Comparative analysis of trabecular bone structure and orientation in South African hominin tali

Cortical bone architecture of hominid intermediate phalanges reveals functional signals of locomotion and manipulation

OBJECTIVES

Reconstruction of fossil hominin manual behaviors often relies on comparative analyses of extant hominid hands to understand the relationship between hand use and skeletal morphology. In this context, the intermediate phalanges remain understudied. Thus, here we investigate cortical bone morphology of the intermediate phalanges of extant hominids and compare it to the cortical structure of the proximal phalanges, to investigate the relationship between cortical bone structure and inferred loading during manual behaviors.

MATERIALS AND METHODS

Using micro-CT data, we analyze cortical bone structure of the intermediate phalangeal shaft of digits 2-5 in Pongo pygmaeus (n = 6 individuals), Gorilla gorilla (n = 22), Pan spp. (n = 23), and Homo sapiens (n = 23). The R package morphomap is used to study cortical bone distribution, cortical thickness and cross-sectional properties within and across taxa.

RESULTS

Non-human great apes generally have thick cortical bone on the palmar shaft, with Pongo only having thick cortex on the peaks of the flexor sheath ridges, while African apes have thick cortex along the entire flexor sheath ridge and proximal to the trochlea. Humans are distinct in having thicker dorsal shaft cortex as well as thick cortex at the disto-palmar region of the shaft.

DISCUSSION

Variation in cortical bone distribution and properties of the intermediate phalanges is consistent with differences in locomotor and manipulative behaviors in extant great apes. Comparisons between the intermediate and proximal phalanges reveals similar patterns of cortical bone distribution within each taxon but with potentially greater load experienced by the proximal phalanges, even in knuckle-walking African apes. This study provides a comparative context for the reconstruction of habitual hand use in fossil hominins and hominids.

Trabecular bone structure of the proximal capitate in extant hominids and fossil hominins with implications for midcarpal joint loading and the dart-thrower's motion

OBJECTIVES

This research examines whether the distribution of trabecular bone in the proximal capitates of extant hominids, as well as several fossil hominin taxa, is associated with the oblique path of the midcarpal joint known as the dart-thrower's motion (DTM).

MATERIALS AND METHODS

We analyzed proximal capitates from extant (Pongo n = 12; Gorilla n = 11; Pan n = 10; fossil and recent Homo sapiens n = 29) and extinct (Australopithecus sediba n = 2; Homo naledi n = 1; Homo floresiensis n = 2; Neandertals n = 3) hominids using a new canonical holistic morphometric analysis, which quantifies and visualizes the distribution of trabecular bone using relative bone volume as a fraction of total volume (rBV/TV).

RESULTS

Homo sapiens and Neandertals had a continuous band of high rBV/TV that extended across the scaphoid, lunate, and hamate subarticular regions, but other fossil hominins and extant great apes did not. A. sediba expressed a distinct combination of human-like and Pan-like rBV/TV distribution. Both H. floresiensis and H. naledi had high rBV/TV on the ulnar-side of the capitate but low rBV/TV on the radial-side.

CONCLUSION

The proximal capitates of H. sapiens and Neandertals share a distinctive distribution of trabecular bone that suggests that these two species of Homo regularly load(ed) their midcarpal joints along the full extent of the oblique path of the DTM. The observed pattern in A. sediba suggests that human-like stress at the capito-scaphoid articular surface was combined with Pan-like wrist postures, whereas the patterns in H. floresiensis and H. naledi suggest their midcarpal joints were loaded differently from that of H. sapiens and Neandertals.

Variation and covariation of external shape and cross-sectional geometry in the human metacarpus

OBJECTIVES

Analyses of external bone shape using geometric morphometrics (GM) and cross-sectional geometry (CSG) are frequently employed to investigate bone structural variation and reconstruct activity in the past. However, the association between these methods has not been thoroughly investigated. Here, we analyze whole bone shape and CSG variation of metacarpals 1-5 and test covariation between them.

MATERIALS AND METHODS

We analyzed external metacarpal shape using GM and CSG of the diaphysis at three locations in metacarpals 1-5. The study sample includes three modern human groups: crew from the shipwrecked Mary Rose (n = 35 metacarpals), a Pre-industrial group (n = 50), and a Post-industrial group (n = 31). We tested group differences in metacarpal shape and CSG, as well as correlations between these two aspects of metacarpal bone structure.

