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Beaudet A. The Australopithecus assemblage from Sterkfontein Member 4 (South Africa) and the concept of variation in palaeontology. Evol Anthropol 2023. [PMID: 36632711 DOI: 10.1002/evan.21972] [Citation(s) in RCA: 1] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/23/2021] [Revised: 09/06/2022] [Accepted: 12/17/2022] [Indexed: 01/13/2023]
Abstract
Interpreting morphological variation within the early hominin fossil record is particularly challenging. Apart from the fact that there is no absolute threshold for defining species boundaries in palaeontology, the degree of variation related to sexual dimorphism, temporal depth, geographic variation or ontogeny is difficult to appreciate in a fossil taxon mainly represented by fragmentary specimens, and such variation could easily be conflated with taxonomic diversity. One of the most emblematic examples in paleoanthropology is the Australopithecus assemblage from the Sterkfontein Caves in South Africa. Whereas some studies support the presence of multiple Australopithecus species at Sterkfontein, others explore alternative hypotheses to explain the morphological variation within the hominin assemblage. In this review, I briefly summarize the ongoing debates surrounding the interpretation of morphological variation at Sterkfontein Member 4 before exploring two promising avenues that would deserve specific attention in the future, that is, temporal depth and nonhuman primate diversity.
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Affiliation(s)
- Amélie Beaudet
- Department of Archaeology, University of Cambridge, Cambridge, UK.,School of Geography, Archaeology and Environmental Studies, University of the Witwatersrand, Johannesburg, South Africa.,Institut Català de Paleontologia Miquel Crusafont, Universitat Autònoma de Barcelona, Sabadell, Cerdanyola del Vallès, Barcelona, Spain
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2
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Harper CM, Ruff CB, Sylvester AD. Calcaneal shape variation in humans, nonhuman primates, and early hominins. J Hum Evol 2021; 159:103050. [PMID: 34438297 DOI: 10.1016/j.jhevol.2021.103050] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/14/2020] [Revised: 06/29/2021] [Accepted: 06/29/2021] [Indexed: 01/08/2023]
Abstract
The foot has played a prominent role in evaluating early hominin locomotion. The calcaneus, in particular, plays an important role in weight-bearing. Although the calcanei of early hominins have been previously scrutinized, a three-dimensional analysis of the entire calcaneal shape has not been conducted. Here, we investigate the relationship between external calcaneal shape and locomotion in modern Homo sapiens (n = 130), Gorilla (n = 86), Pan (n = 112), Pongo (n = 31), Papio (n = 28), and hylobatids (Hylobates, Symphalangus; n = 32). We use these results to place the calcanei attributed to Australopithecus sediba, A. africanus, A. afarensis, H. naledi, and Homo habilis/Paranthropus boisei into a locomotor context. Calcanei were scanned using either surface scanning or micro-CT and their external shape analyzed using a three-dimensional geometric morphometric sliding semilandmark analysis. Blomberg's K statistic was used to estimate phylogenetic signal in the shape data. Shape variation was summarized using a principal components analysis. Procrustes distances between all taxa as well as distances between each fossil and the average of each taxon were calculated. Blomberg's K statistic was small (K = 0.1651), indicating weak phylogenetic effects, suggesting variation is driven by factors other than phylogeny (e.g., locomotion or body size). Modern humans have a large calcaneus relative to body size and display a uniquely convex cuboid facet, facilitating a rigid midfoot for bipedalism. More arboreal great apes display relatively deeper cuboid facet pivot regions for increased midfoot mobility. Australopithecus afarensis demonstrates the most human-like calcaneus, consistent with obligate bipedalism. Homo naledi is primarily modern human-like, but with some intermediate traits, suggesting a different form of bipedalism than modern humans. Australopithecus africanus, A. sediba, and H. habilis/P. boisei calcanei all possess unique combinations of human and nonhuman ape-like morphologies, suggesting a combination of bipedal and arboreal behaviors.
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Affiliation(s)
- Christine M Harper
- The Johns Hopkins University School of Medicine, Center for Functional Anatomy and Evolution, 1830 East Monument Street, Room 302, Baltimore, MD 21205, United States; Cooper Medical School of Rowan University, Department of Biomedical Sciences, 401 S Broadway, Room 453, Camden, NJ 08103, United States.
