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 demonstrates 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 ape-like arboreal positional behaviors, as previously suggested based on multiple lines of evidence from other parts of the skeleton and reconstructed paleobiology of A. sediba.

Data were collected on 3D models using AMIRA and analyzed in R Studio.

The 3D landmarks are as follows:

1. Superior vertebral body – ventral transverse midline
2. Superior vertebral body – central transverse midline
3. Superior vertebral body – dorsal transverse midline
4. Superior vertebral body – lateral sagittal midline
5. Superior vertebral body – ventro-lateral point
6. Superior vertebral body – dorso-lateral point (at ventral pedicle base)
7. Pedicle – superior midpoint
8. Pedicle – superior dorsal point (at ventral base of prezygapophysis)
9. Pedicle – medial midpoint
10. Pedicle – lateral midpoint
11. Pedicle – inferior midpoint
12. Pars interarticularis – dorsal midpoint
13. Pars interarticularis – ventral midpoint
14. Inferior vertebral body – ventral transverse midline
15. Inferior vertebral body – central transverse midline
16. Inferior vertebral body – dorsal transverse midline
17. Inferior vertebral body – lateral sagittal midline
18. Inferior vertebral body - ventro-lateral point
19. Inferior vertebral body – dorso-lateral point
20. Lumbar transverse process (LTP) – superior medial point (based of LTP at pedicle)
21. Lumbar transverse process – inferior medial point (based of LTP at pedicle)
22. Lumbar transverse process – superior mediolateral midpoint
23. Lumbar transverse process – inferior mediolateral midpoint
24. Lumbar transverse process – superior lateral point
25. Lumbar transverse process – inferior lateral point
26. Superior articular facet – cranial-most point
27. Mammillary process – dorsal-most extension
28. Superior articular facet – midpoint
29. Superior articular facet – caudal-most point
30. Spinous process – superior ventral point (at spinous process base)
31. Spinous process – superior sagittal midpoint
32. Spinous process – superior dorsal point (tip of spinous process)
33. Spinous process – inferior dorsal point (tip of spinous process)
34. Spinous process – inferior sagittal midpoint
35. Spinous process – inferior ventral point (at spinous process base)
36. Lamina – inferior midpoint
37. Inferior articular facet – cranial-most point
38. Inferior articular facet – midpoint
39. Inferior articular facet – caudal-most point
40. Inferior articular facet – medial-most point
41. Inferior articular facet – lateral-most point
42. Superior articular facet – medial-most point
43. Superior articular facet – lateral-most point
44. Vertebral body – ventral midpoint
45. Vertebral body – lateral midpoint
46. Lumbar transverse process – ventral extension of base at its midpoint
47. Spinous process – lateral-most extension of the spinous process tip
48. Lamina – superior midpoint