RESULTS

GM analysis demonstrated metacarpus external shape variation is predominately related to changes in diaphyseal width and articular surface size. Differences in external shape were found between the non-pollical metacarpals of the Mary Rose and Pre-industrial groups and between the third metacarpals of the Pre- and Post-industrial groups. CSG results suggest the Mary Rose and Post-industrial groups have stronger metacarpals than the Pre-industrial group. Correlating CSG and external shape showed significant relationships between increasing external robusticity and biomechanical strength across non-pollical metacarpals (r: 0.815-0.535; p ≤ 0.05).

DISCUSSION

Differences in metacarpal cortical structure and external shape between human groups suggest differences in the type and frequency of manual activities. Combining these results with studies of entheses and kinematics of the hand will improve reconstructions of manual behavior in the past.

Challenges and perspectives on functional interpretations of australopith postcrania and the reconstruction of hominin locomotion

In 1994, Hunt published the 'postural feeding hypothesis'-a seminal paper on the origins of hominin bipedalism-founded on the detailed study of chimpanzee positional behavior and the functional inferences derived from the upper and lower limb morphology of the Australopithecus afarensis A.L. 288-1 partial skeleton. Hunt proposed a model for understanding the potential selective pressures on hominins, made robust, testable predictions based on Au. afarensis functional morphology, and presented a hypothesis that aimed to explain the dual functional signals of the Au. afarensis and, more generally, early hominin postcranium. Here we synthesize what we have learned about Au. afarensis functional morphology and the dual functional signals of two new australopith discoveries with relatively complete skeletons (Australopithecus sediba and StW 573 'Australopithecus prometheus'). We follow this with a discussion of three research approaches that have been developed for the purpose of drawing behavioral inferences in early hominins: (1) developments in the study of extant apes as models for understanding hominin origins; (2) novel and continued developments to quantify bipedal gait and locomotor economy in extant primates to infer the locomotor costs from the anatomy of fossil taxa; and (3) novel developments in the study of internal bone structure to extract functional signals from fossil remains. In conclusion of this review, we discuss some of the inherent challenges of the approaches and methodologies adopted to reconstruct the locomotor modes and behavioral repertoires in extinct primate taxa, and notably the assessment of habitual terrestrial bipedalism in early hominins.

Morphologies in-between: The impact of the first steps on the human talus

OBJECTIVE

The development of bipedalism is a very complex activity that contributes to shaping the anatomy of the foot. The talus, which starts ossifying in utero, may account for the developing stages from the late gestational phase onwards. Here, we explore the early development of the talus in both its internal and external morphology to broaden the knowledge of the anatomical changes that occur during early development.

MATERIALS AND METHODS

The sample consists of high-resolution microCT scans of 28 modern juvenile tali (from 36 prenatal weeks to 2 years), from a broad chronological range from the Late Roman period to the 20th century. We applied geometric morphometric and whole-bone trabecular analysis to investigate the early talar morphological changes.

RESULTS

In the youngest group (<6 postnatal months), the immature external shell is accompanied by an isotropic internal structure, with thin and densely packed trabeculae. After the initial attempts of locomotion, bone volume fraction decreases, while anisotropy and trabecular thickness increase. These internal changes correspond to the maturation of the external shell, which is now more defined and shows the development of the articular surfaces.

DISCUSSION

The internal and external morphology of the human talus reflects the diverse load on the foot during the initial phases of the bipedal locomotion, with the youngest group potentially reflecting the lack of readiness of the human talus to bear forces and perform bipedal walking. These results highlight the link between mechanical loading and bone development in the human talus during the acquisition of bipedalism, providing new insight into the early phases of talar development.

Human talar ontogeny: Insights from morphological and trabecular changes during postnatal growth

Objectives

The study of the development of human bipedalism can provide a unique perspective on the evolution of morphology and behavior across species. To generate new knowledge of these mechanisms, we analyze changes in both internal and external morphology of the growing human talus in a sample of modern human juveniles using an innovative approach.

Materials and Methods

The sample consists of high‐resolution microCT scans of 70 modern juvenile tali, aged between 8 postnatal weeks and 10 years old, from a broad chronological range from Middle/Late Neolithic, that is, between 4800 and 4500 BCE, to the 20th century. We applied geometric morphometric and whole‐bone trabecular analysis (bone volume fraction, degree of anisotropy, trabecular number, thickness, and spacing) to all specimens to identify changes in the external and internal morphology during growth. Morphometric maps were also generated.

