Gracility of the modernHomo sapiensskeleton is the result of decreased biomechanical loading

Abstract

The postcranial skeleton of modern Homo sapiens is relatively gracile compared with other hominoids and earlier hominins. This gracility predisposes contemporary humans to osteoporosis and increased fracture risk. Explanations for this gracility include reduced levels of physical activity, the dissipation of load through enlarged joint surfaces, and selection for systemic physiological characteristics that differentiate modern humans from other primates. This study considered the skeletal remains of four behaviorally diverse recent human populations and a large sample of extant primates to assess variation in trabecular bone structure in the human hip joint. Proximal femur trabecular bone structure was quantified from microCT data for 229 individuals from 31 extant primate taxa and 59 individuals from four distinct archaeological human populations representing sedentary agriculturalists and mobile foragers. Analyses of mass-corrected trabecular bone variables reveal that the forager populations had significantly higher bone volume fraction, thicker trabeculae, and consequently lower relative bone surface area compared with the two agriculturalist groups. There were no significant differences between the agriculturalist and forager populations for trabecular spacing, number, or degree of anisotropy. These results reveal a correspondence between human behavior and bone structure in the proximal femur, indicating that more highly mobile human populations have trabecular bone structure similar to what would be expected for wild nonhuman primates of the same body mass. These results strongly emphasize the importance of physical activity and exercise for bone health and the attenuation of age-related bone loss. trabecular bone | gracilization | human evolution | biomechanics | mobility C ompared with other hominoids and extinct hominin species, more recent humans possess relatively gracile postcranial skeletons (1-9). One of the consequences of this gracility in contemporary humans is an increased fracture risk associated with age-related bone loss and osteoporosis [hip fractures alone are projected to reach 6.26 million per year globally by 2050 (10)] (11-15). The etiology of this relative gracility remains uncertain, and this uncertainty hinders the development of strategies for mitigating fracture risk and morbidity. The progressive gracilization of the Homo postcranial skeleton was originally detected in cortical bone structure (1, 2), but has now been demonstrated in the trabecular bone microstructure of joints (12,14,(16)(17)(18)(19), where osteoporotic fracture risk is highest (20). Most notably, in an analysis of thoracic vertebral bodies, Cotter et al. (12) found that young adult humans have significantly lower trabecular bone volume fraction (BV/TV) and thinner vertebral shells than similarly sized apes. Griffin et al. (16) also found significantly lower BV/TV in the human first and second metatarsal heads compared with hominoid primates. The results of these studies are corroborated by work on the hominoid...

Gradual decline in mobility with the adoption of food production in Europe

“…This finding is consistent with recently reported results based on limb bone trabecular structure (10,26), in which significant differences in bone density were found between Holocene foragers and farmers but not between farmers and a modern industrial sample. Our results for a much more comprehensive Holocene sample indicate that moderate variation in activity level, i.e., between the Iron/Roman period and 20th century, does not affect relative bone strength significantly.…”

“…Paradoxically, in many cases increased sedentism also may have led to declines in overall health and increased morbidity within populations by facilitating transmission of infectious diseases and through other negative consequences of more dense settlements (3,4,6). Reductions in mechanical loading of the skeleton associated with a more sedentary lifestyle may have contributed to the etiology of modern skeletal disorders such as osteoporosis (8)(9)(10). Declines in mobility also had significant effects on sociopolitical organization, including sexual division of labor, social hierarchy, and territoriality (1,11,12).…”

“…Earlier studies identified declines in skeletal robusticity (strength relative to body size) in Homo throughout the Pleistocene (24,25), but recent analyses suggest that the major decrease in robusticity occurred later, at the end of the Pleistocene, between early anatomically modern H. sapiens and Holocene populations (26,27). This suggestion in turn strongly implicates increased sedentism as a major driver in producing the more gracile modern human skeleton (10,25,26) and focuses attention on the Holocene as the critical period during which this transformation took place. However, the timing and pace of this change relative to major subsistence and lifestyle transitions cannot be determined from these studies, given their sparse sampling of terminal Pleistocene and Holocene populations.…”

Inner structural organization of the distal humerus in Paranthropus and Homo

“…It seems nonetheless interesting to note that, besides the ambiguous evidence from TM 1517g because of its low bone-matrix contrast, the two specimens from Swartkrans share the presence of more numerous and thick struts, locally resulting into a more closed network of plate-like structures from the accumulation of more material in the cell wall (Dalstra and Huiskes, 1995;Gibson, 1985;Stauber and Müller, 2006). At any comparable site, struts are invariably thicker in the fossil specimens than in the extant human humeri available to us, which is consistent with the process of skeletal gracilisation and trabecular bone density reduction occurred in recent humans (Chirchir et al, 2015(Chirchir et al, , 2017Ryan and Shaw, 2015). It is also noticeable that the medullary cavity of the OH 80-10 distal humerus fragment from the P. boisei partial skeleton of Olduvai Gorge, Tanzania, is filled by trabecular bone (Domínguez-Rodrigo et al, 2013).…”

Shoes Are Not Paleo

“…Barefoot endurance running (the hunting strategy which elevated humans to the top of the food chain) provides impact-based feedback to our sensitive soles, which the brain relies on in order to assess movement efficiency and calibrate muscular engagement. The absence of this feedback likely contributes to modern humans' failure to develop and to maintain the physical robustness of our ancestors (Ryan, 2014).…”