New fossils of Australopithecus sediba reveal a nearly complete lower back
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Evaluation Summary:
This paper is a very valuable contribution to paleoanthropology especially to those who study the evolution of human posture and locomotion. Overall, the quality of some of the fossils is unusually high for this chronology, and the data are properly presented and analysed. However, the authors should be more cautious in their interpretation of the data which does not fully support their conclusions.
(This preprint has been reviewed by eLife. We include the public reviews from the reviewers here; the authors also receive private feedback with suggested changes to the manuscript. Reviewer #2 agreed to share their name with the authors.)
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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 .
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Author Response:
Reviewer #1:
This manuscript presents some new fossil remains from the lower back of one of the specimens of Australopithecus sediba, the Malapa Hominin 2 (MH2). The authors identified portions of four lumbar vertebrae (L1-L4), which refit with some previously found vertebrae. All in this, produce a nearly complete lower back of a female individual, which an invaluable finding to understand the functional morphology and evolution of purported adaptations to bipedalism in fossil hominins. They find that MH2 had both a lumbar lordosis and an intervertebral articular facets width from the upper to lower lumbar column ("pyramidal configuration") similar to that of modern humans. Also, they find that the overall vertebral shape is more similar to that of modern humans compared with that of great apes. These fossils allow …
Author Response:
Reviewer #1:
This manuscript presents some new fossil remains from the lower back of one of the specimens of Australopithecus sediba, the Malapa Hominin 2 (MH2). The authors identified portions of four lumbar vertebrae (L1-L4), which refit with some previously found vertebrae. All in this, produce a nearly complete lower back of a female individual, which an invaluable finding to understand the functional morphology and evolution of purported adaptations to bipedalism in fossil hominins. They find that MH2 had both a lumbar lordosis and an intervertebral articular facets width from the upper to lower lumbar column ("pyramidal configuration") similar to that of modern humans. Also, they find that the overall vertebral shape is more similar to that of modern humans compared with that of great apes. These fossils allow other researchers to test existing hypotheses about the evolutionary process implied in the acquisition of obligate bipedalism.
Some of the conclusions of this paper are supported by the data, but other interpretations of the results need to be modified.
Strengths:
The paper present very important fossils and also includes a large comparative sample of lumbar vertebrae from modern humans and great apes. Also, includes other fossil remains from other species of hominins such as Neandertals, Australopithecus afarensis, and Australopithecus africanus. It also includes analyses from two methodologies, geometric morphometrics, and unidimensional linear and angular variables. This complete approach produces interesting and complementary results that give support to their conclusions.
Weaknesses:
The weaknesses of the paper are the lack of hypotheses and clear objectives of the work. Also, the methods are not explained in detail, which makes the paper hard to follow in some parts and difficult to replicate. The lack of hypotheses makes difficult to understand the use of some analysis. Finally, the interpretation of some results is not fully justified by the data, the authors need to focus on what all the results indicate, and not only on some of them.
We appreciate this reviewer’s careful feedback and have taken their advice of adding hypotheses to make the objective of the manuscript clearer (aside from describing new fossil material). We have also justified or modified our interpretations based on comparative data of living species and previously known fossil hominin material. We hope that our revisions address this reviewer’s concerns.
Reviewer #2:
Williams et al. present newly discovered lumbar vertebrae of MH2 so that now almost the complete lumbar spine of this important australopithecine specimen is known. This allows better inferences for posture and locomotion in these early hominins than what was previously possible, particularly for lumbar lordosis. This study could, however, benefit from using the correct anatomical and taxonomical terminology (e.g., "costal process" instead of lumbar transverse process, and "australopithecines" instead of australopiths), and a more inclusive consideration of the literature. For example, it might help to discuss that Oreopithecus has been said to show a similar pattern of lumbar vertebrae wedging angles and pyramidal configuration of the articular processes, and that the same inferences for a human-like degree of lordosis have already been made previously based on the pelvic incidence of an alternative reconstruction of MH2. Moreover, lumbar lordosis (particular the wedging angles) should also be compared to the Homo erectus specimen KNM-WT 15000, and to the 95% range of variation of modern humans rather than to the 95% confidence interval for the mean of modern human lumbar lordosis. Finally, the authors could be more precise in saying to which vertebra of their great ape comparative sample they have compared the middle lumbar vertebra of MH2 and how they justify this.
We appreciate this reviewer’s detailed and helpful feedback, and we have done our best to address their concerns. We take their point about the Terminologia Anatomica and have replaced “lumbar transverse processes” with “costal processes,” although we use “transverse” in parentheses on its first usage in both the abstract and the main text. This is because the leading textbooks in human osteology and human evolutionary anatomy (White et al. and Aiello & Dean) uses “transverse” and not “costal” process, and we have previously published using “lumbar transverse process.” We feel that the term “australopithecine” term is both outdated and incorrect taxonomically in usage, but we understand the reviewer’s dislike of the colloquial term “australopith;” therefore, we refer only to “members of the genus Australopithecus,” “early hominins included in this study,” etc. in the revision. Regarding our consideration of the literature, we did not previously cite the Oreopithecus literature because one of us (Russo) previously tested and rejected the hypothesis that Oreopithecus possessed hominin-like lumbar vertebrae (Russo & Shapiro, 2017). We now cite both the original paper and its refutation. We do regret missing the Tardieu et al. (2017) paper on pelvic incidence, which we have now incorporated in our revision, along with some other reference not included in the previous version. We have not included KNM-WT 15000 in our wedging angle and multivariate analyses because it is a juvenile individual with incomplete growth and fusion of the vertebral bodies. Our comparative sample used here does not include juveniles. We do include KNM-WT 15000 in our inter- articular facet comparison since the articular processes are complete. For the wedging angle figure, we have now added 95% confidence intervals of the data (2 standard deviations of the mean) for individual human lumbar vertebrae. Finally, we clarify and justify which lumbar vertebra is identified as “middle” in cases of great apes with four lumbar vertebrae (L2).