Results

During the first year of life, the talus has an immature and globular shape, with a dense, compact, and rather isotropic trabecular architecture, with numerous trabeculae packed closely together. This pattern changes while children acquire a more mature gait, and the talus tends to have a lower bone volume fraction, a higher anisotropy, and a more mature shape.

Discussion

The changes in talar internal and external morphologies reflect the different loading patterns experienced during growth, gradually shifting from an “unspecialized” morphology to a more complex one, following the development of bipedal gait. Our research shows that talar plasticity, even though genetically driven, may show mechanical influences and contribute to tracking the main locomotor milestones.

Unique foot posture in Neanderthals reflects their body mass and high mechanical stress

Neanderthal foot bone proportions and morphology are mostly indistinguishable from those of Homo sapiens, with the exception of several distinct Neanderthal features in the talus. The biomechanical implications of these distinct talar features remain contentious, fueling debate around the adaptive meaning of this distinctiveness. With the aim of clarifying this controversy, we test phylogenetic and behavioral factors as possible contributors, comparing tali of 10 Neanderthals and 81 H. sapiens (Upper Paleolithic and Holocene hunter-gatherers, agriculturalists, and postindustrial group) along with the Clark Howell talus (Omo, Ethiopia). Variation in external talar structures was assessed through geometric morphometric methods, while bone volume fraction and degree of anisotropy were quantified in a subsample (n = 45). Finally, covariation between point clouds of site-specific trabecular variables and surface landmark coordinates was assessed. Our results show that although Neanderthal talar external and internal morphologies were distinct from those of H. sapiens groups, shape did not significantly covary with either bone volume fraction or degree of anisotropy, suggesting limited covariation between external and internal talar structures. Neanderthal external talar morphology reflects ancestral retentions, along with various adaptations to high levels of mobility correlated to their presumably unshod hunter-gatherer lifestyle. This pairs with their high site-specific trabecular bone volume fraction and anisotropy, suggesting intense and consistently oriented locomotor loading, respectively. Relative to H.sapiens, Neanderthals exhibit differences in the talocrural joint that are potentially attributable to cultural and locomotor behavior dissimilarity, a talonavicular joint that mixes ancestral and functional traits, and a derived subtalar joint that suggests a predisposition for a pronated foot during stance phase. Overall, Neanderthal talar variation is attributable to mobility strategy and phylogenesis, while H. sapiens talar variation results from the same factors plus footwear. Our results suggest that greater Neanderthal body mass and/or higher mechanical stress uniquely led to their habitually pronated foot posture.

Energetic and endurance constraints on great ape quadrupedalism and the benefits of hominin bipedalism

Bipedal walking was one of the first key behavioral traits that defined the evolution of early hominins. While it is not possible to identify specific selection pressures underlying bipedal evolution, we can better understand how the adoption of bipedalism may have benefited our hominin ancestors. Here, we focus on how bipedalism relaxes constraints on nonhuman primate quadrupedal limb mechanics, providing key advantages during hominin evolution. Nonhuman primate quadrupedal kinematics, especially in our closest living relatives, the great apes, are dominated by highly flexed limb joints, often associated with high energy costs, and are constrained by the need to reduce loads on mobile, but less stable forelimb joints. Bipedal walking would have allowed greater hind limb joint extension, which is associated with reduced energy costs and increased endurance. We suggest that relaxing these constraints provided bipedal hominins important benefits associated with long distance foraging and mobility.

Locomotory behaviour of early tetrapods from Blue Beach, Nova Scotia, revealed by novel microanatomical analysis

Evidence for terrestriality in early tetrapods is fundamentally contradictory. Fossil trackways attributed to early terrestrial tetrapods long predate the first body fossils from the Late Devonian. However, the Devonian body fossils demonstrate an obligatorily aquatic lifestyle. Complicating our understanding of the transition from water to land is a pronounced gap in the fossil record between the aquatic Devonian taxa and presumably terrestrial tetrapods from the later Early Carboniferous. Recent work suggests that an obligatorily aquatic habit persists much higher in the tetrapod tree than previously recognized. Here, we present independent microanatomical data of locomotor capability from the earliest Carboniferous of Blue Beach, Nova Scotia. The site preserves limb bones from taxa representative of Late Devonian to mid-Carboniferous faunas as well as a rich trackway record. Given that bone remodels in response to functional stresses including gravity and ground reaction forces, we analysed both the midshaft compactness profiles and trabecular anisotropy, the latter using a new whole bone approach. Our findings suggest that early tetrapods retained an aquatic lifestyle despite varied limb morphologies, prior to their emergence onto land. These results suggest that trackways attributed to early tetrapods be closely scrutinized for additional information regarding their creation conditions, and demand an expansion of sampling to better identify the first terrestrial tetrapods.

