The study of thoracic biomechanics in Homo sapiens provides highly valuable information to areas such as respiratory medicine, physiology, or ergonomics (De Troyer et al., 2005; Ratnovsky et al., 2008; Beyer et al., 2013, 2016, 2017; García-Martínez et al., 2016, 2019; Torres-Tamayo et al., 2018; Sanchís-Gimeno et al., 2020). Methods from most of the cited publications combine CT scans with three-dimensional (3D) geometric morphometrics, enabling precise analyses of the breathing function in the ribcage and lungs of modern humans. Even though these data are also of great evolutionary interest (Bastir et al., 2017), a comparative study of thoracic breathing biomechanics in fossil hominins is challenging because of the incomplete fossil record, the unknown soft tissue morphology, and the impossibility of performing in vivo analyses (Franciscus and Churchill, 2002; García-Martínez et al., 2014; Latimer et al., 2016; Gómez-Olivencia et al., 2018; Bastir et al., 2020; Ward et al., 2020). Moreover, the robust, caudally deep and wide thorax of fossil Homo may differ biomechanically from that of modern humans because of the skeletal gracility and narrow and flat ribcage of the latter (Jellema et al., 1993; Chirchir et al., 2015; Ryan and Shaw, 2015; Latimer et al., 2016; Gómez-Olivencia et al., 2018; Bastir et al., 2020, 2022). In previous research, a first breathing kinematic simulation was carried out in a thorax reconstruction of the ca. 1.5 Ma Homo erectus s.l. KNM-WT 15000 specimen (Bastir et al., 2020) using a reference sample of healthy modern humans. This article suggested a greater mediolateral rib displacement and less vertical expansion during inspiration in KNM-WT 15000 than in H. sapiens. Such breathing kinematics have also been proposed for other fossil hominins such as Neanderthals, especially after the reconstruction and study of the exceptionally well-preserved thoracic skeleton of Kebara 2 (Gómez-Olivencia et al., 2009, 2018; García-Martínez et al., 2014, 2020).

Understanding rib kinematics in recent humans would not be possible without describing the action of the respiratory muscles, especially the diaphragm. As the most important muscle for respiration (De Troyer and Wilson, 2016), it increases the volume of the thoracic cavity with its contraction, changing its 3D shape and creating a drop in pleural pressure that causes the inflation of the lungs and entry of air into them. Diaphragmatic contraction leads to a caudal displacement of the muscle—particularly of the central tendon area—that is stopped due to the increased pressure acting on it by the compressing abdominal and mediastinal organs. This makes the central tendon a fixed point that allows the diaphragmatic muscle fibers to lift the ribs laterally. In addition, the diaphragmatic displacement also occurs mediolaterally toward the sternum and anterior abdominal and thoracic walls, eventually resulting in maximum ventilation of the inferior lung lobes (Gray, 1918; Ding et al., 2010; De Troyer and Wilson, 2016).

Previous studies have determined that diaphragmatic function in H. sapiens is actively supported by other respiratory muscles, such as the intercostals and scalenes, and with the pectorals and some shoulder and abdominal wall muscles acting as auxiliary breathing muscles (Gray, 1918; Wade, 1954; De Troyer and Estenne, 1984; Ratnovsky et al., 2008). Nevertheless, the reduced rib torsion and wider and antero-posteriorly deeper lower thorax of extinct Homo could suggest different thoracic musculoskeletal contributions to breathing kinematics (García-Martínez et al., 2018a; Bastir et al., 2020). These could be characterized by a powerful diaphragm that performs most of the breathing muscular activity with a smaller contribution of the other respiratory muscles (Franciscus and Churchill, 2002; Gómez-Olivencia et al., 2009, 2018; García-Martínez et al., 2014, 2017; Bastir et al., 2015, 2020; Latimer et al., 2016). Although García-Martínez et al. (2017) described that the groove caused by diaphragmatic insertion on rib 12 is more pronounced in Neanderthals than in modern humans—possibly indicating a more intense contractile action of this respiratory muscle in fossil hominins—the diaphragm is a soft tissue structure that is not preserved in the fossil record. Thus, little is known yet about its morphology and even less about its possible kinematics during breathing in extinct Homo species.

To shed light on this issue, this study aims to provide the first estimates of the upper diaphragmatic morphology of KNM-WT 15000 (H. erectus s.l.) and Kebara 2 (Homo neanderthalensis). We first compare their shapes and surface areas to those of modern humans and, eventually, propose functional interpretations of breathing kinematics in fossil hominins.