Recent origin of low trabecular bone density in modern humans

Humans are unique, compared with our closest living relatives (chimpanzees) and early fossil hominins, in having an enlarged body size and lower limb joint surfaces in combination with a relatively gracile skeleton (i.e., lower bone mass for our body size). Some analyses have observed that in at least a few anatomical regions modern humans today appear to have relatively low trabecular density, but little is known about how that density varies throughout the human skeleton and across species or how and when the present trabecular patterns emerged over the course of human evolution. Here, we test the hypotheses that (i) recent modern humans have low trabecular density throughout the upper and lower limbs compared with other primate taxa and (ii) the reduction in trabecular density first occurred in early Homo erectus, consistent with the shift toward a modern human locomotor anatomy, or more recently in concert with diaphyseal gracilization in Holocene humans. We used peripheral quantitative CT and microtomography to measure trabecular bone of limb epiphyses (long bone articular ends) in modern humans and chimpanzees and in fossil hominins attributed to Australopithecus africanus, Paranthropus robustus/early Homo from Swartkrans, Homo neanderthalensis, and early Homo sapiens. Results show that only recent modern humans have low trabecular density throughout the limb joints. Extinct hominins, including pre-Holocene Homo sapiens, retain the high levels seen in nonhuman primates. Thus, the low trabecular density of the recent modern human skeleton evolved late in our evolutionary history, potentially resulting from increased sedentism and reliance on technological and cultural innovations

A Comparative Study of Trabecular Bone Mass Distribution in Cursorial and Non-Cursorial Limb Joints

Skeletal design among cursorial animals is a compromise between a stable body that can withstand locomotor stress and a light design that is energetically inexpensive to grow, maintain, and move. Cursors have been hypothesized to reduce distal musculoskeletal mass to maintain a balance between safety and energetic cost due to an exponential increase in energetic demand observed during the oscillation of the distal limb. Additionally, experimental research shows that the cortical bone in distal limbs experiences higher strains and remodeling rates, apparently maintaining lower mass at the expense of a smaller safety factor. This study tests the hypothesis that the trabecular bone mass in the distal limb epiphyses of cursors is relatively lower than that in the proximal limb epiphyses to minimize the energetic cost of moving the limb. This study utilized peripheral quantitative computed tomography scanning to measure the trabecular mass in the lower and upper limb epiphyses of hominids, cercopithecines, and felids that are considered cursorial and non-cursorial. One-way ANOVA with Tukey post hoc corrections was used to test for significant differences in trabecular mass across limb epiphyses. The results indicate that overall, both cursors and non-cursors exhibit varied trabecular mass in limb epiphyses and, in certain instances, conform to a proximal-distal decrease in mass irrespective of cursoriality. Specifically, hominid and cercopithecine hind limb epiphyses exhibit a proximal-distal decrease in mass irrespective of cursorial adaptations. These results suggest that cursorial mammals employ other energy saving mechanisms to minimize energy costs during running

Limited Trabecular Bone Density Heterogeneity in the Human Skeleton

There is evidence for variation in trabecular bone density and volume within an individual skeleton, albeit in a few anatomical sites, which is partly dependent on mechanical loading. However, little is known regarding the basic variation in trabecular bone density throughout the skeleton in healthy human adults. This is because research on bone density has been confined to a few skeletal elements, which can be readily measured using available imaging technology particularly in clinical settings. This study comprehensively investigates the distribution of trabecular bone density within the human skeleton in nine skeletal sites (femur, proximal and distal tibia, third metatarsal, humerus, ulna, radius, third metacarpal, and axis) in a sample of N=20 individuals (11 males and 9 females). pQCT results showed that the proximal ulna (mean = 231.3 mg/cm3) and axis vertebra (mean = 234.3 mg/cm3) displayed significantly greater (p<0.01) trabecular bone density than other elements, whereas there was no significant variation among the rest of the elements (p>0.01). The homogeneity of the majority of elements suggests that these sites are potentially responsive to site-specific genetic factors. Secondly, the lack of correlation between elements (p>0.05) suggests that density measurements of one anatomical region are not necessarily accurate measures of other anatomical regions

Relative fibular strength and locomotor behavior in OH 35 and KNM-WT 15000

Relative fibular/tibial strength has been demonstrated to be related to the degree of arboreality/ terrestriality in anthropoid primates. In this study fibular/tibial strength was determined in OH 35, a Homo habilis (or possibly Paranthropus boisei), (1.8 myr) and KNM-WT 15000, a juvenile Homo erectus, (1.5 myr), and was compared to modern humans (n=79), chimpanzees (n=16), gorillas (n=16) and orangutans (n=11). Ontogenetic changes in fibular/tibial strength were also analyzed due to KNM-WT 15000’s juvenile status. Cross-sectional properties were derived from multi-plane radiography and either CT sections of casts (fossils) or external molds (extant). RMA regressions were run on polar second moment of area (J), a measure of torsional and average bending rigidity, of the fibula against that of the tibia for all extant species. Fossils were analyzed using their relative deviations from each regression line, expressed in SEE units. Great apes differed significantly from humans in regression line elevation, with relatively stronger fibulae. OH 35 fell in the center of the great ape distribution, within 1 SEE of each great ape taxon, but 1.9 SEE from humans. KNM-WT 15000 was more than 2 SEE from all great apes and within 0.6 SEE of humans. This was not a result of his age, as fibular/tibial strength slightly decreases with age in humans. OH 35 has some human-like features; however, the relative strength of the two bones aligns the specimen with great apes, suggesting a significant degree of arboreality. KNM-WT 15000 is demonstrated to be fully modern, complimenting other evidence for complete terrestrial bipedality

