New fossils of Australopithecus sediba reveal a nearly complete lower back

Abstract

Adaptations of the lower back to bipedalism are frequently discussed but infrequently demonstrated in early fossil hominins. Newly discovered lumbar vertebrae contribute to a near-complete lower back of Malapa Hominin 2 (MH2), offering additional insights into posture and locomotion in Australopithecus sediba. We show that MH2 possessed a lower back consistent with lumbar lordosis and other adaptations to bipedalism, including an increase in the width of intervertebral articular facets from the upper to lower lumbar column (‘pyramidal configuration’). These results contrast with some recent work on lordosis in fossil hominins, where MH2 was argued to demonstrate no appreciable lordosis (‘hypolordosis’) similar to Neandertals. Our three-dimensional geometric morphometric (3D GM) analyses show that MH2’s nearly complete middle lumbar vertebra is human-like in overall shape but its vertebral body is somewhat intermediate in shape between modern humans and great apes. Additionally, it bears long, cranially and ventrally oriented costal (transverse) processes, implying powerful trunk musculature. We interpret this combination of features to indicate that A. sediba used its lower back in both bipedal and arboreal positional behaviors, as previously suggested based on multiple lines of evidence from other parts of the skeleton and reconstructed paleobiology of A. sediba.

Inferring lumbar lordosis in Neandertals and other hominins

“…Baatstrup disease) in Shaindar 3 ( 50 ) and La Chapelle-aux-Saints 1 ( 23 ) further suggest that Neandertals demonstrated lumbar lordosis. Finally, given that sexual dimorphism in lumbar wedging is present in modern humans and potentially also in Australopithecus ( 10 , 11 ), female Neandertals would be expected to demonstrate more dorsal (lordotic) wedging than the inferred male individuals included in this study.…”

“…Outside of H. sapiens , other hominin species are also thought to have been characterized by lumbar lordosis, inferred necessarily from fossilized remains ( 5 , 10 , 11 , 19 , 20 ). Interestingly, the presence of lumbar lordosis in one of our closest relatives, Neandertals, is debated.…”

Homo naledi lumbar vertebrae and a new 3D method to quantify vertebral wedging

“…88‐232‐234/153, U.W. 88‐126/138 ( A. sediba ) (Williams et al, 2021), SK 3981b ( Paranthropus robustus or early Homo ), A.L. 288‐1aa, A.L.…”

“…Hominins evolved lumbar lordosis, or dorsal concavity of the lower lumbar column, as an adaptation to bipedalism at some point relatively early in their evolutionary history (Been et al, 2012; Latimer & Ward, 1993; Whitcome et al, 2007; Williams et al, 2013; Williams et al, 2021). Lumbar lordosis allows an upright biped to efficiently balance the upper body over the hip joints and lower limbs.…”

African apes and the evolutionary history of orthogrady and bipedalism

“…It is also possible that early hominins retained knuckle‐walking features, either as part of an active locomotor repertoire that potentially also included facultative bipedalism, or as primitive retentions. Indeed, hominins seem to have retained adaptations to arboreality for several million years (Stern & Susman, 1983; Susman et al, 1984; Lovejoy, Latimer, et al 2009; Lovejoy, Simpson, et al 2009; Lovejoy, Suwa, Simpson, et al 2009; Lovejoy, Suwa, Spurlock, et al 2009; White, Asfaw, et al, 2009; White et al, 2015; Rein et al, 2017; Dunmore et al, 2020; Georgiou et al, 2020; Prabhat et al, 2021; Prang et al, 2021; Williams et al, 2021).…”