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Evaluation Summary:
This paper is a very valuable contribution to paleoanthropology especially to those who study the evolution of human posture and locomotion. Overall, the quality of some of the fossils is unusually high for this chronology, and the data are properly presented and analysed. However, the authors should be more cautious in their interpretation of the data which does not fully support their conclusions.
(This preprint has been reviewed by eLife. We include the public reviews from the reviewers here; the authors also receive private feedback with suggested changes to the manuscript. Reviewer #2 agreed to share their name with the authors.)
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Reviewer #1 (Public Review):
This manuscript presents some new fossil remains from the lower back of one of the specimens of Australopithecus sediba, the Malapa Hominin 2 (MH2). The authors identified portions of four lumbar vertebrae (L1-L4), which refit with some previously found vertebrae. All in this, produce a nearly complete lower back of a female individual, which an invaluable finding to understand the functional morphology and evolution of purported adaptations to bipedalism in fossil hominins. They find that MH2 had both a lumbar lordosis and an intervertebral articular facets width from the upper to lower lumbar column ("pyramidal configuration") similar to that of modern humans. Also, they find that the overall vertebral shape is more similar to that of modern humans compared with that of great apes. These fossils allow …
Reviewer #1 (Public Review):
This manuscript presents some new fossil remains from the lower back of one of the specimens of Australopithecus sediba, the Malapa Hominin 2 (MH2). The authors identified portions of four lumbar vertebrae (L1-L4), which refit with some previously found vertebrae. All in this, produce a nearly complete lower back of a female individual, which an invaluable finding to understand the functional morphology and evolution of purported adaptations to bipedalism in fossil hominins. They find that MH2 had both a lumbar lordosis and an intervertebral articular facets width from the upper to lower lumbar column ("pyramidal configuration") similar to that of modern humans. Also, they find that the overall vertebral shape is more similar to that of modern humans compared with that of great apes. These fossils allow other researchers to test existing hypotheses about the evolutionary process implied in the acquisition of obligate bipedalism.
Some of the conclusions of this paper are supported by the data, but other interpretations of the results need to be modified.
Strengths:
The paper present very important fossils and also includes a large comparative sample of lumbar vertebrae from modern humans and great apes. Also, includes other fossil remains from other species of hominins such as Neandertals, Australopithecus afarensis, and Australopithecus africanus. It also includes analyses from two methodologies, geometric morphometrics, and unidimensional linear and angular variables. This complete approach produces interesting and complementary results that give support to their conclusions.
Weaknesses:
The weaknesses of the paper are the lack of hypotheses and clear objectives of the work. Also, the methods are not explained in detail, which makes the paper hard to follow in some parts and difficult to replicate. The lack of hypotheses makes difficult to understand the use of some analysis. Finally, the interpretation of some results is not fully justified by the data, the authors need to focus on what all the results indicate, and not only on some of them.
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Reviewer #2 (Public Review):
Williams et al. present newly discovered lumbar vertebrae of MH2 so that now almost the complete lumbar spine of this important australopithecine specimen is known. This allows better inferences for posture and locomotion in these early hominins than what was previously possible, particularly for lumbar lordosis. This study could, however, benefit from using the correct anatomical and taxonomical terminology (e.g., "costal process" instead of lumbar transverse process, and "australopithecines" instead of australopiths), and a more inclusive consideration of the literature. For example, it might help to discuss that Oreopithecus has been said to show a similar pattern of lumbar vertebrae wedging angles and pyramidal configuration of the articular processes, and that the same inferences for a human-like degree …
Reviewer #2 (Public Review):
Williams et al. present newly discovered lumbar vertebrae of MH2 so that now almost the complete lumbar spine of this important australopithecine specimen is known. This allows better inferences for posture and locomotion in these early hominins than what was previously possible, particularly for lumbar lordosis. This study could, however, benefit from using the correct anatomical and taxonomical terminology (e.g., "costal process" instead of lumbar transverse process, and "australopithecines" instead of australopiths), and a more inclusive consideration of the literature. For example, it might help to discuss that Oreopithecus has been said to show a similar pattern of lumbar vertebrae wedging angles and pyramidal configuration of the articular processes, and that the same inferences for a human-like degree of lordosis have already been made previously based on the pelvic incidence of an alternative reconstruction of MH2. Moreover, lumbar lordosis (particular the wedging angles) should also be compared to the Homo erectus specimen KNM-WT 15000, and to the 95% range of variation of modern humans rather than to the 95% confidence interval for the mean of modern human lumbar lordosis. Finally, the authors could be more precise in saying to which vertebra of their great ape comparative sample they have compared the middle lumbar vertebra of MH2 and how they justify this.
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