One of the defining features of humans is our ability to walk comfortably on two legs. To achieve this, our skeletons have evolved certain physical characteristics. For example, the lower part of the human spine has a forward curve that supports an upright posture; whereas the lower backs of chimpanzees and other apes – which walk around on four limbs and spend much of their time in trees – lack this curvature. Studying the fossilized back bones of ancient human remains can help us to understand how we evolved these features, and whether our ancestors moved in a similar way.

Australopithecus sediba was a close-relative of modern humans that lived about two million years ago. In 2008, fossils from an adult female were discovered at a cave site in South Africa called Malapa. However, the fossils of the lower back region were incomplete, so it was unclear whether the female – referred to as Malapa Hominin 2 (MH2) – had a forward-curving spine and other adaptations needed to walk on two legs.

Here, Williams et al. report the discovery of new A. sediba fossils from Malapa. The new fossils are mainly bones from the lower back, and they fit together with the previously discovered MH2 fossils, providing a nearly complete lower spine. Analysis of the fossils suggested that MH2 would have had an upright posture and comfortably walked on two legs, and the curvature of their lower back was similar to modern females. However, other aspects of the bones’ shape suggest that as well as walking, A. sediba probably spent a significant amount of time climbing in trees.

The findings of Williams et al. provide new insights in to our evolutionary history, and ultimately, our place in the natural world around us. Our lower back is prone to injury and pain associated with posture, pregnancy and exercise (or lack thereof). Therefore, understanding how the lower back evolved may help us to learn how to prevent injuries and maintain a healthy back.

Bipedal locomotion is thought to be one of the earliest and most extensive adaptations in the hominin lineage, potentially evolving initially 6–7 million years (Ma) ago. Human-like bipedalism evolved gradually, however, and early hominins appear to have been facultative bipeds on the ground and competent climbers in the trees (Senut et al., 2001; White et al., 2015; Prang, 2019; Prang et al., 2021). How long climbing adaptations persisted in hominins and when adaptations to obligate terrestrial bipedalism evolved are major outstanding questions in paleoanthropology. Australopithecus sediba – an early Pleistocene (~2 Ma) hominin from the site of Malapa, Gauteng province, South Africa – has featured prominently in these discussions, as well as those concerning the origins of the genus Homo (Berger et al., 2010; Berger, 2012; Irish et al., 2013; Dembo et al., 2015; Kimbel and Rak, 2017; De Ruiter et al., 2018; Williams et al., 2018a; Du and Alemseged, 2019).

Previous studies support the hypothesis that A. sediba possessed adaptations to arboreal locomotion and lacked traits reflecting a form of obligate terrestriality observed in later hominins (Schmid et al., 2013; Prang, 2015a; Prang, 2015b; Prang, 2016; Holliday et al., 2018). Malapa Hominin 2 (MH2) metacarpals are characterized by trabecular density most similar to orangutans, which suggests power grasping capabilities (Dunmore et al., 2020), and the MH2 ulna was estimated to reflect a high proportion of forelimb suspension in the locomotor repertoire of A. sediba (Rein et al., 2017). Evidence from the lower limb also suggests that A. sediba lacked a robust calcaneal tuber (Prang, 2015a) and a longitudinal arch (Prang, 2015b), both thought to be adaptations to obligate, human-like bipedalism, and demonstrates evidence for a mobile subtalar joint proposed to be adaptively significant for vertical climbing and other arboreal locomotor behaviors (Prang, 2016; DeSilva et al., 2013; Zipfel et al., 2011; DeSilva et al., 2018). The upper thorax (Schmid et al., 2013), scapula (Churchill et al., 2013; Churchill et al., 2018), and cervical vertebrae (Meyer et al., 2017) of A. sediba suggest shoulder and arm elevation indicative of arboreal positional behaviors requiring overhead arm positions, and the limb joint size proportions are ape-like (Prabhat et al., 2021). Furthermore, analysis of dental calculus from Malapa Hominin 1 (MH1) indicates that this individual’s diet was high in C₃ plants like fruit and leaves, similar to savannah chimpanzees and Ardipithecus ramidus (Henry et al., 2012).