Does trabecular bone structure within the metacarpal heads of primates vary with hand posture?

Reconstructing function from hominin fossils is complicated by disagreements over how to interpret primitively inherited, ape-like morphology. This has led to considerable research on aspects of skeletal morphology that may be sensitive to activity levels during life. We quantify trabecular bone morphology in three volumes of interest (dorsal, central, and palmar) in the third metacarpal heads of extant primates that differ in hand function: Pan troglodytes, Pongo pygmaeus, Papio anubis, and Homo sapiens. Results show that bone volume within third metacarpal heads generally matches expectations based on differences in function, providing quantitative support to previous studies. Pongo shows significantly low bone volume in the dorsal region of the metacarpal head. Humans show a similar pattern, as manipulative tasks mostly involve flexed and neutral metacarpo-phalangeal joint postures. In contrast, Pan and Papio have relatively high bone volume in dorsal and palmar regions, which are loaded during knuckle-walking/digitigrady and climbing, respectively. Regional variation in degree of anisotropy did not match predictions. Although trabecular morphology may improve behavioral inferences from fossils, more sophisticated quantitative strategies are needed to explore trabecular spatial distributions and their relationships to hand function. Résumé Reconstruire la fonction à partir de fossiles homininés est une tâche compliquée par les désaccords concernant l’interprétation de la morphologie primitive des grands singes. Ceci a conduit à de nombreuses recherches sur différents aspects de la morphologie du squelette, qui pourraient être sensibles aux niveaux d’activité durant la vie d’un individu. Nous quantifions ici la morphologie de l’os trabéculaire selon trois volumes d’intérêt (dorsal, central et palmaire) mesurés dans les têtes de métacarpiens de primates actuels qui diffèrent par la fonction de leurs mains : Pan troglodytes, Pongo pygmaeus, Papio anubis et Homo sapiens. Les résultats montrent que le volume osseux dans les têtes des troisièmes métacarpiens correspond généralement aux prévisions basées sur les différences de fonction, étayant de manière quantitative les précédentes études. Pongo montre un volume osseux significativement faible dans la région dorsale de la tête des métacarpiens. Les humains montrent un patron similaire, compatible avec le fait que les tâches de manipulation impliquent principalement les positions fléchies et neutres des articulations métacarpo-phalangiennes. À l’inverse, Pan et Papio ont un volume osseux relativement élevé dans les régions dorsale et palmaire, qui sont soumises à des contraintes lors de la locomotion sur les phalanges/digitigradie et de l’escalade. La variation régionale du degré d’anisotropie ne correspond pas à nos attentes. Bien que la morphologie trabéculaire puisse aider à améliorer les reconstructions comportementales des fossiles, des stratégies quantitatives plus sophistiquées sont nécessaires pour explorer la distribution spatiale trabéculaire et sa relation avec la fonction de la main

Recent origin of low trabecular bone density in modern humans

Humans are unique, compared with our closest living relatives (chimpanzees) and early fossil hominins, in having an enlarged body size and lower limb joint surfaces in combination with a relatively gracile skeleton (i.e., lower bone mass for our body size). Some analyses have observed that in at least a few anatomical regions modern humans today appear to have relatively low trabecular density, but little is known about how that density varies throughout the human skeleton and across species or how and when the present trabecular patterns emerged over the course of human evolution. Here, we test the hypotheses that (i) recent modern humans have low trabecular density throughout the upper and lower limbs compared with other primate taxa and (ii) the reduction in trabecular density first occurred in early Homo erectus, consistent with the shift toward a modern human locomotor anatomy, or more recently in concert with diaphyseal gracilization in Holocene humans. We used peripheral quantitative CT and microtomography to measure trabecular bone of limb epiphyses (long bone articular ends) in modern humans and chimpanzees and in fossil hominins attributed to Australopithecus africanus, Paranthropus robustus/early Homo from Swartkrans, Homo neanderthalensis, and early Homo sapiens. Results show that only recent modern humans have low trabecular density throughout the limb joints. Extinct hominins, including pre-Holocene Homo sapiens, retain the high levels seen in nonhuman primates. Thus, the low trabecular density of the recent modern human skeleton evolved late in our evolutionary history, potentially resulting from increased sedentism and reliance on technological and cultural innovations