| | - Christopher B Ruff
- The Johns Hopkins University School of Medicine, Center for Functional Anatomy and Evolution, 1830 East Monument Street, Room 302, Baltimore, MD 21205, United States
| | - Adam D Sylvester
- The Johns Hopkins University School of Medicine, Center for Functional Anatomy and Evolution, 1830 East Monument Street, Room 302, Baltimore, MD 21205, United States
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3
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Harper CM, Ruff CB, Sylvester AD. Scaling and relative size of the human, nonhuman ape, and baboon calcaneus. Anat Rec (Hoboken) 2021; 305:100-122. [PMID: 33843151 DOI: 10.1002/ar.24642] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/28/2020] [Revised: 02/28/2021] [Accepted: 03/06/2021] [Indexed: 01/04/2023]
Abstract
Among human and nonhuman apes, calcaneal morphology exhibits significant variation that has been related to locomotor behavior. Due to its role in weight-bearing, however, both body size and locomotion may impact calcaneal morphology. Determining how calcaneal morphologies vary as a function of body size is thus vital to understanding calcaneal functional adaptation. Here, we study calcaneus allometry and relative size in humans (n = 120) and nonhuman primates (n = 278), analyzing these relationships in light of known locomotor behaviors. Twelve linear measures and three articular facet surface areas were collected on calcaneus surface models. Body mass was estimated using femoral head superoinferior breadth. Relationships between calcaneal dimensions and estimated body mass were analyzed across the sample using phylogenetic least squares regression analyses (PGLS). Differences between humans and pooled nonhuman primates were tested using RMA ANCOVAs. Among (and within) genera residual differences from both PGLS regressions and isometry were analyzed using ANOVAs with post hoc multiple comparison tests. The relationships between all but two calcaneus dimensions and estimated body mass exhibit phylogenetic signal at the smallest taxonomic scale. This signal disappears when reanalyzed at the genus level. Calcaneal morphology varies relative to both body size and locomotor behavior. Humans have larger calcanei for estimated body mass relative to nonhuman primates as a potential adaptation for bipedalism. More terrestrial taxa exhibit longer calcaneal tubers for body mass, increasing the triceps surae lever arm. Among nonhuman great apes, more arboreal taxa have larger cuboid facet surface areas for body mass, increasing calcaneocuboid mobility.
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Affiliation(s)
- Christine M Harper
- The Johns Hopkins University School of Medicine, Center for Functional Anatomy and Evolution, Baltimore, Maryland, USA.,Cooper Medical School of Rowan University, Department of Biomedical Sciences, Camden, New Jersey, USA
| | - Christopher B Ruff
- The Johns Hopkins University School of Medicine, Center for Functional Anatomy and Evolution, Baltimore, Maryland, USA
| | - Adam D Sylvester
- The Johns Hopkins University School of Medicine, Center for Functional Anatomy and Evolution, Baltimore, Maryland, USA
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4
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DeSilva JM, McNutt E, Zipfel B, Ward CV, Kimbel WH. Associated Australopithecusafarensis second and third metatarsals (A.L. 333-133) from Hadar, Ethiopia. J Hum Evol 2020; 146:102848. [PMID: 32717476 DOI: 10.1016/j.jhevol.2020.102848] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/11/2020] [Revised: 06/05/2020] [Accepted: 06/05/2020] [Indexed: 10/23/2022]
Affiliation(s)
- Jeremy M DeSilva
- Department of Anthropology, Dartmouth College, Hanover, NH, 03755, USA.
| | - Ellison McNutt
- Keck School of Medicine, University of Southern California, Los Angeles, CA, 90033, USA
| | - Bernhard Zipfel
- Evolutionary Studies Institute, University of the Witwatersrand, Johannesburg, South Africa
| | - Carol V Ward
- Integrative Anatomy Program, Department of Pathology and Anatomical Sciences, School of Medicine, University of Missouri, Columbia, MO, 65212, USA
| | - William H Kimbel
- Institute of Human Origins and School of Human Evolution and Social Change, Arizona State University, Tempe, AZ, 85287, USA
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DeSilva J, McNutt E, Benoit J, Zipfel B. One small step: A review of Plio‐Pleistocene hominin foot evolution. AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 2018; 168 Suppl 67:63-140. [DOI: 10.1002/ajpa.23750] [Citation(s) in RCA: 33] [Impact Index Per Article: 5.5] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/03/2018] [Revised: 10/01/2018] [Accepted: 10/05/2018] [Indexed: 01/03/2023]
Affiliation(s)
- Jeremy DeSilva
- Department of AnthropologyDartmouth College Hanover New Hampshire
- Evolutionary Studies Institute and School of GeosciencesUniversity of the Witwatersrand Johannesburg South Africa
| | - Ellison McNutt
- Department of AnthropologyDartmouth College Hanover New Hampshire
| | - Julien Benoit
- Evolutionary Studies Institute and School of GeosciencesUniversity of the Witwatersrand Johannesburg South Africa
| | - Bernhard Zipfel
- Evolutionary Studies Institute and School of GeosciencesUniversity of the Witwatersrand Johannesburg South Africa