Despite the presence of climbing adaptations, A. sediba also demonstrates clear evidence for bipedal locomotion. The knee and ankle possess human-like adaptations to bipedalism, demonstrating a valgus angle of the femur and a human-like ankle joint (Zipfel et al., 2011; DeSilva et al., 2013; DeSilva et al., 2018). Evidence for strong dorsal (lordotic) wedging of the two lower lumbar vertebrae suggests the presence of a lordotic (ventrally convex) lower back (Williams et al., 2013; Williams et al., 2018b). However, the initial recovery of just the last two lumbar vertebrae of MH2 limited interpretations of spinal curvature, and a study of the MH2 pelvis reconstruction (Kibii et al., 2011) suggests that A. sediba was characterized by a small lordosis angle estimated from calculated pelvic incidence (Been et al., 2014). A separate pelvis reconstruction of MH2 produces a pelvic incidence angle more in line with other hominins (Tardieu et al., 2017). The presence of a long, mobile lower back and a Homo-like lower thorax morphology indicating the presence of a waist further suggest bipedal adaptations in A. sediba (Schmid et al., 2013; Williams et al., 2013). However, missing and incomplete lumbar vertebrae prevented comparative analysis of overall lower back morphology and allowed only limited interpretations of A. sediba back posture and implications for positional behavior.

Here, we report the discovery of portions of four lumbar vertebrae from two ex situ breccia blocks that were excavated from an early 20th century mining road and dump at Malapa. The former mining road is represented by a trackway located in the northern section of the site approximately 2 m north of the main pit that yielded the original A. sediba finds (Dirks et al., 2010; Figure 1). The trackway traverses the site in an east-west direction and was constructed using breccia and soil removed from the main pit by the historic limestone miners. Specimens U.W.88–232, −233,–234, and –281 were recovered in 2015 from the upper section of layer 2 (at a depth of 10 cm) and formed part of the foundation layer of the mining road. The trackway can be distinguished from the surrounding deposits by a section of compacted soil (comprising quartz, cherts, and flowstone) and breccia that extends between layers 1 and 2. Breccia recovered from the trackway, including the block containing U.W.88–232, −233,–234, and –281 similarly presented with quantities of embedded quartz fragments and grains. The breccia block containing specimen U.W.88–280, along with U.W.88–43, –44, and –114 (Williams et al., 2013; Williams et al., 2018b), were recovered from the miner’s dump comprised of excess material (soil and breccia) used for the construction of the miner’s road. The composition of the road matrix and associated breccia, as well as the breccia initially recovered from the mine dump, corresponds to the facies D and E identified in the main pit (Dirks et al., 2010). Facies D includes a fossil-rich breccia deposit that contained the fossil material associated with MH2 (Dirks et al., 2010; Val et al., 2018). Therefore, the geological evidence suggests that the material used for the construction of the miner’s road was sourced on-site, and most probably originated from the northern section of the main pit.

Figure 1

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The newly discovered vertebrae (second and third lumbar) are preserved in articulation with each other (Figure 2, Figure 2—figure supplement 1) and refit at multiple contacts with the previously known penultimate (fourth) lumbar vertebra (Figure 3). Together, the new and previously known (Williams et al., 2013; Williams et al., 2018b) vertebral elements form a continuous series from the antepenultimate thoracic vertebra through the fifth sacral element, with only the first lumbar vertebra missing major components of morphology (Figure 3—figure supplement 1). The presence of a nearly complete lower back of MH2 allows us to more comprehensively evaluate the functional morphology and evolution of purported adaptations to bipedalism in A. sediba and test the hypotheses that the following fundamental features are similar to modern humans (Homo sapiens) and distinct from extant great apes: (1) lumbar lordosis, (2) progressive widening of the articular facets and laminae (pyramidal configuration) of the lower back, and (3) overall middle lumbar vertebra shape. Specifically, for these hypotheses, we predict that measurements of combined lumbar wedging (representing degree of lordosis ascertained from available lumbar vertebrae) will fall within the human range (H1), that the configuration of the articular facets and laminae will progressively widen caudally (rather than remaining constant or becoming increasingly narrow) as seen in modern humans (H2), and that the most complete lumbar vertebra of MH2 (U.W.88–233) will fall within the human range of variation in shape analyses (H3).

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The five new fossils, U.W.88–232, U.W.88–233, U.W.88–234, U.W.88–280, and U.W.88–281, are described below and shown in Figure 4. Measurements are included in Table 1. A depiction of the anatomical features mentioned in the descriptions below and throughout the manuscript is shown in Figure 4—figure supplement 1.