Cortical and trabecular bone structure of the hominoid capitate

Morphological variation in the hominoid capitate has been linked to differences in habitual locomotor activity due to its importance in movement and load transfer at the midcarpal joint proximally and carpometacarpal joints distally. Although the shape of bones and their articulations are linked to joint mobility, the internal structure of bones has been shown experimentally to reflect, at least in part, the loading direction and magnitude experienced by the bone. To date, it is uncertain whether locomotor differences among hominoids are reflected in the bone microarchitecture of the capitate. Here, we apply a whole‐bone methodology to quantify the cortical and trabecular architecture (separately and combined) of the capitate across bipedal (modern Homo sapiens), knuckle‐walking (Pan paniscus, Pan troglodytes, Gorilla sp.), and suspensory (Pongo sp.) hominoids (n = 69). It is hypothesized that variation in bone microarchitecture will differentiate these locomotor groups, reflecting differences in habitual postures and presumed loading force and direction. Additionally, it is hypothesized that trabecular and cortical architecture in the proximal and distal regions, as a result of being part of mechanically divergent joints proximally and distally, will differ across these portions of the capitate. Results indicate that the capitate of knuckle‐walking and suspensory hominoids is differentiated from bipedal Homo primarily by significantly thicker distal cortical bone. Knuckle‐walking taxa are further differentiated from suspensory and bipedal taxa by more isotropic trabeculae in the proximal capitate. An allometric analysis indicates that size is not a significant determinate of bone variation across hominoids, although sexual dimorphism may influence some parameters within Gorilla. Results suggest that internal trabecular and cortical bone is subjected to different forces and functional adaptation responses across the capitate (and possibly other short bones). Additionally, while separating trabecular and cortical bone is normal protocol of current whole‐bone methodologies, this study shows that when applied to carpals, removing or studying the cortical bone separately potentially obfuscates functionally relevant signals in bone structure.

Trabecular bone properties in the ilium of the Middle Paleolithic/Middle Stone Age Border Cave 3 Homo sapiens infant and the onset of independent gait

The Border Cave 3 (BC3) infant skeleton has been understudied, despite its importance as an example of a well-preserved and fairly complete immature skeleton of early Homo sapiens which potentially provides a rare window into various aspects of ontogenetic development, including locomotor activity (e.g., timing of gait events). Trabecular structure in the BC3 ilium was evaluated to investigate whether it matches that of an equivalently aged infant from a postindustrialized society. Microcomputed tomography (μCT) scans were acquired from the BC3 infant and from an ontogenetic series of 25 postindustrial infants that were divided into three age classes (ACs) ranging from neonates to toddlers (<36 months). All ilia were qualitatively compared and then digitally subdivided into 10 volumes of interest (VOIs) based on anatomical reference points. The VOIs were quantified and ontogenetic differences in trabecular structure were statistically evaluated. Across the comparative ontogenetic series, trabecular architectural properties overlapped in all regions. However, trabecular thickness increased significantly after the first year of life. The BC3 infant demonstrated generally similar trabecular structure to that observed in the age-equivalent postindustrial infants (AC2), including relatively strong development of the trabecular chiasma qualitatively. However, some interesting distinctions were observed in BC3, such as low strut thickness compared with infants from the postindustrial sample, that bear further exploration in future studies. Evaluation of only one individual from the Middle Stone Age (MSA), coupled with the relatively small comparative sample, limit our ability to distinguish more meaningful biological differences in trabecular structure throughout ontogeny from idiosyncratic characteristics. Nonetheless, results of this study extend ongoing research on infant locomotor and morphological development to archeological populations in the Middle Stone Age. Further cross-cultural studies consisting of larger comparative postindustrial samples may provide additional information on trabecular structure in the infant ilium during this important developmental timeframe.

