The costal skeleton of Shanidar 3 and a reappraisal of Neandertal thoracic morphology

Abstract

For over a century, Neandertal rib remains have engendered frequent discussions of "barrel-shaped" thoraces, largely in the absence of systematic comparison and hard data. We present here a description of the relatively complete ribcage of the Near Eastern Shanidar 3 Neandertal. We also furnish metric and non-metric comparisons of the Shanidar 3 ribs with other Near Eastern and European Neandertals, the Nariokotome (Homo erectus/ergaster) specimen, Levantine archaic/early modern humans, early and later European modern humans, and a sample of recent Euroamerican males. It is clear from these comparisons that Neandertals share with modern humans the fundamentally human thoracic "bauplan" that first evolved in the early Pleistocene. Yet it is also apparent that the ribcage of Neandertals differ in several anatomical details from those of fully modern humans. Rib curvature, posterior angle, mid-shaft cross-sectional size and shape, and muscle scarring varies considerably among Neandertals and across all samples when considered in isolated ribs. However, normalized metric and discrete patterning across the greater thorax clearly distinguishes Neandertals from our comparative samples. This is most marked in the inferior thorax where Neandertals (and probably earlier Homo) exhibit larger, more rounded and rugose ribs, and a greater costal area (thoracic volume). Greater lower rib cross-sectional robusticity and muscle scarring indicates relatively elevated ventilatory levels. Greater thoracic volume in Neandertals probably reflects greater body mass compared with modern humans since lung volume scales isometrically to body mass among mammals. Neandertal and modern human pulmonary capacity, normalized for body mass differences, was therefore roughly equivalent in the context of detailed differences in thoracic shape. To the extent that cold-climate adaptation is involved, Near Eastern Neandertals appear less "hyper-polar" in thoracic shape than their European counterparts as is also true for several other body proportion measures that are clinally distributed across the known Neandertal range.