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Wedging angles and inferred lumbar lordosis

Wedging of articulated vertebrae contribute to the multiple sagittal curvatures of the human spine, with dorsal wedging of lower lumbar vertebrae contributing to a ventrally convex curvature of the lumbar spine (lumbar lordosis). This sinusoidal configuration passively balances the upper body over the pelvis and allows for the unique system of weight bearing and force transmission found in members of the human lineage (Davis, 1961; Robinson, 1972; Pal and Routal, 1987; Latimer and Ward, 1993; Shapiro, 1993a; Lovejoy, 2005; Whitcome et al., 2007; Masharawi et al., 2010; Been et al., 2014; Tardieu et al., 2017). Wedging angles for individual lumbar vertebrae (L2-L5) and combined L2-L5 wedging were calculated for A. sediba and the comparative sample and are presented in (Figure 5, Figure 5—figure supplement 1, Figure 5—figure supplement 2) and Table 2 and Table 3. MH2 possesses the greatest (i.e., most negative) combined wedging value of any adult early hominin (–6.8°). Although all fossil hominins fall within the 95% prediction intervals of modern humans, only MH2 falls outside the 95% prediction intervals of great apes in combined L2-L5 wedging (Figure 5).

Table 2

Level	Group/fossil	Human ♂ (48)	Human ♀ (31)	Pan (43)	Gorilla (31)	Pongo (10)
L2	Mean (stdev)	4.4 (3.3)	2.1 (2.1)	4.6 (3.0)	2.3 (2.7)	5.3 (4.7)
	95% PI	–2.1, 10.9	–2.0, 6.2	–1.3,10.5	–3.0, 7.6	–3.9, 14.5
	Min, max	–3.4, 12.8	–2.1, 6.0	–1.1, 12.4	–4.0, 8.3	–0.4, 14.4
L3	Mean (stdev)	2.4 (3.1)	1.3 (2.6)	4.5 (3.1)	2.6 (2.0)	6.1 (2.0)
	95% PI	–3.7, 8.5	–3.8, 6.4	–1.6, 10.6	–1.3, 6.5	2.2, 10.0
	Min, max	–4.8, 11.2	–3.4, 6.6	–2.4, 10.9	–2.2, 7.5	3.2, 8.6
L4	Mean (stdev)	–0.5 (2.8)	–1.5 (2.9)	3.5 (3.4)	1.3 (2.5)	4.8 (3.8)
	95% PI	–6.0, 5.0	–7.2, 4.2	–3.2, 10.2	–3.6, 6.2	–2.6, 12.2
	Min, max	–8.3, 4.8	–7.9, 4.6	–3.6, 10.9	–3.9, 7.1	–1.0, 11.3
L5	Mean (stdev)	–5.9 (2.9)	–6.5 (3.1)	–0.5 (2.8)	–0.8 (2.2)	2.0 (4.1)
	95% PI	−11.6,–0.2	−12.6,–0.4	–6.0, 5.0	–5.1, 3.5	–6.0, 10.0
	Min,max	–11.6, 1.8	–12.3, 2.2	–5.9, 7.0	–6.2, 3.5	–5.2, 8.0
Sum	Mean (stdev)	0.4 (7.8)	–4.9 (8.1)	12.1 (8.8)	5.4 (5.7)	18.1 (9.5)
	95% PI	–15.0, 15.7	–20.8, 11.0	–5.1, 29.3	–5.8, 16.6	8.6, 27.6
	Min, max	–15.6, 15.5	–18.2, 11.2	–5.2, 31.7	–8.1, 15.7	0.04, 36.0

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Lumbar wedging angles and combined wedging angles values of extant taxa (Excel file).

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Table 3

	L2	L3	L4	L5	Combined
Kebara 2	8.1	6.9	4.5	–10.6	8.9
Shanidar 3	8.0	5.1	0.1	–4.9	8.3
La Chapelle-aux-Saints 1	–	4.7	0.0	–7.8	–
KNM-WT 15000	–	–	–8.3	–11.8	–
LES1	3.0	–	–	–	–
SK 3981b	–	–	–	–3.5	–
MH2	4.1	1.4	–1.6	–11.2	–7.3
Sts 14	2.3	1.7	–0.9	–6.9	–3.8
StW 431	2.0	2.3	0.9	–4.2	1.1
StW 8	5.2	3.6	–	–	–
StW 572	4.8	–	–	–	–
StW 656/600	–	4.2	–	–6.2	–
A.L. 288–1	–	7.2	–	–	–
A.L. 333–73	–	2.8	–	–	–

Patterns of change across lumbar levels demonstrate that MH2’s vertebrae transition from ventral (kyphotic) to dorsal (lordotic) wedging between the L3 and L4 levels; however, all adult fossil hominins fall within the 95% prediction intervals of modern humans (Figure 5, Figure 5—figure supplement 1). As shown previously (Williams et al., 2013), the last lumbar vertebra of MH2 is strongly dorsally wedged like that of the Kebara 2 Neandertal and the juvenile specimen KNM-WT 15000, whereas other fossil hominins do not demonstrate this pattern. Although vertebral wedging is characterized by high levels of variation within groups, especially in combined L2-L5 wedging (Figure 5, Table 2), the pattern of lumbar wedging angles observed in MH2 (i.e., transition from penultimate to ultimate lumbar level) and its combined L2-L5 wedging fall within the modern human 95% PIs and outside those of great apes (Figure 5, Figure 5—figure supplement 1, Figure 5—figure supplement 2). The hypothesis that A. sediba is human-like in lumbar wedging, therefore, cannot be rejected.

