On the chest size of Kebara 2

Abstract

Chest size is important for reconstructing Neandertal paleobiology. The large size of the Neandertal skeletal thorax has been interpreted about cold adaptation, increased body mass, specific body shape, and increased activity levels (Franciscus and Churchill, 2002, Churchill, 2006, Gómez-Olivencia et al., 2009). Large Neandertal chests should also be expected because chest and lung sizes are correlated developmentally (Thurlbeck, 1982, Bastir et al., 2013a), because lung size scales isometrically with body mass across mammals (Stahl, 1967), and because body mass was larger in Neandertals than in modern humans (Ruff et al., 1997, Froehle and Churchill, 2009).

The notion of large chests has recently been reinforced by Gómez-Olivencia et al. (2009), who measured the ribs of the Kebara 2 Neandertal male using arc lengths. Their study suggested that the upper ribs of Kebara 2 are within the range of modern humans, and the middle (ribs 4, 5, 7) and lower (ribs 8, 10) thoracic ribs exceed the range of modern human variation. Consequently, this predicts that the total thorax size (the sum of the sizes of each of the ribs) should be larger in Kebara than in modern humans.

However, Gómez-Olivencia et al. (2009) also identified a problem in the original reconstruction of the 6th and 7th ribs of Kebara due to a misidentification of fragments fitted together by Arensburg (1991), who stated that the ribs of “Kebara man are quite similar in metric and morphological respects to those of ribs in present human populations” (Arensburg, 1991: 142). Nevertheless, misidentifications such as those identified by Gómez Olivencia et al. (2009) could affect size assessments.

Bastir et al. (2012) measured the reconstruction of the Kebara rib cage (Sawyer and Maley, 2005) using 3D geometric morphometrics, and no evidence for such enlarged chest size was found, supporting Arensburg's (1991) opinion. Thus, the size of one of the best-preserved Neandertal male thoracic skeletons is currently unclear.

Recent advances in virtual morphological methods and geometric morphometrics can be used to clarify this situation. First, standard measurements of rib morphology, such as chords and arcs can be improved using 3D landmarks and geometric morphometrics because 3D coordinates of landmarks at rib heads, sternal extremities, and rib shafts quantify all features of rib morphology together (García-Martínez et al., 2012, García-Martínez et al., 2013, Bastir et al., 2013b). This method provides a size measurement (centroid size, Zelditch et al., 2004) along the extent of the entire rib from the head to its sternal extremities. Second, once scanned by computed tomography (CT) or surface scanners, virtual morphological reconstructions can be carried out and measured on the computer screen, without modifying the original precious fossils or historical reconstructions.

This study aimed to carry out a new virtual reconstruction of lower thoracic elements of Kebara 2 and to compare the new reconstruction with the original (Arensburg, 1991), testing several hypotheses on differences in rib and chest size between the Kebara 2 Neandertal and modern humans in light of Neandertal paleobiology and paleophysiology.