The pectoral girdle of StW 573 ('Little Foot') and its implications for shoulder evolution in the Hominina

The ca. 3.67 Ma adult skeleton known as 'Little Foot' (StW 573), recovered from Sterkfontein Member 2 breccia in the Silberberg Grotto, is remarkable for its morphology and completeness. Preservation of clavicles and scapulae, including essentially complete right-side elements, offers opportunities to assess morphological and functional aspects of a nearly complete Australopithecus pectoral girdle. Here we describe the StW 573 pectoral girdle and offer quantitative comparisons to those of extant hominoids and selected homininans. The StW 573 pectoral girdle combines features intermediate between those of humans and other apes: a long and curved clavicle, suggesting a relatively dorsally positioned scapula; an enlarged and uniquely proportioned supraspinous fossa; a relatively cranially oriented glenoid fossa; and ape-like reinforcement of the axillary margin by a stout ventral bar. StW 573 scapulae are as follows: smaller than those of some homininans (i.e., KSD-VP-1/1 and KNM-ER 47000A), larger than others (i.e., A.L. 288-1, Sts 7, and MH2), and most similar in size to another australopith from Sterkfontein, StW 431. Moreover, StW 573 and StW 431 exhibit similar structural features along their axillary margins and inferior angles. As the StW 573 pectoral girdle (e.g., scapular configuration) has a greater affinity to that of apes-Gorilla in particular-rather than modern humans, we suggest that the StW 573 morphological pattern appears to reflect adaptations to arboreal behaviors, especially those with the hand positioned above the head, more than human-like manipulatory capabilities. When compared with less complete pectoral girdles from middle/late Miocene apes and that of the penecontemporaneous KSD-VP-1/1 (Australopithecus afarensis), and mindful of consensus views on the adaptiveness of arboreal positional behaviors soliciting abducted glenohumeral joints in early Pliocene taxa, we propose that the StW 573 pectoral girdle is a reasonable model for hypothesizing pectoral girdle configuration of the crown hominin last common ancestor.

Trabecular organization of the proximal femur in Paranthropus robustus: Implications for the assessment of its hip joint loading conditions

Reconstruction of the locomotor repertoire of the australopiths (Australopithecus and Paranthropus) has progressively integrated information from the mechanosensitive internal structure of the appendicular skeleton. Recent investigations showed that the arrangement of the trabecular network at the femoral head center is biomechanically compatible with the pattern of cortical bone distribution across the neck, both suggesting a full commitment to bipedalism in australopiths, but associated with a slightly altered gait kinematics compared to Homo involving more lateral deviation of the body center of mass over the stance limb. To provide a global picture in Paranthropus robustus of the trabecular architecture of the proximal femur across the head, neck and greater trochanter compartments, we applied techniques of virtual imaging to the variably preserved Early Pleistocene specimens SK 82, SK 97, SK 3121, SKW 19 and SWT1/LB-2 from the cave site of Swartkrans, South Africa. We also assessed the coherence between the structural signals from the center of the head and those from the trabecular network of the inferolateral portion of the head and the inferior margin of the neck, sampling the so-called vertical bundle, which in humans represents the principal compressive system of the joint. Our analyses show a functionally related trabecular organization in Pa. robustus that closely resembles the extant human condition, but which also includes some specificities in local textural arrangement. The network of the inferolateral portion of the head shows a humanlike degree of anisotropy and a bone volume fraction intermediate between the extant human and the African ape patterns. These results suggest slight differences in gait kinematics between Pa. robustus and extant humans. The neck portion of the vertical bundle revealed a less biomechanically sensitive signal. Future investigations on the australopith hip joint loading environment should more carefully investigate the trabecular structure of the trochanteric region and possible structural covariation between cortical bone distribution across the neck and site-specific trabecular properties of the arcuate bundle.

Combinations of trabecular and cortical bone properties distinguish various loading modalities between athletes and controls

OBJECTIVES

Variation in trabecular and cortical bone properties is often used to infer habitual behavior in the past. However, the structures of both types of bone are rarely considered together and may even contradict each other in functional interpretations. We examine trabecular and cortical bone properties in various athletes and sedentary controls to clarify the associations between combinations of cortical and trabecular bone properties and various loading modalities.

MATERIALS AND METHODS

We compare trabecular and cortical bone properties using peripheral quantitative computed tomography scans of the tibia between groups of 83 male athletes (running, hockey, swimming, cricket) and sedentary controls using Bayesian multilevel models. We quantify midshaft cortical bone rigidity and area (J, CA), midshaft shape index (Imax/Imin), and mean trabecular bone mineral density (BMD) in the distal tibia.

RESULTS

All groups show unique combinations of biomechanical properties. Cortical bone rigidity is high in sports that involve impact loading (cricket, running, hockey) and low in nonimpact loaded swimmers and controls. Runners have more anteroposteriorly elliptical midshafts compared to other groups. Interestingly, all athletes have greater trabecular BMD compared to controls, but do not differ credibly among each other.