Configuration of the neural arch

The recovery of new lumbar vertebrae of MH2 allows for the quantification and comparison of inter-articular facet width increase in A. sediba. Humans are characterized by a pyramidal configuration of the articular facets such that they increase in transverse width progressively down the lumbar column (i.e., from cranial to caudal) (Latimer and Ward, 1993; Ward and Latimer, 2005). Using an index of the last lumbar-sacrum inter-articular maximum distance relative to that of lumbar vertebrae three levels higher (L2-L3 in hominins, L1-L2 in chimpanzees and gorillas), we show that Australopithecus africanus (Sts 14 and StW 431; average = 1.42) and A. sediba (1.43) fall at the low end of the range of modern human variation in this trait (Figure 6). We note that A.L. 288–1 (Australopithecus afarensis) falls at the low end of human variation near other australopiths if the preserved lumbar vertebra (A.L. 288-1aa/ak/al) is treated as an L3 (Latimer and Ward, 1993; Lovejoy, 2005; Johanson et al., 1982; Meyer et al., 2015), but outside the range of human variation and within that of orangutans if it is treated as an L2 (Cook et al., 1983). Homo erectus and Neandertals fall well within the range of modern human variation. The presence of a pyramidal configuration of the lumbar articular facets is therefore present in MH2, supporting our hypothesis that A. sediba was adapted to a human-like configuration of the neural arch.

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Inter-articular facet ratios of fossil hominins and extant taxa (Excel file).

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Middle lumbar vertebra (L3) comparative morphology

The new middle lumbar vertebra, U.W.88–233, is complete, and although the neural arch is compressed ventrally into the vertebral foramen space, it can be reasonably reconstructed from µCT data (see Materials and methods). We used three-dimensional geometric morphometrics (3D GM) to evaluate the shape affinities of U.W.88–233 among humans, great apes, and fossil hominins. The results of our principal components analysis (PCA) on Procrustes-aligned shape coordinates reveal that A. sediba falls within or near the human distribution on the first three principal components (PC1–3) (Figure 7). PC1 explains 31% of the variance in the dataset, and along it hominins are characterized by more sagittally oriented and concave superior articular facets (SAF), more dorsally oriented costal processes, a dorsoventrally shorter and cranially oriented spinous process, craniocaudally shorter, dorsoventrally longer vertebral body, and more caudally positioned SAF and IAF relative to the vertebral body compared to great apes.

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PC2 explains 13% of the variance and contrasts long spinous processes and relatively neutrally wedged (~0° ± 1°) vertebral bodies of hominins and African apes with the shorter spinous processes and strongly ventrally wedged vertebral bodies of orangutans. PC3 explains 8% of the variance and largely contrasts dorsoventrally longer vertebral bodies with caudally oriented spinous processes in gorillas with dorsoventrally shorter vertebral bodies and less caudally oriented spinous processes in chimpanzees and orangutans; hominins fall intermediate between these groups.

PC4 explains 5% of the variance, and contrasts A. sediba and A. africanus with both humans and great apes. Sts 14 and especially U.W.88–233 are characterized by longer, taller, more cranially oriented costal processes that do not taper distally and more sagittally oriented (as opposed to more coronally oriented) articular facets (Figure 7, Figure 7—figure supplement 1). We removed great apes and reran the PCA to ensure that their presence is not affecting the relationship of fossil hominins to modern humans. This hominin-only PCA essentially reproduced the results of PC4 on PC2 (Figure 7—figure supplement 2), confirming that Sts 14 and U.W.88–233 are distinct from modern humans in costal process morphology. Therefore, although the A. sediba middle lumbar vertebra is somewhat human-like in overall shape, its vertebral body is intermediate in shape between great apes and modern humans and its costal processes are robust, cranially oriented, and cranially positioned on the pedicles.

To provide a more in-depth comparison of the morphometric affinities of U.W.88–233, we plot Procrustes distances between U.W.88–233 and Sts 14, Shanidar 3, the modern human sample, and the chimpanzee sample (Figure 7—figure supplement 1A). We also show pairwise comparisons of Procrustes distances for middle lumbar vertebra shape within both human and chimpanzee samples, and between human and chimpanzee samples (Figure 7—figure supplement 1C). These analyses demonstrate that U.W.88–233 is most similar to Sts 14 among fossil and extant specimens included.

