The hominoid proximal radius: re-interpreting locomotor behaviors in early hominins

Specimens as individuals: Four interventions and recommendations for great ape skeletal collections research and curation

Extensive discourse surrounds the ethics of human skeletal research and curation, but there has yet to be a similar discussion of the treatment of great ape skeletal remains, despite the clear interest in their ethical treatment when alive. Here we trace the history of apes who were killed and collected for natural history museums during the early 20th century and showcase how the guiding research questions of the colonial era continue to influence scholarship. We discuss best practices for improving industry and academic standards of research on, and the curation of, ape remains. The suggested interventions involve restoring individual identity and narrative to great apes while engaging with contextual reflexivity and decolonial theory. The resulting recommendations include contextualizing the individual, piecing individuals back together, challenging/questioning the captive-wild dichotomy, and collaborative international conversations. Our objective is to encourage a conversation regarding ethical and theoretical considerations in great ape skeletal remains research.

Taxonomic attribution of the KNM-ER 1500 partial skeleton from the Burgi Member of the Koobi Fora Formation, Kenya

Paranthropus boisei is well represented in the eastern African fossil record by craniodental remains, but very few postcranial fossils can be securely attributed to this taxon. For this reason, KNM-ER 1500 from East Turkana, Kenya, is especially important. KNM-ER 1500 is a badly weathered and fragmented postcranial skeleton associated with a small piece of mandibular corpus. It derives from the Burgi Member, which has yielded diagnostic craniodental fossils attributable to P. boisei, Homo habilis, Homo rudolfensis and Homo erectus. Although it has been proposed that KNM-ER 1500 may be attributable to P. boisei based on the small mandibular fragment, this hypothesis remained challenging to test. Here we re-examine the preserved portions of KNM-ER 1500 and reassess support for its taxonomic attribution. There are compelling features of the mandible, proximal femur, and especially the proximal radius that support attribution of KNM-ER 1500 to P. boisei. These features include the absolute width of the mandible and its lack of a lateral intertoral sulcus, an anteroposteriorly compressed femoral neck with a distinctive posteroinferior marginal ridge, the rim of the radial head that is proximodistally uniform in thickness around its circumference, and a long radial neck that is elliptical in cross section. No feature serves to align KNM-ER 1500 with Homo to the exclusion of Paranthropus. KNM-ER 1500 was a small-bodied individual and attributing this specimen to P. boisei confirms that significant postcranial-size dimorphism was present in this species.

A proximal radius of Barberapithecus huerzeleri from Castell de Barberà: Implications for locomotor diversity among pliopithecoids

Pliopithecoids are a diverse group of Miocene catarrhine primates from Eurasia. Their positional behavior is still unknown, and many species are known exclusively from dentognathic remains. Here, we describe a proximal radius (IPS66267) from the late Miocene of Castell de Barberà (Vallès-Penedès Basin, NE Iberian Peninsula) that represents the first postcranial specimen of the pliopithecoid Barberapithecus huerzeleri. A body mass estimate based on the radius is compared with dental estimates, and its morphology is compared with that of extant and fossil anthropoids by qualitative means as well as by landmark-based three-dimensional geometric morphometrics. The estimated body mass of ∼5 kg for IPS66267 closely matches the dental estimates for the (female) holotype, thereby discounting an alternative attribution to the large-bodied hominoid recorded at Castell de Barberà. In multiple features (oval and moderately tilted head with a pronounced lateral lip and a restricted articular area for the capitulum; proximodistally expanded proximal radioulnar joint; and short, robust, and anteroposteriorly compressed neck), the specimen differs from hominoids and resembles instead extant nonateline monkeys and stem catarrhines. The results of the morphometric analysis further indicate that the Barberapithecus proximal radius shows closer similarities with nonsuspensory arboreal cercopithecoids and the dendropithecid Simiolus. From a locomotor viewpoint, the radius of Barberapithecus lacks most of the features functionally related to climbing and/or suspensory behaviors and displays instead a proximal radioulnar joint that would have been particularly stable under pronation. On the other hand, the Barberapithecus radius differs from other stem catarrhines in the less anteroposteriorly compressed and less tilted radial head with a deeper capitular fovea, suggesting a somewhat enhanced mobility at the elbow joint. We conclude that pronograde arboreal quadrupedalism was the main component of the locomotor repertoire of Barberapithecus but that, similar to other crouzeliids, it might have displayed better climbing abilities than pliopithecids.