DISCUSSION

Results suggest that cortical midshaft hypertrophy is associated with impact loading while trabecular BMD is positively associated with both impact and nonimpact loading. Midshaft shape is associated with directionality of loading. Individuals from the different categories overlap substantially, but group means differ credibly, suggesting that nuanced group-level inferences of habitual behavior are possible when combinations of trabecular and cortical bone are analyzed.

Automated resolution independent method for comparing in vivo and dry trabecular bone

OBJECTIVES

Variation in human trabecular bone morphology can be linked to habitual behavior, but it is difficult to investigate in vivo due to the radiation required at high resolution. Consequently, functional interpretations of trabecular morphology remain inferential. Here we introduce a method to link low- and high-resolution CT data from dry and fresh bone, enabling bone functional adaptation to be studied in vivo and results compared to the fossil and archaeological record.

MATERIALS AND METHODS

We examine 51 human dry bone distal tibiae from Nile Valley and UK and two pig tibiae containing soft tissues. We compare low-resolution peripheral quantitative computed tomography (pQCT) parameters and high-resolution micro CT (μCT) in homologous single slices at 4% bone length and compare results to our novel Bone Ratio Predictor (BRP) method.

RESULTS

Regression slopes between linear attenuation coefficients of low-resolution pQCT images and bone area/total area (BA/TA) of high-resolution μCT scans differ substantially between geographical subsamples, presumably due to diagenesis. BRP accurately predicts BA/TA (R² = .97) and eliminates the geographic clustering. BRP accurately estimates BA/TA in pigs containing soft tissues (R² = 0.98) without requiring knowledge of true density or phantom calibration of the scans.

DISCUSSION

BRP allows automated comparison of image data from different image modalities (pQCT, μCT) using different energy settings, in archeological bone and wet specimens. The method enables low-resolution data generated in vivo to be compared with the fossil and archaeological record. Such experimental approaches would substantially improve behavioral inferences based on trabecular bone microstructure.

Using point clouds to investigate the relationship between trabecular bone phenotype and behavior: An example utilizing the human calcaneus

OBJECTIVES

The objective of this study is to demonstrate a new method for analyzing trabecular bone volume fraction and degree of anisotropy in three dimensions.

METHODS

We use a combination of automatic mesh registration, point-cloud correspondence registration, and P-value corrected univariate statistical tests to compare bone volume fraction and degree of anisotropy on a point by point basis across the entire calcaneus of two human groups with different subsistence strategies.

RESULTS

We found that the patterns of high and low bone volume fraction and degree of anisotropy distribution between the Black Earth (hunter-gatherers) and Norris Farms (mixed-strategy agriculturalists) are very similar, but differ in magnitude. The hunter-gatherers exhibit higher levels of bone volume fraction and less anisotropic trabecular bone organization. Additionally, patterns of bone volume fraction and degree of anisotropy in the calcaneus correspond well with biomechanical expectations of relative forces experienced during walking and running.

CONCLUSIONS

We conclude that comparing site-specific, localized differences in trabecular bone variables such as bone volume fraction and degree of anisotropy in three-dimensions is a powerful analytical tool. This method makes it possible to determine where similarities and differences between groups are located within the whole skeletal element of interest. The visualization of multiple variables also provides a way for researchers to see how the trabecular bone variables interact within the morphology, and allows for a more nuanced understanding of how they relate to one another and the broader mechanical environment.

Morphometric analysis of the hominin talus: Evolutionary and functional implications

The adoption of bipedalism is a key benchmark in human evolution that has impacted talar morphology. Here, we investigate talar morphological variability in extinct and extant hominins using a 3D geometric morphometric approach. The evolutionary timing and appearance of modern human-like features and their contributions to bipedal locomotion were evaluated on the talus as a whole, each articular facet separately, and multiple combinations of facets. Distinctive suites of features are consistently present in all fossil hominins, despite the presence of substantial interspecific variation, suggesting a potential connection of these suites to bipedal gait. A modern human-like condition evolved in navicular and lateral malleolar facets early in the hominin lineage compared with other facets, which demonstrate more complex morphological variation within Homininae. Interestingly, navicular facet morphology of Australopithecus afarensis is derived in the direction of Homo, whereas more recent hominin species such as Australopithecus africanus and Australopithecus sediba retain more primitive states in this facet. Combining the navicular facet with the trochlea and the posterior calcaneal facet as a functional suite, however, distinguishes Australopithecus from Homo in that the medial longitudinal arch had not fully developed in the former. Our results suggest that a more everted foot and stiffer medial midtarsal region are adaptations that coincide with the emergence of bipedalism, whereas a high medial longitudinal arch emerges later in time, within Homo. This study provides novel insights into the emergence of talar morphological traits linked to bipedalism and its transition from a facultative to an obligate condition.