To include other fossil hominins with broken processes, we ran a second 3D GM analysis excluding the majority of costal process and spinous process landmarks. This analysis, which includes landmarks on the vertebral body, SAF and IAF, and the bases of the costal and spinous processes, produces a similar pattern compared to the analysis on the full landmark set (Figure 7). Humans and great apes separate along PC1, which is largely explained by vertebral body heights (including vertebral wedging) and SI position of the articular facets relative to the vertebral body. U.W.88–233, like other early hominins included in this study, falls intermediately between modern humans and great apes along PC1.

We used a Procrustes distance-based analysis of variance (ANOVA) to evaluate the effect of centroid size on lumbar shape (Goodall, 1991). The results show significant effects of centroid size (F = 9.83; p < 0.001), genus (with hominins pooled; F = 27.7; p < 0.001), and an interaction between genus and centroid size (F = 1.48; p = 0.01), implying unique shape allometries within genera (Table 4). We plotted standardized shape scores derived from a multivariate regression of shape on centroid size against centroid size to visualize shape changes (Drake and Klingenberg, 2008; Figure 7—figure supplement 3). In general, larger centroid sizes are associated with 3D shape changes including dorsoventrally longer and more caudally projecting spinous processes, more cranially oriented and less sagittally oriented costal processes, and less caudally projecting IAF. Importantly, however, the cranially oriented costal processes of U.W.88–233 (and Sts 14) appear not to be explained by centroid size given its relatively small size and overlap with Pan in standardized shape scores (Figure 7—figure supplement 3).

Table 4

	Df	SS	MS	R2	F	Z	Pr (>F)
Centroid size	1	0.11917	0.11917	0.0452	9.8252	5.5237	<0.0001
Genus	3	1.00793	0.33598	0.38234	27.7009	12.0794	<0.0001
Centroid size:genus	3	0.0537	0.0179	0.02037	1.4759	2.2858	0.0109
Residuals	120	1.45545	0.01213	0.55209
Total	127	2.63626

The recovery of two new lumbar vertebrae and portions of other lumbar vertebrae of the adult female A. sediba (MH2), together with previously known vertebrae, form a nearly complete lumbar column (Figure 3, Figure 3—figure supplement 1) and allows us to test hypotheses based on more limited material. As we outline below, A. sediba demonstrates evidence for lumbar lordosis in the combined pattern of bony wedging of lumbar vertebral bodies, as well as progressive widening of neural arch structures moving caudally (‘pyramidal configuration’) of lumbar vertebrae and the sacrum, which does not allow us to reject the hypotheses that A. sediba has human-like adaptations to bipedalism. However, the hypothesis that A. sediba’s middle lumbar vertebra (L3) is human-like is not fully supported: although U.W.88–233 is somewhat human-like in overall shape, its costal processes are long and cranially oriented, unlike modern humans, and its vertebral body is intermediate in shape between those of modern humans (and Neandertals) and great apes.

Williams et al., 2013, predicted strong lumbar lordosis (‘hyperlordosis’) in MH2 based on the combined wedging values of the penultimate and ultimate lumbar vertebrae. In contrast, Been et al., 2014, estimated lumbar lordosis angle using pelvic incidence from a pelvis reconstruction (Kibii et al., 2011) and found MH2 to produce the least lordotic lumbar column of the sampled members of the genus Australopithecus in their sample, falling well below modern human values and within the distribution of Neandertals. Neandertals are thought to be ‘hypolordotic’, or characterized by a relatively straight, non-lordotic lumbar column (Been et al., 2014; Been et al., 2017; but see Haeusler et al., 2019). However, Tardieu et al., 2017, report a human-like degree of pelvic incidence (and therefore lumbar lordosis) in a new reconstruction of the MH2 pelvis. Therefore, current interpretations of lumbar curvature of A. sediba range from hyperlordotic to hypolordotic. Here, we report that the pattern of vertebral wedging of MH2 and most other fossil hominins are similar to both modern humans and extant great apes except at the last lumbar level, where MH2 is markedly more dorsally wedged (Figure 5, Figure 5—figure supplement 1, Figure 5—figure supplement 2). Like the Neandertal Kebara 2, the strong dorsal (lordotic) wedging of MH2’s last lumbar vertebra is likely countering a strong ventral (kyphotic) wedging in the upper lumbar column (Figure 5). However, MH2 demonstrates much less ventral wedging than Neandertals and produces a human-like combined wedging angles value, falling outside the 95% prediction intervals of great apes. Therefore, it seems likely that MH2 and possibly the juvenile H. erectus individual KNM-WT 15000 demonstrate strong dorsal wedging at the last lumbar level for different reasons than Kebara 2. It was suggested previously that the morphology of the MH2 last lumbar is part of a kinematic chain linked to hyperpronation of the foot (DeSilva et al., 2013; Williams et al., 2013). With the absence of soft tissue contributions to the kinematic chain (i.e., intervertebral discs), formal biomechanical testing is beyond the scope of the current paper; however, our results suggest that MH2 was probably neither hypolordotic nor hyperlordotic and produces a combined wedging angles value more similar to modern humans than great apes.