Load bearing performance of mechanical joints inspired by elbow of quadrupedal mammals

One of the biggest issues of the mechanical cylindrical joints is related to premature wear appearing. Application of bioinspiration principles in an engineering context taking advantage of smart solutions offered by nature in terms of kinematic joints could be a way of solving those problems. This work is focussed on joints of one degrees of freedom in rotation (revolute or ginglymus joints in biological terms), as this is one of the most common type of mechanical joints. This type of joints can be found in the elbow of some quadrupedal mammals. The articular morphology of the elbow of these animals differs in the presence/absence of a trochlear sulcus. In this study, bio-inspired mechanical joints based on these morphologies (with/without trochlear sulcus) were designed and numerically tested. Their load bearing performance was numerically analysed. This was done through contact simulations using the finite element method under different external loading conditions (axial load, radial load and turnover moment). Results showed that the tested morphologies behave differently in transmission of external mechanical loads. It was found that bio-inspired joints without trochlea sulcus showed to be more specialized in the bearing of turnover moments. Bio-inspired joints with trochlea sulcus are more suitable for supporting combined loads (axial and radial load and turnover moments). Learning about the natural rules of mechanical design can provide new insights to improve the design of current mechanical joints.

Ontogenetic and morphological variation in primate long bones reflects signals of size and behavior

OBJECTIVES

Many primates change their locomotor behavior as they mature from infancy to adulthood. Here we investigate how long bone cross-sectional geometry in Pan, Gorilla, Pongo, Hylobatidae, and Macaca varies in shape and form over ontogeny, including whether specific diaphyseal cross sections exhibit signals of periosteal adaptation or canalization.

MATERIALS AND METHODS

Diaphyseal cross sections were analyzed in an ontogenetic series across infant, juvenile, and adult subgroups. Three-dimensional laser-scanned long bone models were sectioned at midshaft (50% of biomechanical length) and distally (20%) along the humerus and femur. Traditional axis ratios acted as indices of cross-sectional circularity, while geometric morphometric techniques were used to study cross-sectional allometry and ontogenetic trajectory.

RESULTS

The humeral midshaft is a strong indicator of posture and locomotor profile in the sample across development, while the mid-femur appears more reflective of shifts in size. By comparison, the distal diaphyses of both limb elements are more ontogenetically constrained, where periosteal shape is largely static across development relative to size, irrespective of a given taxon's behavior or ecology.

DISCUSSION

Primate limb shape is not only highly variable between taxa over development, but at discrete humeral and femoral diaphyseal locations. Overall, periosteal shape of the humeral and femoral midshaft cross sections closely reflects ontogenetic transitions in behavior and size, respectively, while distal shape in both bones appears more genetically constrained across intraspecific development, regardless of posture or size. These findings support prior research on tradeoffs between function and safety along the limbs.

Cross-sectional properties of the humeral diaphysis of Paranthropus boisei: Implications for upper limb function

A ∼1.52 Ma adult upper limb skeleton of Paranthropus boisei (KNM-ER 47000) recovered from the Koobi Fora Formation, Kenya (FwJj14E, Area 1A) includes most of the distal half of a right humerus (designated KNM-ER 47000B). Natural transverse fractures through the diaphysis of KNM-ER 470000B provide unobstructed views of cortical bone at two sections typically used for analyzing cross-sectional properties of hominids (i.e., 35% and 50% of humerus length from the distal end). Here we assess cross-sectional properties of KNM-ER 47000B and two other P. boisei humeri (OH 80-10, KNM-ER 739). Cross-sectional properties for P. boisei associated with bending/torsional strength (section moduli) and relative cortical thickness (%CA; percent cortical area) are compared to those reported for nonhuman hominids, AL 288-1 (Australopithecus afarensis), and multiple species of fossil and modern Homo. Polar section moduli (Z_p) are assessed relative to a mechanically relevant measure of body size (i.e., the product of mass [M] and humerus length [HL]). At both diaphyseal sections, P. boisei exhibits %CA that is high among extant hominids (both human and nonhuman) and similar to that observed among specimens of Pleistocene Homo. High values for Z_p relative to size (M × HL) indicate that P. boisei had humeral bending strength greater than that of modern humans and Neanderthals and similar to that of great apes, A. afarensis, and Homo habilis. Such high humeral strength is consistent with other skeletal features of P. boisei (reviewed here) that suggest routine use of powerful upper limbs for arboreal climbing.

