Differential Growth and Development of the Upper and Lower Human Thorax

Abstract

The difficulties in quantifying the 3D form and spatial relationships of the skeletal components of the ribcage present a barrier to studies of the growth of the thoracic skeleton. Thus, most studies to date have relied on traditional measurements such as distances and indices from single or few ribs. It is currently known that adult-like thoracic shape is achieved early, by the end of the second postnatal year, with the circular cross-section of the newborn thorax transforming into the ovoid shape of adults; and that the ribs become inclined such that their anterior borders come to lie inferior to their posterior. Here we present a study that revisits growth changes using geometric morphometrics applied to extensive landmark data taken from the ribcage. We digitized 402 (semi) landmarks on 3D reconstructions to assess growth changes in 27 computed tomography-scanned modern humans representing newborns to adults of both sexes. Our analyses show a curved ontogenetic trajectory, resulting from different ontogenetic growth allometries of upper and lower thoracic units. Adult thoracic morphology is achieved later than predicted, by diverse modifications in different anatomical regions during different ontogenetic stages. Besides a marked increase in antero-posterior dimensions, there is an increase in medio-lateral dimensions of the upper thorax, relative to the lower thorax. This transforms the pyramidal infant thorax into the barrel-shaped one of adults. Rib descent is produced by complex changes in 3D curvature. Developmental differences between upper and lower thoracic regions relate to differential timings and rates of maturation of the respiratory and digestive systems, the spine and the locomotor system. Our findings are relevant to understanding how changes in the relative rates of growth of these systems and structures impacted on the development and evolution of modern human body shape.

The relevance of the first ribs of the El Sidrón site (Asturias, Spain) for the understanding of the Neandertal thorax

“…These data were compared with measurements on 3D reconstructions of CT scans of a growth series of isolated first ribs of twenty-seven modern humans ranging from newborns to adults of both sexes (Bastir et al, 2013b;Bastir et al, 2013c). This data set was also used to study covariation between the first ribs and the remaining ones in anatomical connection with the spine in order to quantify thorax shape covariation.…”

“…Twenty 3D-landmarks and semilandmarks were measured on each rib (García-Martínez et al, 2012;Bastir et al, 2013b;Bastir et al, 2013c;García-Martínez et al, 2013). Semilandmarks were slid to the GPA average to minimize bending energy between each specimen and the GPAconsensus.…”

“…Hypotheses about the paleobiological significance of Neanderthal thorax morphology have referred to different factors ranging from cold adaptations, energetics and activity levels, increased body mass (Franciscus and Churchill, 2002;Churchill, 2006) to questions related to the evolution of overall body shape (Jellema et al, 1993;Ruff, 2002;Gómez-Olivencia et al, 2009;Ruff, 2010;García-Martínez et al, 2012;Bastir et al, 2013a;Bastir et al, 2013b;Bastir et al, 2013c). …”

“…The question of size and shape differences is also important in the light of recent evidence that supports a division of the thorax into an upper and a lower part for functional, developmental and evolutionary reasons (Arensburg, 1991;Bastir et al, 2013c;Schmid et al, 2013). While the upper part (ribs 1 to 5) has been related to thoracic breathing and to upper limb locomotion the lower part (rib 6-12) reflect features related to diaphragmatic breathing and posture and body shape as well as subt-horacic organ content.…”

“…In addition, recent methodological developments can be used for rigorous quantification of rib curvature, which is a key-factor of overall thoracic morphology and variation (García-Martínez et al, 2012;Bastir et al, 2013b;Bastir et al, 2013c;García-Martínez et al, 2013).…”