Abstract
Reconstructing the morphology of the Neanderthal rib cage not only provides information about the general evolution of human body shape but also aids understanding of functional anatomy and energetics. Despite this paleobiological importance there is still debate about the nature and extent of variations in the size and shape of the Neandertal thorax. The El Sidrón Neandertals can be used to contribute to this debate, providing new costal remains ranging from fully preserved and undistorted ribs to highly fragmented elements. Six first ribs are particularly well preserved and offer the opportunity to analyze thorax morphology in Neandertals. The aims of this paper are to present this new material, to compare the ontogenetic trajectories of the first ribs between Neandertals and modern humans, and, using geometric morphometrics, to test the hypothesis of morphological integration between the first rib and overall thorax morphology. The first ribs of the El Sidrón adult Neandertals are smaller in centroid size and tend to be less curved when compared with those of modern humans, but are similar to Kebara 2. Our results further show that the straightening of the first ribs is significantly correlated with a straightening of the ribs of the upper thorax (R = 0.66; p < 0.0001) in modern humans, suggesting modularity in the upper and lower thorax units as reported in other hominins. It also supports the hypothesis that the upper thorax of Neandertals differs in shape from modern humans with more anteriorly projecting upper ribs during inspiration. These differences could have biomechanical consequences and account for stronger muscle attachments in Neandertals. Different upper thorax shape would also imply a different spatial arrangement of the shoulder girdle and articulation with the humerus (torsion) and its connection to the upper thorax. Future research should address these inferences in the context of Neandertal overall body morphology.
Highlights
IntroductionDespite its importance the extent of differences in size and shape of the Neandertal thorax is still not entirely clear (Franciscus and Churchill, 2002; Churchill, 2006; Gómez‐Olivencia et al, 2009)
Hypotheses about the paleobiological significance of Neanderthal thorax morphology have referred to different factors ranging from cold adaptations, energetics and activity levels, increased body mass (Franciscus and Churchill, 2002; Churchill, 2006) to questions related to the evolution of overall body shape (Jellema et al, 1993; Ruff, 2002; Gómez‐Olivencia et al, 2009; Ruff, 2010; García‐Martínez et al, 2012; Bastir et al, 2013a; Bastir et al, 2013b; Bastir et al, 2013c)
In a pioneering quantitative analysis and by introducing arcs and chords to the measurement of isolated ribs of the Shanidar 3 Neanderthal, Franciscus and Churchill (2002) suggested that the lower Neanderthal thorax is larger in volume, with more rounded cross sections of the lower ribs than in modern humans
Summary
Despite its importance the extent of differences in size and shape of the Neandertal thorax is still not entirely clear (Franciscus and Churchill, 2002; Churchill, 2006; Gómez‐Olivencia et al, 2009). Other researcher has suggested that the ribs of the lower thorax in Kebara are relatively large (Gómez‐Olivencia et al, 2009; García Martínez et al, accepted). This evidence, together with a complete reconstruction of a Neanderthal skeleton (Sawyer and Maley, 2005) suggests a wider lower thorax in Neanderthals than in modern humans
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