A two-million-year-long hydroclimatic context for hominin evolution in southeastern Africa

The past two million years of eastern African climate variability is currently poorly constrained, despite interest in understanding its assumed role in early human evolution^1-4. Rare palaeoclimate records from northeastern Africa suggest progressively drier conditions^2,5 or a stable hydroclimate⁶. By contrast, records from Lake Malawi in tropical southeastern Africa reveal a trend of a progressively wetter climate over the past 1.3 million years^7,8. The climatic forcings that controlled these past hydrological changes are also a matter of debate. Some studies suggest a dominant local insolation forcing on hydrological changes^9-11, whereas others infer a potential influence of sea surface temperature changes in the Indian Ocean^8,12,13. Here we show that the hydroclimate in southeastern Africa (20-25° S) is controlled by interplay between low-latitude insolation forcing (precession and eccentricity) and changes in ice volume at high latitudes. Our results are based on a multiple-proxy reconstruction of hydrological changes in the Limpopo River catchment, combined with a reconstruction of sea surface temperature in the southwestern Indian Ocean for the past 2.14 million years. We find a long-term aridification in the Limpopo catchment between around 1 and 0.6 million years ago, opposite to the hydroclimatic evolution suggested by records from Lake Malawi. Our results, together with evidence of wetting at Lake Malawi, imply that the rainbelt contracted toward the Equator in response to increased ice volume at high latitudes. By reducing the extent of woodland or wetlands in terrestrial ecosystems, the observed changes in the hydroclimate of southeastern Africa-both in terms of its long-term state and marked precessional variability-could have had a role in the evolution of early hominins, particularly in the extinction of Paranthropus robustus.

Functional associations between support use and forelimb shape in strepsirrhines and their relevance to inferring locomotor behavior in early primates

The evolution of primates is intimately linked to their initial invasion of an arboreal environment. However, moving and foraging in this milieu creates significant mechanical challenges related to the presence of substrates differing in their size and orientation. It is widely assumed that primates are behaviorally and anatomically adapted to movement on specific substrates, but few explicit tests of this relationship in an evolutionary context have been conducted. Without direct tests of form-function relationships in living primates it is impossible to reliably infer behavior in fossil taxa. In this study, we test a hypothesis of co-variation between forelimb morphology and the type of substrates used by strepsirrhines. If associations between anatomy and substrate use exist, these can then be applied to better understand limb anatomy of extinct primates. The co-variation between each forelimb long bone and the type of substrate used was studied in a phylogenetic context. Our results show that despite the presence of significant phylogenetic signal for each long bone of the forelimb, clear support use associations are present. A strong co-variation was found between the type of substrate used and the shape of the radius, with and without taking phylogeny into account, whereas co-variation was significant for the ulna only when taking phylogeny into account. Species that use a thin branch milieu show radii that are gracile and straight and have a distal articular shape that allows for a wide range of movements. In contrast, extant species that commonly use large supports show a relatively robust and curved radius with an increased surface area available for forearm and hand muscles in pronated posture. These results, especially for the radius, support the idea that strepsirrhine primates exhibit specific skeletal adaptations associated with the supports that they habitually move on. With these robust associations in hand it will be possible to explore the same variables in extinct early primates and primate relatives and thus improve the reliability of inferences concerning substrate use in early primates.