First metatarsal trabecular bone structure in extant hominoids and Swartkrans hominins

Changes in first metatarsal (MT1) morphology within the hominin clade are crucial for reconstructing the evolution of a forefoot adapted for human-like gait. Studies of the external morphology of the MT1 in humans, non-human apes, and fossil hominins have documented changes in its robusticity, epiphyseal shape and its articulation with the medial cuneiform. Here, we test whether trabecular structure in the MT1 reflects different loading patterns in the forefoot across extant large apes and humans, and within this comparative context, infer locomotor behavior in two fossil hominins from Swartkrans, South Africa. Microtomographic scans were collected from the MT1 of Pongo sp. (n = 6), Gorilla gorilla (n = 10), Pan troglodytes (n = 10), Homo sapiens (n = 11), as well as SKX 5017 (Paranthropus robustus), and SK 1813 (Hominin gen. sp. indet.). Trabecular structure was quantified within the head and base using a 'whole-epiphysis' approach with medtool 4.2. We found that modern humans displayed relatively higher bone volume fraction (BV/TV) in the dorsal region of each epiphysis and a higher overall degree of anisotropy (DA), whereas great apes showed higher BV/TV in the plantar regions, reflecting dorsiflexion at the metatarsophalangeal (MTP) joint in the former and plantarflexion in the latter. Both fossils displayed low DA, with SKX 5017 showing a hyper-dorsal concentration of trabecular bone in the head (similar to humans), while SK 1813 showed a more central trabecular distribution not seen in either humans or non-human apes. Additionally, we found differences between non-human apes, modern humans, and the fossil taxa in trabecular spacing (Tb.Sp.), number (Tb.N.), and thickness (Tb.th.). While low DA in both fossils suggests increased mobility of the MT1, differences in their trabecular distributions could indicate variable locomotion in these Pleistocene hominins (recognizing that the juvenile status of SK 1813 is a potential confounding factor). In particular, evidence for consistent loading in hyper-dorsiflexion in SKX 5017 would suggest locomotor behaviors beyond human-like toe off during terrestrial locomotion.

Cancellous bone and theropod dinosaur locomotion. Part I-an examination of cancellous bone architecture in the hindlimb bones of theropods

This paper is the first of a three-part series that investigates the architecture of cancellous ('spongy') bone in the main hindlimb bones of theropod dinosaurs, and uses cancellous bone architectural patterns to infer locomotor biomechanics in extinct non-avian species. Cancellous bone is widely known to be highly sensitive to its mechanical environment, and has previously been used to infer locomotor biomechanics in extinct tetrapod vertebrates, especially primates. Despite great promise, cancellous bone architecture has remained little utilized for investigating locomotion in many other extinct vertebrate groups, such as dinosaurs. Documentation and quantification of architectural patterns across a whole bone, and across multiple bones, can provide much information on cancellous bone architectural patterns and variation across species. Additionally, this also lends itself to analysis of the musculoskeletal biomechanical factors involved in a direct, mechanistic fashion. On this premise, computed tomographic and image analysis techniques were used to describe and analyse the three-dimensional architecture of cancellous bone in the main hindlimb bones of theropod dinosaurs for the first time. A comprehensive survey across many extant and extinct species is produced, identifying several patterns of similarity and contrast between groups. For instance, more stemward non-avian theropods (e.g. ceratosaurs and tyrannosaurids) exhibit cancellous bone architectures more comparable to that present in humans, whereas species more closely related to birds (e.g. paravians) exhibit architectural patterns bearing greater similarity to those of extant birds. Many of the observed patterns may be linked to particular aspects of locomotor biomechanics, such as the degree of hip or knee flexion during stance and gait. A further important observation is the abundance of markedly oblique trabeculae in the diaphyses of the femur and tibia of birds, which in large species produces spiralling patterns along the endosteal surface. Not only do these observations provide new insight into theropod anatomy and behaviour, they also provide the foundation for mechanistic testing of locomotor hypotheses via musculoskeletal biomechanical modelling.