Modern humans are characterized by a pyramidal configuration of the lumbar inter-articular facet joints such that the upper lumbar articular facet joints (and associated laminae) are transversely more narrowly spaced than those of the lower lumbar vertebrae and especially compared to the lumbosacral inter-articular facet joints (Latimer and Ward, 1993; Sanders, 1998; Ward and Latimer, 2005). Together with vertebral body and intervertebral disc wedging, this progressive widening facilitates the adoption of lordotic posture during ontogeny and allows for the imbrication of the IAF of a superjacent vertebra onto the laminae of the subjacent vertebra during hyperextension of the lower back (Latimer and Ward, 1993; Williams et al., 2013). Like modern humans, known fossil hominin lumbar vertebrae bear ‘imbrication pockets’, mechanically induced fossae positioned just caudal to the SAF on the lamina (Latimer and Ward, 1993; Williams et al., 2013), providing direct evidence for lumbar hyperextension and lordosis. Inadequate spacing of lower lumbar inter-articular facets in modern humans can result in spondylolysis, fracture of the pars interarticularis, and potential separation of the affected vertebra’s spinous process and inferior articular processes (Ward and Latimer, 2005). Lack of the progressive widening of inter-articular facets of lower lumbar vertebrae in our closest living relatives, the African apes, begs the question of when the pyramidal configuration evolved and to what extent various fossil hominins demonstrated this trait. Although a human-like pattern of interfacet distance was once claimed for the European late Miocene ape Oreopithecus (Köhler and Moya-Sola, 1997), Russo and Shapiro, 2013, demonstrated that measures for changes in both interfacet distance and laminar width in this extinct ape fall within ranges of extant apes (and other suspensory mammals) and outside those of humans. Among hominins, Latimer and Ward, 1993, documented the presence of a pyramidal configuration in H. erectus. It has also been demonstrated qualitatively in A. afarensis and A. africanus (Robinson, 1972; Lovejoy, 2005), and its presence in A. sediba could be inferred previously based on the articulated penultimate and ultimate lumbar vertebrae and sacrum of MH2 (Williams et al., 2013). Here, we show that MH2 and other Australopithecus specimens fall at the low end of modern human variation and differ from great apes in having significantly wider inter-articular facets at the lumbosacral junction than higher in the lumbar column (Figure 6).

The overall morphologies of lumbar vertebrae are informative with regard to locomotion and posture in primates (Slijper, 1946; Shapiro, 1993b; Sanders and Bodenbender, 1994; Granatosky et al., 2014; Williams and Russo, 2015). Hominoids are characterized by derived vertebral morphologies related to orthogrady and antipronograde positional behaviors, and early hominins have been found to largely resemble modern humans in lumbar vertebra shape, with some retained primitive morphologies (Robinson, 1972; Schmid, 1991; Shapiro, 1993a). Our 3D GM results show that the middle lumbar vertebra of A. sediba (U.W.88–233; L3) falls with modern humans (L3) to the exclusion of great apes (L2) in overall shape (Figure 7A–B). However, it bears long, cranially and ventrally oriented costal processes unlike those of modern humans (Figure 7C, Figure 7—figure supplement 1, Figure 7—figure supplement 2), and the vertebral body is somewhat intermediate in shape between modern humans and great apes (Figure 7D).

To evaluate the potential effect of centroid size in driving differences in middle lumbar vertebra shape, we ran a Procrustes distance-based ANOVA on generalized Procrustes analysis (GPA) shape scores to test whether shape differences between this fossil specimen and any extant taxon, such as H. sapiens, could be explained by differences in size (‘allometry’). Since body mass scales as the cube of linear dimensions and the physiological cross-sectional area of skeletal muscle – a major determinant of isometric force production – scales as the square of linear dimensions, larger-bodied individuals should be relatively weaker with all else held equal. Thus to compensate, we would expect to see changes in bony morphology based on differences in body size. We find that the spinous and costal processes are longer in specimens with larger centroid sizes (Figure 7—figure supplement 3). These changes would increase the moment arms of the erector spinae and quadratus lumborum muscles, respectively, resulting in greater moments that contribute to lumbar extension, ventral flexion, and lateral flexion to cope with increases in body mass. Our results suggest that, while we detect a statistically significant effect of centroid size on middle lumbar vertebral shape within each group, the differences in costal process size and orientation observed between A. sediba and modern humans appear not to be explained by size alone.

