A citation network analysis of lithic microwear research

The introduction of lithic microwear research into the wider archaeological community by Keeley (1980) was concurrent with the development of the processual paradigm and the adoption of the scientific method. Subsequently, lithic microwear research has benefited from over 35 years of innovation, including the introduction of novel methodological and analytical procedures. The present study employs a citation network to objectively analyse the development of microwear research. Given developments in technology, as well as the institutional isolation of early microwear research, the present analysis considers the citation network that stems from Keeley's seminal 1980 volume. The 363 papers identified as having cited Keeley (1980) in the subsequent 35 years were treated as individual nodes within the citation network. Before analysis, nodes were assigned attributes, including the type of research published and whether they were supportive of three key aspects of Keeley's experimental program: the ability to determine the function of the tool and to ascertain the type of worked material from microwear, as well as the use of high-powered microscopy techniques. Emergent properties of the papers, including closeness centrality, indegree and betweenness centrality, are used to test for significant differences between paper attributes. Similarly a clustering algorithm is used to objectively define distinct clusters of important papers within the discipline. Results indicate that a small number of nodes in the network maintain statistically significant influence on the form of the citation network. These important nodes and the distinct ‘schools of thought’ identified are discussed in the context of Keeley's initial contribution to the sub-field

The unexpected importance of the fifth digit during stone tool production

Unique anatomical features of the human hand facilitate our ability to proficiently and forcefully perform precision grips and in-hand manipulation of objects. Extensive research has been conducted into the role of digits one to three during these manual behaviours, and the origin of the highly derived first digit anatomy that facilitates these capabilities. Stone tool production has long been thought a key influence in this regard. Despite previous research stressing the unique derived morphology of the human fifth digit little work has investigated why humans alone display these features. Here we examine the recruitment frequency, loading magnitude, and loading distribution of all digits on the non-dominant hand of skilled flintknappers during four technologically distinct types of Lower Palaeolithic stone tool production. Our data reveal the fifth digit to be heavily and frequently recruited during all studied behaviours. It occasionally incurred pressures, and was used in frequencies, greater or equal to those of the thumb, and frequently the same or greater than those of the index finger. The fifth digit therefore appears key to >2 million years of stone tool production activities, a behaviour that likely contributed to the derived anatomy observed in the modern human fifth ray

Morphological integration and shape covariation between the trapezium and first metacarpal among extant hominids

Objectives: The shape of the trapezium and first metacarpal (Mc1) markedly influence thumb mobility, strength, and the manual abilities of extant hominids. Previous research has typically focused solely on trapezium‐Mc1 joint shape. Here we investigate how morphological integration and shape covariation between the entire trapezium (articular and non‐articular surfaces) and the entire Mc1 reflect known differences in thumb use in extant hominids. Materials and Methods: We analyzed shape covariation in associated trapezia and Mc1s across a large, diverse sample of Homo sapiens (n = 40 individuals) and other extant hominids (Pan troglodytes, n = 16; Pan paniscus, n = 13; Gorilla gorilla gorilla, n = 27; Gorilla beringei, n = 6; Pongo pygmaeus, n = 14; Pongo abelii, n = 9) using a 3D geometric morphometric approach. We tested for interspecific significant differences in degree of morphological integration and patterns of shape covariation between the entire trapezium and Mc1, as well as within the trapezium‐Mc1 joint specifically. Results: Significant morphological integration was only found in the trapezium‐Mc1 joint of H. sapiens and G. g. gorilla. Each genus showed a specific pattern of shape covariation between the entire trapezium and Mc1 that was consistent with different intercarpal and carpometacarpal joint postures. Discussion: Our results are consistent with known differences in habitual thumb use, including a more abducted thumb during forceful precision grips in H. sapiens and a more adducted thumb in other hominids used for diverse grips. These results will help to infer thumb use in fossil hominins