Trabecular bone patterning across the human hand

Hand bone morphology is regularly used to link particular hominin species with behaviors relevant to cognitive/technological progress. Debates about the functional significance of differing hominin hand bone morphologies tend to rely on establishing phylogenetic relationships and/or inferring behavior from epigenetic variation arising from mechanical loading and adaptive bone modeling. Most research focuses on variation in cortical bone structure, but additional information about hand function may be provided through the analysis of internal trabecular structure. While primate hand bone trabecular structure is known to vary in ways that are consistent with expected joint loading differences during manipulation and locomotion, no study exists that has documented this variation across the numerous bones of the hand. We quantify the trabecular structure in 22 bones of the human hand (early/extant modern Homo sapiens) and compare structural variation between two groups associated with post-agricultural/industrial (post-Neolithic) and foraging/hunter-gatherer (forager) subsistence strategies. We (1) establish trabecular bone volume fraction (BV/TV), modulus (E), degree of anisotropy (DA), mean trabecular thickness (Tb.Th) and spacing (Tb.Sp); (2) visualize the average distribution of site-specific BV/TV for each bone; and (3) examine if the variation in trabecular structure is consistent with expected joint loading differences among the regions of the hand and between the groups. Results indicate similar distributions of trabecular bone in both groups, with those of the forager sample presenting higher BV/TV, E, and lower DA, suggesting greater and more variable loading during manipulation. We find indications of higher loading along the ulnar side of the forager sample hand, with high site-specific BV/TV distributions among the carpals that are suggestive of high loading while the wrist moves through the 'dart-thrower's' motion. These results support the use of trabecular structure to infer behavior and have direct implications for refining our understanding of human hand evolution and fossil hominin hand use.

Systemic patterns of trabecular bone across the human and chimpanzee skeleton

Aspects of trabecular bone architecture are thought to reflect regional loading of the skeleton, and thus differ between primate taxa with different locomotor and postural modes. However, there are several systemic factors that affect bone structure that could contribute to, or be the primary factor determining, interspecific differences in bone structure. These systemic factors include differences in genetic regulation, sensitivity to loading, hormone levels, diet, and activity levels. Improved understanding of inter-/intraspecific variability, and variability across the skeleton of an individual, is required to interpret properly potential functional signals present within trabecular structure. Using a whole-region method of analysis, we investigated trabecular structure throughout the skeleton of humans and chimpanzees. Trabecular bone volume fraction (BV/TV), degree of anisotropy (DA) and trabecular thickness (Tb.Th) were quantified from high resolution micro-computed tomographic scans of the humeral and femoral head, third metacarpal and third metatarsal head, distal tibia, talus and first thoracic vertebra. We found that BV/TV is, in most anatomical sites, significantly higher in chimpanzees than in humans, suggesting a systemic difference in trabecular structure unrelated to local loading regime. Differences in BV/TV between the forelimb and hindlimb did not clearly reflect differences in locomotor loading in the study taxa. There were no clear systemic differences between the taxa in DA and, as such, this parameter might reflect function and relate to differences in joint loading. This systemic approach reveals both the pattern of variability across the skeleton and between taxa, and helps identify those features of trabecular structure that may relate to joint function.

Trabecular and cortical bone structure of the talus and distal tibia in Pan and Homo

OBJECTIVES

Internal bone structure, both cortical and trabecular bone, remodels in response to loading and may provide important information regarding behavior. The foot is well suited to analysis of internal bone structure because it experiences the initial substrate reaction forces, due to its proximity to the substrate. Moreover, as humans and apes differ in loading of the foot, this region is relevant to questions concerning arboreal locomotion and bipedality in the hominoid fossil record.

MATERIALS AND METHODS

We apply a whole-bone/epiphysis approach to analyze trabecular and cortical bone in the distal tibia and talus of Pan troglodytes and Homo sapiens. We quantify bone volume fraction (BV/TV), degree of anisotropy (DA), trabecular thickness (Tb.Th), bone surface to volume ratio (BS/BV), and cortical thickness and investigate the distribution of BV/TV and cortical thickness throughout the bone/epiphysis.

RESULTS

We find that Pan has a greater BV/TV, a lower BS/BV and thicker cortices than Homo in both the talus and distal tibia. The trabecular structure of the talus is more divergent than the tibia, having thicker, less uniformly aligned trabeculae in Pan compared to Homo. Differences in dorsiflexion at the talocrural joint and in degree of mobility at the talonavicular joint are reflected in the distribution of cortical and trabecular bone.

DISCUSSION

Overall, quantified trabecular parameters represent overall differences in bone strength between the two species, however, DA may be directly related to joint loading. Cortical and trabecular bone distributions correlate with habitual joint positions adopted by each species, and thus have potential for interpreting joint position in fossil hominoids.