion through ontogeny as the normal walking individual. The lumbar vertebra of the nonambulatory individual also presents only a slight dorsal wedge, whereas the normal individual demonstrates significant dorsal (lordotic) wedging. The ontogeny of vertebral body shape, therefore, allows for the plasticity of vertebral wedging. Modified from ref. (37).