Femoral neck and shaft structure in Homo naledi from the Dinaledi Chamber (Rising Star System, South Africa)

Long legs and small joints: The locomotor capabilities of Homo naledi

The lower limb of Homo naledi presents a suite of primitive, derived and unique morphological features that pose interesting questions about the nature of bipedal movement in this species. The exceptional representation of all skeletal elements in H. naledi makes it an excellent candidate for biomechanical analysis of gait dynamics using modern kinematic software. However, virtual gait analysis software requires 3D models of the entire lower limb kinematic chain. No single H. naledi individual preserves all lower limb elements, and what material is preserved is fragmentary. As an antecedent to future kinematic analysis, a 3D lower limb skeleton was reconstructed from the most complete fossil bones of different H. naledi individuals. As both juvenile and adult H. naledi were used, we tested if the knee joint remained congruent throughout ontogeny in a sample of great apes (N = 143) and modern humans (N = 70). The reconstruction and subsequent comparative analysis reveal that H. naledi had remarkably small joint sizes for their body size, a hyper-elongated tibia, and a high crural index (90.2). We consider that the lower limb morphology of H. naledi could have improved locomotor economy, but the exceptionally small joints cast doubt on its capabilities for long distance travel, including endurance running. The unusual mixture of primitive and derived traits in H. naledi remains intriguing and might indicate that this hominin engaged both in bipedal walking and climbing, demonstrating that kinematic diversity in hominins persisted well into the Middle Pleistocene.

Comparing Methods for Calculating Cross-Sectional Properties From High-Resolution CT Scans in Dry Bone: The Case of the Non-Adult Femoral Midneck

OBJECTIVE

This paper explores the various protocols for deriving endosteal and periosteal contours of the femoral midneck cross-section in non-adult individuals using EPmacroJ.

MATERIALS AND METHODS

We analyzed femoral midneck cross-sectional contours in a total sample of 55 femora belonging to medieval individuals ranging in age between 2 and 20 years. Femoral midneck cross-sections were taken on high-resolution CT images and analyzed by EPJMacro in FIJI. Cross-sectional properties were obtained from periosteal and endosteal contours derived by manual, automatic, spline, and ellipse techniques. Agreement between the manual and the other techniques was evaluated using percent prediction error (%PE) and following the Bland-Altman approach.

RESULTS

All the evaluated techniques used for deriving the periosteal and endosteal contours at the femoral midneck cross-section in non-adult individuals have proven to be interchangeable with manual processing, except for the spline technique for the endosteal contour. The narrowest limits of agreement were obtained for the total area, moderate limits of agreement were found for the cortical and medullary areas, and the widest limits of agreement were observed in the section moduli.

DISCUSSION

The automatic processing of the periosteal and endosteal contours shows a high level of agreement with the manual processing of both contours. Semi-automatic approaches (spline and ellipse) provide less agreement, especially in section moduli. This suggests that inaccuracies in deriving the endosteal contour significantly affect section moduli. The best approach for extracting the geometric properties of femoral midneck cross-sections is a combination of automatic and manual methods, although automatic methods alone are also effective.

What we know and do not know after the first decade of Homo naledi

It has been just over 10 years since the first fossils attributed to Homo naledi were recovered from the Rising Star Cave system in South Africa's Cradle of Humankind. The hominin fossil evidence for H. naledi displays a distinctive combination of primitive and derived morphology, yet for a time-averaged fossil sample it is remarkable for its relatively low level of variation. Thus-unusually for palaeoanthropology-there has been little pushback against the decision to recognize a single novel taxon for all of the material recovered from the Rising Star Cave system. However, almost everything else claimed about H. naledi-its age, burial context and behaviour-has been controversial. Here we examine the strength of the evidence for these claims.

Biomechanical and taxonomic diversity in the Early Pleistocene in East Africa: Structural analysis of a recently discovered femur shaft from Olduvai Gorge (bed I)

Recent Plio-Pleistocene hominin findings have revealed the complexity of human evolutionary history and the difficulties involved in its interpretation. Moreover, the study of hominin long bone remains is particularly problematic, since it commonly depends on the analysis of fragmentary skeletal elements that in many cases are merely represented by small diaphyseal portions and appear in an isolated fashion in the fossil record. Nevertheless, the study of the postcranial skeleton is particularly important to ascertain locomotor patterns. Here we report on the discovery of a robust hominin femoral fragment (OH 84) at the site of Amin Mturi Korongo dated to 1.84 Ma (Olduvai Bed I). External anatomy and internal bone structure of OH 84 were analyzed and compared with previously published data for modern humans and chimpanzees, as well as for Australopithecus, Paranthropus and Homo specimens ranging from the Late Pliocene to Late Pleistocene. Biomechanical analyses based on transverse cross-sections and the comparison of OH 84 with another robust Olduvai specimen (OH 80) suggest that OH 84 might be tentatively allocated to Paranthropus boisei. More importantly, the identification of a unique combination of traits in OH 84 could indicate both terrestrial bipedalism and an arboreal component in the locomotor repertoire of this individual. If interpreted correctly, OH 84 could thus add to the already mounting evidence of substantial locomotor diversity among Early Pleistocene hominins. Likewise, our results also highlight the difficulties in accurately interpreting the link between form and function in the human fossil record based on fragmentary remains, and ultimately in distinguishing between coeval hominin groups due to the heterogeneous pattern of inter- and intraspecific morphological variability detected among fossil femora.

The taxonomic attribution of African hominin postcrania from the Miocene through the Pleistocene: Associations and assumptions

Postcranial bones may provide valuable information about fossil taxa relating to their locomotor habits, manipulative abilities and body sizes. Distinctive features of the postcranial skeleton are sometimes noted in species diagnoses. Although numerous isolated postcranial fossils have become accepted by many workers as belonging to a particular species, it is worthwhile revisiting the evidence for each attribution before including them in comparative samples in relation to the descriptions of new fossils, functional analyses in relation to particular taxa, or in evolutionary contexts. Although some workers eschew the taxonomic attribution of postcranial fossils as being less important (or interesting) than interpreting their functional morphology, it is impossible to consider the evolution of functional anatomy in a taxonomic and phylogenetic vacuum. There are 21 widely recognized hominin taxa that have been described from sites in Africa dated from the Late Miocene to the Middle Pleistocene; postcranial elements have been attributed to 17 of these. The bones that have been thus assigned range from many parts of a skeleton to isolated elements. However, the extent to which postcranial material can be reliably attributed to a specific taxon varies considerably from site to site and species to species, and is often the subject of considerable debate. Here, we review the postcranial remains attributed to African hominin taxa from the Late Miocene to the Middle and Late Pleistocene and place these assignations into categories of reliability. The catalog of attributions presented here may serve as a guide for making taxonomic decisions in the future.

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.