Long costal processes give the psoas major and quadratus lumborum muscles an effective leverage in acting on the vertebral column, increasing their moment arms and torque generation capabilities to assist the erector spinae in lateral flexion of the spine, back extension, and stabilizing the trunk during upright posture and bipedalism and ape-like vertical climbing (Robinson, 1972; Waters and Morris, 1972; Schmid, 1991; Sanders, 1998; Figure 7—figure supplement 2). Psoas major acts with iliacus as a powerful flexor of the thigh and trunk, while quadratus lumborum is a trunk extensor and a lateral flexor of the vertebral column and pelvis unilaterally (Robinson, 1972; Drake et al., 2019). The lumbar vertebral morphology of A. sediba, therefore, is that of a biped equipped with especially powerful trunk musculature for stabilizing the hip and back during walking and/or vertical climbing. Further work on back morphology and function in A. sediba and other early hominins is required to explore the efficacy of these possible functional explanations for the observed morphology of MH2’s lumbar vertebrae.

Previous work has shown that the adult, presumed female individual from Malapa (MH2) demonstrates clear adaptations to bipedal locomotion (Zipfel et al., 2011; DeSilva et al., 2013; DeSilva et al., 2018; Williams et al., 2013; Williams et al., 2018b), as do other Australopithecus specimens, despite their retention of features linked to suspensory behavior and other arboreal proclivities (Zipfel et al., 2011; Henry et al., 2012; Churchill et al., 2013; Churchill et al., 2018; DeSilva et al., 2013; DeSilva et al., 2018; Prang, 2015a; Prang, 2015b; Prang, 2016; Meyer et al., 2017; Rein et al., 2017; Holliday et al., 2018; Prabhat et al., 2021). The new fossils here reinforce these conclusions, signaling a lower back in MH2 as that of an upright biped equipped with powerful trunk musculature potentially used in both terrestrial and arboreal locomotion. The recovery and study of new fossil material, including juvenile material such that the ontogeny of bipedal features can be examined (Ward et al., 2017; Nalley et al., 2019), along with experimental biomechanical work and additional comparative analyses, will allow for testing hypotheses of form and function in the hominin fossil record.

All data generated or analyzed during this study are included in the manuscript and supporting files. Source data files have been provided for Figures 5 and 6 (Figure 5-source data 1, Figure 6-source data 2), and raw XYZ coordinate files for each specimen are available for download on Dryad (https://doi.org/10.5061/dryad.6m905qg0x).

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Author details

1. Scott A Williams

   1. Center for the Study of Human Origins, Department of Anthropology, New York University, New York, United States
   2. New York Consortium in Evolutionary Primatology, New York, United States
   3. Centre for the Exploration of the Deep Human Journey, University of the Witwatersrand, Johannesburg, South Africa
   4. Evolutionary Studies Institute, University of the Witwatersrand, Johannesburg, South Africa

   Contribution

   Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Methodology, Project administration, Visualization, Writing – original draft, Writing – review and editing

   For correspondence

   sawilliams@nyu.edu

   Competing interests

   No competing interests declared

   "This ORCID iD identifies the author of this article:" 0000-0001-7860-8962

2. Thomas Cody Prang

   Department of Anthropology, Texas A&M University, College Station, United States

   Contribution

   Data curation, Formal analysis, Investigation, Methodology, Visualization, Writing – original draft, Writing – review and editing

   Competing interests

   No competing interests declared

   "This ORCID iD identifies the author of this article:" 0000-0003-3032-8309

3. Marc R Meyer

   Department of Anthropology, Chaffey College, Rancho Cucamonga, United States

   Contribution

   Data curation, Formal analysis, Investigation, Methodology, Visualization, Writing – original draft, Writing – review and editing

   Competing interests

   No competing interests declared

   "This ORCID iD identifies the author of this article:" 0000-0002-3938-0173

4. Thierra K Nalley

   Western University of Health Sciences, College of Osteopathic Medicine of the Pacific, Department of Medical Anatomical Sciences, Pomona, United States

   Contribution

   Data curation, Investigation, Methodology, Visualization, Writing – review and editing

   Competing interests

   No competing interests declared

   "This ORCID iD identifies the author of this article:" 0000-0002-4296-2940

5. Renier Van Der Merwe

   Centre for the Exploration of the Deep Human Journey, University of the Witwatersrand, Johannesburg, South Africa

