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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.

We examine how derived functional signal and phylogenetic inheritance interact in the forelimb of tree sloths, to understand the relative contribution of each in the evolution of a novel morphobehavioural suite. Molecular and craniodental data demonstrate that extant tree sloths evolved suspensory behaviours and associated morphologies from a non-suspensory ancestor independently of one another, making them a useful model system. We find that convergence in univariate traits is expressed mosaically, although the signal is largely functional. Three-dimensional analyses suggest there is greater conservatism of gross morphology in more proximal bones than in more distal elements. Convergence in some univariate scapular traits is independent of the gross morphology of the scapula itself, demonstrating that functionally relevant morphologies were mapped on to a more conserved scapular shape. Our results suggest that morphological homoplasy is expressed in a mosaic manner. The relationship between homoplasy and trait integration may be more nuanced than previously thought, even within a single adaptive system.

The evolution of bipedalism and reduced reliance on arboreality in hominins resulted in larger lower limb joints relative to the joints of the upper limb. The pattern and timing of this transition, however, remains unresolved. Here, we find the limb joint proportions of Australopithecus afarensis, Homo erectus, and Homo naledi to resemble those of modern humans, whereas those of A. africanus, Australopithecus sediba, Paranthropus robustus, Paranthropus boisei, Homo habilis, and Homo floresiensis are more ape-like. The homology of limb joint proportions in A. afarensis and modern humans can only be explained by a series of evolutionary reversals irrespective of differing phylogenetic hypotheses. Thus, the independent evolution of modern human-like limb joint proportions in A. afarensis is a more parsimonious explanation. Overall, these results support an emerging perspective in hominin paleobiology that A. afarensis was the most terrestrially adapted australopith despite the importance of arboreality throughout much of early hominin evolution.

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Significance Here, we demonstrate that a naturally evolving behavior (allonursing) has greater effect on reproductive power (mass per unit of time) and output (litter mass at birth) than does artificial selection (domestication). Additionally, we demonstrate the importance of resource optimization afforded by sociality (rather than resource abundance per se) in shaping a species’ life history profile and its ability to overcome its own physiological constraints.

Scapular breadth is associated with forelimb-dominated suspensory behavior in Atelidae: Comments on Selby and Lovejoy (2017) Recently, Selby and Lovejoy (2017) argued that most apparent synapomorphies in the hominoid scapula are not independent adaptive traits but rather secondary developmental characters driven by a few underlying adaptive changes. According to their argument, chief among these adaptive changes is the mediolateral narrowing of the hominoid scapula. This interpretation is consistent with recent work suggesting that scapular blade shape is the most phylogenetically conserved aspect of the hominoid scapula (Young, Capellini, Roach, & Alemseged, 2015). Selby and Lovejoy (2017) then assert, contrary to previously published work (e.g., Hunt, 1991;Green, 2013), that a mediolaterally narrow scapula is unrelated to suspensory locomotion and therefore that early crown hominoids could not have been suspensory and were instead clambering arboreal quadrupeds. This conclusion is based on their assertion that among extant border height, while both are narrower than either Lagothrix or Cebus (P < .0001). When the ratio of scapular border size relative to geometric mean is calculated, this relationship disappears. Instead, Ateles has the narrowest scapula, followed by Lagothrix, then Alouatta, and finally Cebus (Figure 1B; P < .002 for all pairwise comparisons). Based on published descriptions of atelid positional repertoires (Cant, Youlatos, and Rose, 2001;Defler, 1999;Fontaine 1990;Gebo, 1992), the taxa with the narrowest scapulae are also the most suspensory ( Figure 1B). This pattern holds whether only suspensory locomotion (P < .0001, R 2 5 .81), only suspensory positional behavior (P < .0001, R 2 5 .80), or the full positional repertoire (P < .0001, R 2 5 .82) of each genus is considered.The discrepancy between our results and those reported by Selby and Lovejoy (2017) is driven by the relatively tall vertebral border of Alouatta ( Figure 1C). Further, the relationship we observed between scapular narrowness and suspension is not explained by allometry or other factors potentially associated with suspension, such as the angle of the vertebral spine or long arms. Selby and Lovejoy (2017) found a slightly positively allometric trend in the vertebral border of anthropoids, but the smallest primate analyzed here, Cebus, has the second largest relative vertebral border height while the largest primate, Ateles, Am J Phys Anthropol. 2018;1-3. wileyonlinelibrary.com/journal/ajpa V C 2018 Wiley Periodicals, Inc. | 1

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