Shanidar 3 'rings the bell': Virtual ribcage reconstruction and its implications for understanding the Neanderthal bauplan.

Abstract

The study of the ribcage is fundamental to understanding hominin evolution. However, ribs and vertebrae are scarce in the fossil record. Although Neanderthals are one of the most represented and, therefore, one of the most studied fossil Homo species, it is controversial whether there is a standardized Neanderthal ribcage morphotype that could differ from modern humans. Hence, we used three-dimensional geometric morphometrics to reconstruct and compare the Neanderthal ribcage of Shanidar 3 with another Neanderthal specimen, Kebara 2, and with 58 Homo sapiens individuals of worldwide distribution. Shape differences among the Neanderthal and H. sapiens ribcages were analyzed by a hierarchical cluster using the Euclidean distances among the permuted Procrustes distances between groups. Size differences between the Neanderthal and H. sapiens ribcages were examined using a permutation test on centroid size. To examine the potential for allometry, we performed a linear regression of Procrustes coordinates on centroid size of the sample, followed by a principal component analysis in form space. Our results show that Shanidar 3 has the 'bell-shaped' thorax typically described for Neanderthals. In fact, the shapes of both Shanidar 3 and Kebara 2 ribcages cluster apart from that of H. sapiens, being closer to cold-adapted individuals. The study of the centroid size supports similarities between Neanderthals and cold-adapted H. sapiens since significant size differences were found only between Neanderthals and temperate/tropical recent humans. The linear regression and principal component analysis showed an allometric relationship between ribcage size and shape, suggesting Neanderthals had larger and stockier ribcages than most H. sapiens, although they fall within the H. sapiens range of variation. Finally, ribcage similarities found between Shanidar 3 and Kebara 2, both inhabiting warm Levantine locations during the Upper Pleistocene, could challenge the conventional idea of a cold-adapted bauplan in Neanderthals.