Phenetic and functional analyses of the distal ulna of Australopithecus afarensis and Australopithecus africanus

The morphology of the distal portion of the hominoid ulna is poorly studied despite its important functional role at the wrist joint. There are five qualitatively well-described fossil hominin distal ulnae belonging to Australopithecus afarensis and Australopithecus africanus, but there have been few efforts to quantify their morphology or relate it to their functional abilities. This article presents an effort to do so, using three-dimensional geometric morphometrics to analyze the shape of the distal ulna of the Plio-Pleistocene hominins and an extant comparative sample of great apes and humans. For the extant taxa, results show that the morphology of Pan and Pongo is distinct from that of Homo, and that these differences are likely related to climbing, clambering and below-branch suspension in the former, and the release of the limbs from locomotion and (potentially) tool manufacture in the latter. For the australopiths, results indicate that the A. afarensis sample is relatively heterogeneous. These results are driven by the morphology of A.L. 333-12, which is the largest ulna in the sample and has a unique combination of traits when compared with the other two A. afarensis specimens. Overall, the morphology of all the hominins was most consistent with the pattern displayed by extant great apes, and specifically Pan and Pongo; however, large overlap in shape in the distal ulna in the extant sample indicates that other areas of the skeleton may be more informative for functional analyses.

Morphological integration in the forelimb of musteloid carnivorans

The forelimb forms a functional unit that allows a variety of behaviours and needs to be mobile, yet at the same time stable. Both mobility and stability are controlled, amongst others, at the level of the elbow joint. This joint is composed of the humero-ulnar articulation, mainly involved during parasagittal movements; and the radio-ulnar articulation, mainly allowing rotation. In contrast, the humero-radial articulation allows both movements of flexion-extension and rotation. Here, we study the morphological integration between each bone of the forelimb at the level of the entire arm, as well as at the elbow joint, in musteloid carnivorans. To do so, we quantitatively test shape co-variation using surface 3D geometric morphometric data. Our results show that morphological integration is stronger for bones that form functional units. Different results are obtained depending on the level of investigation: for the entire arm, results show a greater degree of shape co-variation between long bones of the lower arm than between the humerus and either bone of the lower arm. Thus, at this level the functional unit of the lower arm is comprised of the radius and ulna, permitting rotational movements of the lower arm. At the level of the elbow, results display a stronger shape co-variation between bones allowing flexion and stability (humerus and ulna) than between bones allowing mobility (ulna and radius and humerus and radius). Thus, the critical functional unit appears to be the articulation between the humerus and ulna providing the stability of the joint.

First partial skeleton of a 1.34-million-year-old Paranthropus boisei from Bed II, Olduvai Gorge, Tanzania

Recent excavations in Level 4 at BK (Bed II, Olduvai Gorge, Tanzania) have yielded nine hominin teeth, a distal humerus fragment, a proximal radius with much of its shaft, a femur shaft, and a tibia shaft fragment (cataloged collectively as OH 80). Those elements identified more specifically than to simply Hominidae gen. et sp. indet are attributed to Paranthropus boisei. Before this study, incontrovertible P. boisei partial skeletons, for which postcranial remains occurred in association with taxonomically diagnostic craniodental remains, were unknown. Thus, OH 80 stands as the first unambiguous, dentally associated Paranthropus partial skeleton from East Africa. The morphology and size of its constituent parts suggest that the fossils derived from an extremely robust individual who, at 1.338±0.024 Ma (1 sigma), represents one of the most recent occurrences of Paranthropus before its extinction in East Africa.

Zona conoidea of the elbow

The zona conoidea comprises the area of the lateral trochlear ridge of the humerus. The purpose of this study is to reintroduce this term ‘zona conoidea’ to the discussion of the human elbow and to investigate its significance in the development of osteoarthritis of the elbow.

The upper extremities of 12 cadavers were prepared. With the forearm in neutral, pronation and supination, the distance between the bevel of the radial head and zona conoidea was inspected. A total of 12 healthy volunteers had a CT scan. The distance between the zona conoidea and the bevelled rim of the radial head was measured in these positions.

In the anatomical specimens, early osteo-arthritic changes were identified in the posteromedial bevelled rim of the radial head, and the corresponding zona conoidea in supination. Measurement in the CT study showed that in full supination, the distance between the bevel of the radial head and the zona conoidea was at a minimum.

This study suggests that the significant contact between the bevel of the radial head and the zona conoidea in supination is associated with the initiation of osteoarthritis of the elbow in this area.

Articular surface area of the coronoid process and radial head in elbow extension: surface ratio in cadavers and a computed tomography study in vivo

PURPOSE

To quantify the articular surface area ratio of the radial head to the coronoid process to gain a better understanding of the stress distribution across these articulations and possibly to explain the patterns of osteoarthritis that are commonly seen in the elbow.

