@WFS,World Fossil Society,Riffin T Sajeev,Russel Sajeev

The lower back of Malapa Hominin 2 in ventral (left) and dorsal (right) views.
New second and third lumbar vertebrae (U.W.88–232, U.W.88–233) are positioned at the top, and U.W.88–234 contributes to the upper portion of the fourth lumbar vertebra (U.W.88–127/153/234). The fifth lumbar vertebra (U.W.88–126/138) sits atop the sacrum (U.W.88–137/125). The lower back elements are preserved together in four blocks, each containing multiple elements held together in matrix and/or in partial articulation: (1) The vertebral body fragment of L1 (U.W.88–280) is preserved within the matrix of a block containing the lower thoracic vertebrae (U.W.88–43/114 and U.W.88–44) (Figure 2—figure supplement 1, Figure 3—figure supplement 1); (2) L1 inferior neural arch (U.W.88–281; concealed in matrix), L2 (U.W.88–232), L3 (U.W.88–233), and upper neural arch of L4 (U.W.88–234); (3) the L4 (U.W.88–127) and L5 (U.W.88–126) vertebral bodies, and partial S1 body (U.W.88–125); (4) most of the sacrum (U.W.88–137), the neural arch of L5 (U.W.88–153), the inferior portion of the neural arch of L4 (U.W.88–138).

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.

Cite this articleas: eLife 2021;10:e70447 DOI: 10.7554/eLife.70447

@WFS,World Fossil Society,Riffin T Sajeev,Russel Sajeev