Mineral content analysis in the rib cross-sections of Homo sapiens and Pan troglodytes and its implications for the study of Sts 14 costal remains

Introduction: From an evolutionary perspective, the ribcage has changed substantially in the subfamily Homininae. Among many other features, the amount of mineralized tissues of the rib cross-section at the midshaft could be informative about potential biomechanical changes during Homininae evolution. These changes would be related to the different loading stresses that each costal level has to deal with. Nevertheless, this knowledge remains hypothetical and has never been properly addressed. Materials and Methods: This issue was assessed by analysing via micro-CT the internal rib anatomy of the complete sets of ribs belonging to ten Homo sapiens and ten Pan troglodytes adult individuals. Additionally, five fossil ribs of Australopithecus africanus Sts 14 (costal levels 5–9) were also included to evaluate similarities with the two tested extant species. Results: The mineralized area of P. troglodytes rib cross-sections was higher than that of H. sapiens. However, its serial change along the rib sequence (1–12/13) was similar in both species. The mineralized area of the Sts 14 rib cross-sections was closer to H. sapiens than to P. troglodytes for costal levels 7–9 (Sts 14w) but not for levels 5 and 6, where it was distinct from both comparative samples. Discussion: The variation in the amount of mineralized tissues along the rib sequence in H. sapiens and P. troglodytes might indicate a common upper-lower thorax division probably linked to the insertions of the diaphragm. This variation is similar between ribs Sts 14w and the corresponding modern human costal levels, which could be potentially related to closer breathing kinematics in the lower thorax of Sts 14 and H. sapiens.