METHODS

Thirty cadaveric elbows were harvested and dissected to allow measurement of the radial head and coronoid process articular surfaces. The articular surface areas were measured using the Image J program (National Institutes of Health, Chicago, IL). Twelve men were recruited for this study, and all received a computed tomography (CT) scan of the elbow. A 3-dimensional image of the proximal radioulnar articular surface was created using volume rendering. All specimens were measured 3 times by 2 observers.

RESULTS

In the cadaveric measurements, the mean area of the radial head articular fossa was 247.3 +/- 52.6 mm(2) (mean +/- SD). The mean area of the medial facet of the coronoid process was 232.29 +/- 36.5 mm(2), and the mean area of the lateral facet was 141.9 +/- 33.3 mm(2). The articular surface area ratio of radial head to coronoid process was 1:1.5. In the CT measurement, the mean area of the radial head articular fossa was 258.9 +/- 26.3 mm(2). The mean area of the coronoid process articular surface was 376.9 +/- 37.0 mm(2). The articular surface area ratio of radial head to coronoid process was 1:1.46.

CONCLUSIONS

The ratio of articular surface area of radial head to coronoid process is 1:1.51 in cadavers and 1:1.46 using a CT in vivo, which is the reverse of the reported force transmission ratio across the elbow joint.

Chimpanzee nest distribution and site reuse in a dry habitat: implications for early hominin ranging

This paper reports on a 20-month study of chimpanzee nesting patterns in Issa, Ugalla, western Tanzania. Ugalla is one of the driest, most open, and seasonal habitats where chimpanzees are found. The methods used were ethoarchaeological, as the chimpanzees were not habituated and behavioural observations were rare. Systematic data on the spatial and temporal distribution of nests are presented. Places with no nests at the beginning of the study, despite being suitable for nesting, were used as controls. Similar to other chimpanzee study sites, nests were highly concentrated in some parts of the landscape. Issa chimpanzees preferred to nest on slopes. They extensively used the woodland vegetation type of their habitat for nesting throughout the annual cycle. Ninety percent of nest sites were used repeatedly throughout the study period, but none of the control places had nests during this period. The results indicate that chimpanzees ranged more widely during the dry season, when food abundance was lowest, food was available mainly in open vegetation types, and when drinking water was restricted to a few sources. Early hominins in similar habitats may have followed the ranging strategy of Issa chimpanzees. As with a previous study, the distribution of nests was spatially similar to archaeological distributions in early hominin sites. Hominin topography and vegetation type preferences may be misrepresented in the archaeological record. Nest sites may have been the antecedents of carcass processing sites.

Locomotion and posture from the common hominoid ancestor to fully modern hominins, with special reference to the last common panin/hominin ancestor

Based on our knowledge of locomotor biomechanics and ecology we predict the locomotion and posture of the last common ancestors of (a) great and lesser apes and their close fossil relatives (hominoids); (b) chimpanzees, bonobos and modern humans (hominines); and (c) modern humans and their fossil relatives (hominins). We evaluate our propositions against the fossil record in the context of a broader review of evolution of the locomotor system from the earliest hominoids of modern aspect (crown hominoids) to early modern Homo sapiens. While some early East African stem hominoids were pronograde, it appears that the adaptations which best characterize the crown hominoids are orthogrady and an ability to abduct the arm above the shoulder - rather than, as is often thought, manual suspension sensu stricto. At 7-9 Ma (not much earlier than the likely 4-8 Ma divergence date for panins and hominins, see Bradley, 2008) there were crown hominoids in southern Europe which were adapted to moving in an orthograde posture, supported primarily on the hindlimb, in an arboreal, and possibly for Oreopithecus, a terrestrial context. By 7 Ma, Sahelanthropus provides evidence of a Central African hominin, panin or possibly gorilline adapted to orthogrady, and both orthogrady and habitually highly extended postures of the hip are evident in the arboreal East African protohominin Orrorin at 6 Ma. If the traditional idea that hominins passed through a terrestrial 'knuckle-walking' phase is correct, not only does it have to be explained how a quadrupedal gait typified by flexed postures of the hindlimb could have preadapted the body for the hominin acquisition of straight-legged erect bipedality, but we would have to accept a transition from stem-hominoid pronogrady to crown hominoid orthogrady, back again to pronogrady in the African apes and then back to orthogrady in hominins. Hand-assisted arboreal bipedality, which is part of a continuum of orthograde behaviours, is used by modern orangutans to forage among the small branches at the periphery of trees where the core hominoid dietary resource, ripe fruit, is most often to be found. Derivation of habitual terrestrial bipedality from arboreal hand-assisted bipedality requires fewer transitions, and is also kinematically and kinetically more parsimonious.