   Contribution

   Data curation, Writing – original draft, Writing – review and editing

   Competing interests

   No competing interests declared

6. Christopher Yelverton

   1. Evolutionary Studies Institute, University of the Witwatersrand, Johannesburg, South Africa
   2. Department of Chiropractic, Faculty of Health Sciences, University of Johannesburg, Johannesburg, South Africa

   Contribution

   Data curation, Writing – review and editing

   Competing interests

   No competing interests declared

   "This ORCID iD identifies the author of this article:" 0000-0001-5108-3641

7. Daniel García-Martínez

   1. Centre for the Exploration of the Deep Human Journey, University of the Witwatersrand, Johannesburg, South Africa
   2. Centro Nacional de Investigación sobre la Evolución Humana (CENIEH), Burgos, Spain
   3. Departamento de Biodiversidad, Ecología y Evolución, Universidad Complutense de Madrid (UCM), Madrid, Spain

   Contribution

   Data curation, Investigation, Methodology, Writing – review and editing

   Competing interests

   No competing interests declared

   "This ORCID iD identifies the author of this article:" 0000-0001-7518-3866

8. Gabrielle A Russo

   Department of Anthropology, Stony Brook University, Stony Brook, United States

   Contribution

   Formal analysis, Methodology, Writing – original draft, Writing – review and editing

   Competing interests

   No competing interests declared

   "This ORCID iD identifies the author of this article:" 0000-0002-2203-1831

9. Kelly R Ostrofsky

   Department of Anatomy, College of Osteopathic Medicine, New York Institute of Technology, Old Westbury, United States

   Contribution

   Data curation, Writing – review and editing

   Competing interests

   No competing interests declared

   "This ORCID iD identifies the author of this article:" 0000-0002-7158-546X

10. Jeffrey Spear

    1. Center for the Study of Human Origins, Department of Anthropology, New York University, New York, United States
    2. New York Consortium in Evolutionary Primatology, New York, United States

    Contribution

    Investigation, Methodology, Writing – review and editing

    Competing interests

    No competing interests declared

    "This ORCID iD identifies the author of this article:" 0000-0002-0290-7090

11. Jennifer Eyre

    1. Center for the Study of Human Origins, Department of Anthropology, New York University, New York, United States
    2. Department of Anthropology, Bryn Mawr College, Bryn Mawr, United States

    Contribution

    Data curation, Writing – review and editing

    Competing interests

    No competing interests declared

    "This ORCID iD identifies the author of this article:" 0000-0001-6418-6113

12. Mark Grabowski

    Research Centre in Evolutionary Anthropology and Palaeoecology, Liverpool John Moores University, Liverpool, United Kingdom

    Contribution

    Data curation, Methodology, Writing – review and editing

    Competing interests

    No competing interests declared

    "This ORCID iD identifies the author of this article:" 0000-0001-7045-9472

13. Shahed Nalla

    1. Centre for the Exploration of the Deep Human Journey, University of the Witwatersrand, Johannesburg, South Africa
    2. Department of Human Anatomy and Physiology, Faculty of Health Sciences, University of Johannesburg, Johannesburg, South Africa

    Contribution

    Data curation, Writing – review and editing

    Competing interests

    No competing interests declared

    "This ORCID iD identifies the author of this article:" 0000-0002-0957-1067

14. Markus Bastir

    1. Centre for the Exploration of the Deep Human Journey, University of the Witwatersrand, Johannesburg, South Africa
    2. Departamento de Paleobiología, Museo Nacional de Ciencias Naturales (CSIC), Madrid, Spain

    Contribution

    Data curation, Methodology, Writing – review and editing

    Competing interests

    No competing interests declared

    "This ORCID iD identifies the author of this article:" 0000-0002-3141-3401

15. Peter Schmid

    1. Centre for the Exploration of the Deep Human Journey, University of the Witwatersrand, Johannesburg, South Africa
    2. Anthropological Institute and Museum, University of Zurich, Zurich, Switzerland

    Contribution

    Project administration, Writing – review and editing

    Competing interests

    No competing interests declared

16. Steven E Churchill

    1. Centre for the Exploration of the Deep Human Journey, University of the Witwatersrand, Johannesburg, South Africa
    2. Department of Evolutionary Anthropology, Duke University, Durham, United States

    Contribution

    Project administration, Writing – review and editing

    Competing interests

    No competing interests declared

17. Lee R Berger

    Centre for the Exploration of the Deep Human Journey, University of the Witwatersrand, Johannesburg, South Africa

    Contribution

    Conceptualization, Data curation, Funding acquisition, Project administration, Visualization, Writing – review and editing

    Competing interests

    No competing interests declared

    "This ORCID iD identifies the author of this article:" 0000-0002-0367-7629

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