Trabecular distribution of distal femur in extant apes

Extant great apes are often used to model aspects of fossil hominin locomotor behaviours. Comparative investigation of trabecular bone, which (re-)models to reflect loads incurred during life, can provide novel insights into the locomotor reconstruction of fossil taxa [1]. Here we analyze the distal femoral epiphysis of [italics]Homo sapiens[italics] (N = 26), [italics]Gorilla gorilla[italics] (N = 14), [italics]Pan troglodytes verus[italics] (N = 15), and [italics]Pongo[italics] sp. (N = 9) to determine how variation in trabecular structure reflects differences in locomotor behaviours. Canonical holistic morphometric analysis (cHMA) of relative bone volume fraction (rBV/TV) and degree of anisotropy (DA) is used to infer patterns of joint loading in extant taxa. A principal component analysis of rBV/TV and DA distributions show clear separation between taxa. Trabecular distribution in humans is consistent with medial (due to the ground reaction forces) and lateral (due to the resistance of the knee adduction moment provided by the quadriceps and gastrocnemius muscles and lateral collateral ligament) loading. Distribution in non-human apes is consistent with primarily medial loading due to the higher knee adduction moment, varus angle and ground reaction forces. Signals of a more extended knee in female gorillas compared to males (or chimpanzees) may reflect a more extended knee position during vertical climbing and higher arboreality in females [3]. Orangutans showed the most homogenous distribution of trabecular structure across both condyles, consistent with more variable knee joint postures. These results provide the comparative context to interpret knee posture and, in turn, locomotor behaviours in fossil hominins. References: [1] Georgiou, L., Dunmore, C. J., Bardo, A., Buck, L. T., Hublin, J. J., Pahr, D. H., ... & Skinner, M. M. (2020). Evidence for habitual climbing in a Pleistocene hominin in South Africa. Proceedings of the National Academy of Sciences, 117(15), 8416-8423. [2] Bachmann, S., Dunmore, C. J., Skinner, M. M., Pahr, D. H. and Synek, A. (2022). A computational framework for canonical holistic morphometric analysis of trabecular bone. Scientific Reports, 12, 1-13. [3] Isler, K. (2005). 3D‐kinematics of vertical climbing in hominoids. American Journal of Physical Anthropology: The Official Publication of the American Association of Physical Anthropologists, 126(1), 66-81. Acknowledgments: For access to specimens we thank the following individuals/institutions: Max Planck Institute for Evolutionary Anthropology (C. Boesch, J-J. Hublin); Museum für Naturkunde - Leibniz Institute for Evolution and Biodiversity Science (F. Mayer, C. Funk); Powell-Cotton Museum (I. Livne); Royal Museum for Central Africa (E. Gilissen); University of Florence (J. Moggi-Cecchi, S. Bortoluzzi); Johann-Friedrich�Blumenback-Institute for Zoology and Anthropology, Georg-August University, Goettingen (B. Grosskopf); Frankfurt Senckenberg Museum (V. Volpato); University of the Witswatersand (L. Berger, B. Zipfel); Science Academy of the Czech Republic (J. Svoboda). This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No. 819960)

A computational framework for canonical holistic morphometric analysis of trabecular bone

Bone is a remarkable, living tissue that functionally adapts to external loading. Therefore, bone shape and internal structure carry information relevant to many disciplines, including medicine, forensic science, and anthropology. However, morphometric comparisons of homologous regions across different individuals or groups are still challenging. In this study, two methods were combined to quantify such differences: (1) Holistic morphometric analysis (HMA) was used to quantify morphometric values in each bone, (2) which could then be mapped to a volumetric mesh of a canonical bone created by a statistical free-form deformation model (SDM). Required parameters for this canonical holistic morphometric analysis (cHMA) method were identified and the robustness of the method was evaluated. The robustness studies showed that the SDM converged after one to two iterations, had only a marginal bias towards the chosen starting image, and could handle large shape differences seen in bones of different species. Case studies were performed on metacarpal bones and proximal femora of different primate species to confirm prior study results. The differences between species could be visualised and statistically analysed in both case studies. cHMA provides a framework for performing quantitative comparisons of different morphometric quantities across individuals or groups. These comparisons facilitate investigation of the relationship between spatial morphometric variations and function or pathology, or both

Trabecular distribution of distal femur in extant apes and Australopithecus sediba