Cercopithecoid cervical vertebral morphology and implications for the presence of Theropithecus in early Pleistocene Europe

While it is recognized that the overall configuration of the vertebral column, as well as the size and shape of individual vertebrae, differ within and between primate taxa, relatively little is known about the degree to which vertebral morphology reflects a phylogenetic signal or the degree to which vertebral elements can be used in accurate taxonomic classification. Isolated vertebrae are occasionally found in fossil assemblages, and proper taxonomic identification is necessary to make inferences about the animal's biology and place it in a broader phylogenetic and evolutionary context. Recently, three large primate cervical vertebrae (C3, C5, and C6) from Pirro Nord, Italy (early Pleistocene, late Villafranchian) were attributed to the genus Theropithecus based on size comparisons with extant cercopithecoid primates (Rook et al., 2004, J. Hum. Evol. 47, 267-277). These fossils were suggested to indicate an early dispersal of this genus out of Africa around 1.6-1.3 Ma possibly co-incident with early Pleistocene dispersals of Homo. Because of the potential importance of these fossils for interpreting Theropithecus evolution and the relatively few morphological data on primate cervical vertebral morphology, we examined the size and shape of cervical vertebrae in a large sample of extant cercopithecoid taxa (n=106). Specifically, we evaluated whether subfamily and genus level assignments can be made on the basis of isolated cervical elements. Discriminant analyses reveal that scaled shape variables are good discriminators of taxonomic affinity at the subfamily level but are poor discriminators at the genus level. Least-squares regressions show that raw linear dimensions of cervical vertebral morphology are good predictors of body mass in the extant sample. Our regression results produce a likely body mass estimate of 22-38 kg for the Pirro Nord cervical vertebrae. Based on these regression estimates, the poor ability to discriminate cervical vertebrae at the genus level, and paleoenvironmental reconstructions of Pirro Nord, it is unlikely that the Pirro Nord fossils can be confidently attributed to the genus Theropithecus. These findings have important implications for recent interpretations of the nature of Theropithecus dispersal out of Africa.

Bone density spatial patterns in the distal radius reflect habitual hand postures adopted by quadrupedal primates

Primates adopt diverse hand postures during terrestrial and above-branch quadrupedal locomotion--knuckle-walking, digitigrady, and palmigrady--that incorporate varying degrees of wrist dorsiflexion (i.e., extension). Although relationships between hand postures, wrist joint range of motion, and the external properties of wrist bones (e.g., surface morphology) have been examined, the relationship between hand postures and the internal properties of wrist bones (e.g., bone density) remains largely unexplored. Because articular joint surfaces transmit mechanical loads between conjoining limb bones, measures of density (e.g., magnitudes and patterns) in the subchondral cortical plate of bone of the distal radius can be used to evaluate load regimes experienced by the wrist joint in different hand postures. We assessed apparent (i.e. optical) density patterns in several extant catarrhine primate taxa partitioned into different hand posture groups: knuckle-walking apes, digitigrade monkeys, and palmigrade monkeys. Computed tomography osteoabsorptiometry (CT-OAM) was used to construct maximum intensity projection (MIP) maps of apparent densities. High apparent density areas were characterized relative to a dorsal-volar reference plane and compared across hand posture groups. All groups had large percentage areas of high apparent density in the dorsal region of the distal radial articular surface. Only knuckle-walking apes, however, had a large percentage area of high apparent density in the volar region of the distal radial articular surface. These patterns are consistent with radiocarpal articulations in specific hand postures as evidenced by available radiographic data and suggest that the different habitual hand postures adopted by monkeys and African apes during quadrupedal locomotion have different stereotypic loading patterns. This has implications for understanding the functional morphology and evolution of knuckle-walking and digitigrade hand postures in primates.