Rib cage anatomy in Homo erectus suggests a recent evolutionary origin of modern human body shape

Abstract

The tall and narrow body shape of anatomically modern humans (Homo sapiens) evolved via changes in the thorax, pelvis and limbs. It is debated, however, whether these modifications first evolved together in African Homo erectus, or whether H. erectus had a more primitive body shape that was distinct from both the more ape-like Australopithecus species and H. sapiens. Here we present the first quantitative three-dimensional reconstruction of the thorax of the juvenile H. erectus skeleton, KNM-WT 15000, from Nariokotome, Kenya, along with its estimated adult rib cage, for comparison with H. sapiens and the Kebara 2 Neanderthal. Our three-dimensional reconstruction demonstrates a short, mediolaterally wide and anteroposteriorly deep thorax in KNM-WT 15000 that differs considerably from the much shallower thorax of H. sapiens, pointing to a recent evolutionary origin of fully modern human body shape. The large respiratory capacity of KNM-WT 15000 is compatible with the relatively stocky, more primitive, body shape of H. erectus.

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Fig. 1: Virtual quantitative 3D reconstruction of the KNM-WT 15000 thorax.
Fig. 2: PCA of thoracic vertebral shape.
Fig. 3: PCA of rib shape.
Fig. 4: PCA of thorax 3D shapes of fossil hominins and modern humans.
Fig. 5: Scatter plots of maximum widths, depths and heights of KNM-WT 15000 (actual and estimated adult) H. erectus compared to juvenile and adult modern humans and Kebara 2 Neanderthal.
Fig. 6: Functional simulation of respiratory kinematics.

Data availability

Computed tomography scans of fossil material from the KNM-WT 15000 skeleton and the 3D models derived from them are the property of the National Museums of Kenya, to whom application must be made for access. The CT data for modern human thoraces cannot be shared, for ethical and legal reasons related to the protocols of the hospitals and hosting institutions. Interested readers should contact the authors, who will assist in getting in touch with the relevant institutions. All other data and linear measurements of fossil reconstructions are provided within the manuscript and Supplementary information.

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Acknowledgements

We thank the National Museums of Kenya, E. Mbua and F. Kyalo Manthi for permissions to perform CT and surface scanning of the vertebrae and ribs of KNM-WT 15000. H. Pontzer, D. Lieberman and B. Wood provided helpful comments on an earlier draft of this manuscript. We also thank B. Perea-Pérez, D.A. Cáceres-Monllor, A.L. Santos, E. Cunha and M. Almeida for permissions and access to their collections. This research was funded by the Spanish Ministry of Economy and Competitivity (no. CGL 2015-63648-P) to M.B. D.G.-M. was funded by IdEx University of Bordeaux Investments for the Future programme (no. ANR-10-IDEX-03-02) and the European Commission’s Research Infrastructure Action via the Synthesys Projects (nos. SE-TAF-6406, DE-TAF-6404, BE-TAF-5639). Financial support for M.H. was provided by the Swiss National Science Foundation (no. 31003A_176319/1) and the Mäxi Foundation. A.G.-O. received support from the Spanish FEDER/Ministerio de Ciencia e Innovación-AEI (project no. PGC2018-093925-B-C33) and Research Group (no. IT1418-19) from Eusko Jaurlaritza-Gobierno Vasco. A.G.-O. is funded by a Ramón y Cajal fellowship (no. RYC-2017-22558).

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M.B., D.G.-M., S.A.W., N.T.-T. and F.G.-R. wrote the paper. D.G.-M., F.S., A.B., C.V., J.A.S.-G., I.T.-S., B.B., F.G.-R., M.H. and S.N. contributed data. M.B., D.G.-M., N.T.-T., C.A.P., A.R.-L., B.B., E.B. and A.G.-O. analysed data. All authors critically interpreted results. M.B. designed the project.

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Correspondence to Markus Bastir.

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Extended data

Extended Data Fig. 1 Original scans of the KNM-WT 15000 thoracic vertebrae and virtual 3D models and the reconstructed thoracic spine.

a, S (T1), b,T (T2), c, U (T3), d, CA (T4), e, T5: virtual, quantitative virtual 3D reconstruction, f, W, (T6), g, V (T7), h, T8: quantitative virtual 3D reconstruction. i, BI, (T9 quantitative virtual 3D reconstruction), j, X (T10, quantitative virtual 3D reconstruction), k, Y (T11), l, T12: virtually assembled following Haeusler et al.35; m: ventral view of thoracic spine, n: left lateral view of thoracic spine.

Extended Data Fig. 2 Original scans of the KNM-WT 15000 ribs and virtual 3D models for the rib cage reconstruction.

Cranial view of the individual ribs using the level assessment from Haeusler et al.35. The labels displayed in black are originals whereas the labels displayed in red colour are mirror images. (Rec indicates virtual reconstruction).

Extended Data Fig. 3 Mean comparisons of the juvenile and hypothetical adult KNM-WT 15000 thorax with modern humans and the Kebara 2 Neanderthal.

a, The KNM-WT 15000 thorax (red) superimposed on the modern human juvenile mean in Procrustes registration. b, The hypothetical adult KNM-WT 15000 thorax (red) superimposed on the modern human adult mean in Procrustes registration. c, The hypothetical adult KNM-WT 15000 thorax (red) superimposed on the Kebara 2 Neanderthal thorax reconstruction in Procrustes registration. Note the similar (more horizontal) orientation of the ribs in these two specimens due to reduced rib torsion.

Extended Data Fig. 4 Landmarks and semilandmarks of the ribs and thoracic vertebrae.

a, Rib landmarks account for height, thickness and the 3D shape of the cranial and caudal curvatures and torsion of the shaft. b, Vertebral landmarks account for the morphology of the vertebral body (outline of endplates, body height, width and lengths), and the neural arches (curvatures, articulations, neural canal) and the thickness, height and orientations of the transverse and spinous processes. (Landmarks: red, semilandmarks of curves and surfaces: blue).

Extended Data Fig. 5 Statistical validation of spine reconstructions.

a, Scatterplot of PC1 and PC2 of the four original (stars) and 24 reconstructed thoracic spines (dots: reconstructions carried out by researcher 1; open squares: reconstructions carried out by researcher 2), and the convex hulls of each spine. PC1 shows that all (except one) reconstructions plotted towards more positive PC1 scores relative to their original. Inset 3D shapes illustrate variation along PC1. The experiment indicates a systematic underestimation of the thoracic kyphosis following the standardized reconstruction methods. b, The dendrogram shows the high accuracy of the reconstructions, which is independent of the researcher. All reconstructions fall together only with their original spine.

Supplementary information

Supplementary Information

Supplementary methods, Tables 1–7 and references.

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Supplementary Video 1

Breathing simulation of the KNM-WT 15000 juvenile rib cage. Kinematic changes in the KNM-WT 15000 thorax are based on computer simulations of modern human axes and ranges of motion applied to the virtual 3D reconstruction of the KNM-WT 15000 thorax. Note the lateral expansion of the rib cage during inspiration. Motion was simulated for true ribs only.

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Bastir, M., García-Martínez, D., Torres-Tamayo, N. et al. Rib cage anatomy in Homo erectus suggests a recent evolutionary origin of modern human body shape. Nat Ecol Evol 4, 1178–1187 (2020). https://doi.org/10.1038/s41559-020-1240-4

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