Knee morphology of fossil hominins is of particular interest to paleoanthropologists due to longstanding debates about relative degrees of arboreality and terrestrial bipedalism in the hominin clade. In addition to external bone shape, the investigation of trabecular bone in the knee joint can provide insights into in vivo locomotor behavior of hominins [1-2]. The nearly complete right distal femur (U.W. 88-63) of Australopithecus sediba (1.98 Ma) shows a unique combination of condyles that resemble other australopith species and Homo-like anatomy of the patellar surface, which has been used to infer a unique locomotor pattern in this species [3]. Here we analyze the trabecular morphology of distal femoral epiphysis of Homo sapiens (N = 15), Gorilla gorilla (N=14), Pan troglodytes verus (N = 15), Pongo sp. (N = 9), and A. sediba (MH2) in order to 1) establish patterns of joint loading in extant taxa of known locomotor behaviour and 2) investigate joint loading in the knee of A. sediba. A canonical holistic morphometric analysis (cHMA), combining holistic morphometric analysis (HMA) and statistical free-form deformation model (SDM), approach was used to analyze the patterns of trabecular bone distribution following published protocols [4]. A principal component (PC) analysis of relative bone volume (rBV/TV) distribution shows clear separation between extant ape taxa. Positive values on PC1, PC2 and PC3 are mostly driven by rBV/TV concentrated on the patellar surface and on the posterior articular surface of the medial condyle separating humans from great apes (PC1, PC2) and chimpanzees (PC3) from humans, gorillas and orangutans. Negative PC1 is mostly driven by rBV/TV concentrated beneath the insertion of posterior cruciate ligament discriminating non-human apes from humans, negative PC2 by loadings on the patellar surface separating gorillas from others, and negative PC3 by loadings on the patellar surface and on the posterior articulation surface of the medial condyle discriminating orangutans from others. Results suggest that differences between humans and apes are primarily in the patellar articular surface. Relative bone volume in humans is concentrated in the posteroinferior region of the lateral condyle and on the lateral patellar surface, which is consistent with loading in an extended knee position during locomotion. In non-human apes relative bone volume is found to extend from the inferior margin of the patellar articulation to the posterior region of both condyles. However, in gorillas it does not extend as posterosuperiorly in the medial condyle as it does in chimpanzees and orangutans. Trabecular bone is concentrated in the lateral condyles in apes, with the greatest values in the posterosuperior and the posteroinferior regions. Unlike humans, ape like a trabecular concentration at the distal regions of both condyles (i.e., those assumed to be loaded in an extended knee), with the lowest values in orangutans. We suggest that this reflects predominant loading in a more flexed knee posture in great apes compared to humans. Finally, among apes, we found the most homogenous distribution of trabecular bone across both condyles in orangutans, which we relate to their more variable knee joint postures during locomotion. A. sediba shows trabecular concentrations on the patellar surface and on the posterior area of the lateral condyle. Values in the posteroinferior and posterosuperior regions of lateral condyle are generally higher than in medial condyle. We interpret these fossil results as reflective of loading the knee joint with a degree of flexion that differs somewhat from modern humans. However, taphonomic erosion of parts of the condyles hinders a complete assessment of trabecular bone distribution in A. sediba

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

Reconstructing hand use in Australopithecus sediba and Homo naledi: mapping variation in cortical thickness across the proximal and intermediate phalanges

The inferred diversity of manual behaviors within the hominin lineage is exemplified by the mosaic morphology of the hands of Australopithecus sediba and Homo naledi [1 -2]. Derived morphological features present in both fossil hominins point to human-like manipulation abilities, while curved fingers with well-developed flexor sheath ridges (FSRs) suggest the continued use of their hands for climbing and grasping [1-2]. The internal bone structure may reflect habitual locomotor and/or manipulative loading of the fingers due to bone’s ability to (re)model throughout life in response to mechanical load [3], and thus variation in phalangeal cortical bone distribution holds the potential for reconstructing hand use among fossil hominins.We investigated the distribution of phalangeal cortical thickness in extant great apes, A. sediba and H. naledi to inform our reconstruction of fossil manual behaviours. The sample included microCT scans of the proximal and intermediate phalanges of rays II-V of Gorilla gorilla (N=21), Homo sapiens (N=37), Pongo pygmaeus (N=9), Pan sp. (N=24), A. sediba, and H. naledi (Hand 1). We used the R package morphomap [4] to map diaphyseal cortical thickness and conducted a principal component analysis (PCA) on the cortical thickness values to explore patterns across species.Results show clear separation among extant taxa across all phalanges. The extant taxa are separated along PC1, with some overlap between the African apes reflecting a pattern of thickness localised to the FSRs, with a different pattern of overall thickness of the shaft in Pongo and in humans. PC2 reflects differences in the location of greatest thickness; in African apes this occurs in the FSRs throughout the shaft, whereas in Pongo this is distally located with some dorsal thickening, and in humans it is primarily in the dorsodistal region of the shaft. The proximal phalanges of A. sediba consistently fall close to the African apes, reflecting maximum thickness localised to the FSRs, while H. naledi consistently falls close to humans, reflecting thickening of the dorsal region of the shaft and the FSRs distally. The intermediate phalanges of both fossil species fall between the African ape and human distribution, reflecting a thickening of the dorsal region of the bone and the FSRs, which are located in the proximal-to -midshaft region. The distribution of the A. sediba intermediate phalanges differs from those of H. naledi in that the thickening on the palmar surface extends further distally. This is consistent with external morphology ofA. sediba intermediate phalanges that possess unusually long and prominent FSRs.Overall, we show that variation in phalangeal cortical bone distribution clearly separates extant hominoid taxa that differ in their locomotor and manipulative behaviours. Within this comparative context, A. sediba proximal phalanges are most similar to the African ape cortical signal and intermediate phalanges are intermediate between all extant taxa. Along with a high degree of phalangeal curvature and prominent FSRs, this morphology better distributes stress and reduces strain on the bone during flexed finger loading (e.g., during climbing or suspension) [5]. In contrast, while H. naledi phalanges are most similar to humans in their cortical distribution, this appears to contradict their high degree of curvature, which may be a functional indication of arboreal locomotion [5]. The H. naledi phalanges indicate a hand uniquely adapted for arboreal behaviours in the degree of curvature but not cortical distribution, differing to that of extant great apes and A. sediba

Inferring hand use in Australopithecus sediba: Analysis of the external and internal morphology of hominin proximal and intermediate phalanges.

The evolution of hominin hand use is characterized by a transition from locomotion to primarily dexterous hand use. However, our understanding of this transition remains unclear with discoveries of hominin hand fossils increasingly pointing towards a diverse range of manual behaviors. The almost complete right hand of Australopithecus sediba (1.98 Ma) shows a unique mix of primitive and derived morphology; an exceptionally long thumb and broad distal phalanx suggest human-like manipulative abilities, while curved phalanges and well-developed flexor sheath ridges (FSRs) on both the proximal and, unusually, intermediate phalanges, suggest the continued importance of climbing and suspension [1]. We analyzed the external and internal morphology of the A. sediba proximal and intermediate phalanges in comparison to extant apes to draw inferences about locomotor and manipulative capabilities. The study sample included microCT scans of manual proximal and intermediate phalanges of rays II-V of Gorilla gorilla (N =12), Homo sapiens (N =14), Pongo pygmaeus (N =17), Pan sp. (N = 18), and A. sediba (MH2). We quantified the degree of curvature via included angle, and variation in FSR morphology via linear measurements. We also assessed the cross-sectional cortical bone properties throughout the shaft using BoneJ (including cross-sectional area (CSA), polar section modulus (Zpol), and polar second moment of area (J)) and mapped variation in cortical thickness using Morphomap [2]. Results reveal curvature in the proximal and intermediate phalanges is lowest in Homo followed by, in an increasing order, Gorilla, Pan, and Pongo. The proximal and intermediate phalanges of A. sediba have intermediate values of curvature, falling within the African ape range, exceeding the mean values of Gorilla and falling below the mean values of Pan. After controlling for size, Gorilla has the shortest (in length) but the most prominent (deepest) and widest FSRs and humans FSRs are short, lacking depth and width, while Pongo and Pan share FSR morphology that is longer but intermediate in width and depth, for both proximal and intermediate phalanges. In A. sediba, the proximal phalanx FSRs are most similar to Gorilla, but longer, while the intermediate phalanges are distinct in having proximal phalanx-like morphology, such that they lack a median bar, and the FSRs are exceptionally deep, exceeding the mean values of each extant species. The cortical bone analysis indicates that Gorilla has the highest values across all cross-sectional properties (CSA, Zpol, J) while humans have the lowest, and Pongo and Pan are intermediate, for both the proximal and intermediate phalanges. A. sediba is characterized by low cross-sectional values similar to those observed in humans. Initial bone distribution analysis within the extant sample demonstrates that cortical bone is thicker palmarly in great apes and dorsally in humans, in both intermediate and proximal phalanges. Consistent with the human-like CSG properties, A. sediba also has a thicker cortex dorsally. The low CSG properties in the fingers of A. sediba indicate limited strength compared to great apes, which may suggest its hands were not adapted for high loads that are incurred on the hand during climbing or suspension. However, this internal structure is combined with African ape-like curvature, which provides better distribution of the stress experienced during suspension [3], and prominent FSRs on the phalanges, which help reduce strains on the shaft of the bone [3]. Thus, the external morphology of the A. sediba phalanges indicate a hand uniquely adapted for arboreal locomotion but one that lacked the cortical robusticity of extant great apes. This suggests that either arboreal behaviors may not have constituted a large a part of the A. sediba locomotor repertoire, or that the mechanical loads of this behavior were resisted in a different manner than in great apes, or some combination of both