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McNutt EJ, Zipfel B, DeSilva JM. The evolution of the human foot. Evol Anthropol 2018; 27:197-217. [DOI: 10.1002/evan.21713] [Citation(s) in RCA: 31] [Impact Index Per Article: 5.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/25/2017] [Revised: 04/20/2018] [Accepted: 05/30/2018] [Indexed: 11/07/2022]
Affiliation(s)
- Ellison J. McNutt
- Department of Anthropology; Dartmouth College; Hanover New Hampshire
- Ecology, Evolution, Ecosystems, and Society; Dartmouth College; Hanover New Hampshire
| | - Bernhard Zipfel
- Evolutionary Studies Institute and School of Geosciences; University of the Witwatersrand; Johannesburg South Africa
| | - Jeremy M. DeSilva
- Department of Anthropology; Dartmouth College; Hanover New Hampshire
- Evolutionary Studies Institute and School of Geosciences; University of the Witwatersrand; Johannesburg South Africa
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Abstract
The primate foot functions as a grasping organ. As such, its bones, soft tissues, and joints evolved to maximize power and stability in a variety of grasping configurations. Humans are the obvious exception to this primate pattern, with feet that evolved to support the unique biomechanical demands of bipedal locomotion. Of key functional importance to bipedalism is the morphology of the joints at the forefoot, known as the metatarsophalangeal joints (MTPJs), but a comprehensive analysis of hominin MTPJ morphology is currently lacking. Here we present the results of a multivariate shape and Bayesian phylogenetic comparative analyses of metatarsals (MTs) from a broad selection of anthropoid primates (including fossil apes and stem catarrhines) and most of the early hominin pedal fossil record, including the oldest hominin for which good pedal remains exist, Ardipithecus ramidus Results corroborate the importance of specific bony morphologies such as dorsal MT head expansion and "doming" to the evolution of terrestrial bipedalism in hominins. Further, our evolutionary models reveal that the MT1 of Ar. ramidus shifts away from the reconstructed optimum of our last common ancestor with apes, but not necessarily in the direction of modern humans. However, the lateral rays of Ar. ramidus are transformed in a more human-like direction, suggesting that they were the digits first recruited by hominins into the primary role of terrestrial propulsion. This pattern of evolutionary change is seen consistently throughout the evolution of the foot, highlighting the mosaic nature of pedal evolution and the emergence of a derived, modern hallux relatively late in human evolution.
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Hominin hand bone fossils from Sterkfontein Caves, South Africa (1998–2003 excavations). J Hum Evol 2018; 118:89-102. [DOI: 10.1016/j.jhevol.2018.02.014] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/17/2017] [Revised: 02/26/2018] [Accepted: 02/27/2018] [Indexed: 11/23/2022]
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9
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New hominin postcranial remains from locality OMO 323, Shungura Formation, Lower Omo Valley, southwestern Ethiopia. J Hum Evol 2018; 122:23-32. [PMID: 29706229 DOI: 10.1016/j.jhevol.2018.03.011] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/04/2016] [Revised: 03/27/2018] [Accepted: 03/28/2018] [Indexed: 11/23/2022]
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Foot use during vertical climbing in chimpanzees (Pan troglodytes). J Hum Evol 2017; 109:1-10. [DOI: 10.1016/j.jhevol.2017.04.006] [Citation(s) in RCA: 22] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/12/2015] [Revised: 04/15/2017] [Accepted: 04/18/2017] [Indexed: 11/18/2022]
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Kitashiro M, Ogihara N, Kokubo T, Matsumoto M, Nakamura M, Nagura T. Age- and sex-associated morphological variations of metatarsal torsional patterns in humans. Clin Anat 2017. [PMID: 28631280 DOI: 10.1002/ca.22944] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
Abstract
It has been demonstrated that the torsional patterns of the metatarsal heads are associated with the presence or absence of the medial longitudinal arch in hominoid feet. The relatively untwisted second metatarsal is unique in humans, but that of the African apes is much more inverted, suggesting that the torsion of the second metatarsal might represent the overall shape and flatness of the foot. Some clinical studies have recently argued that the onset of foot pathologies such as hallux valgus might be related to the torsional pattern of the metatarsals. However, to date, no studies have systematically investigated the morphological variations of the torsional patterns of human metatarsals. In this study, therefore, the aim was to clarify the age- and sex-associated variations in the torsional patterns of human metatarsals using three-dimensional computed tomography. The torsion angles of the five metatarsals were calculated by defining the dorsopalmar vector of the metatarsal base and the vector corresponding to the rotational axis of the metatarsal head. The present result demonstrated that the second metatarsals of females were significantly more inverted with increasing age. Flat foot is known to be most common in elderly women. Whether there is a cause-effect relationship between second metatarsal torsion and flattening of the medial longitudinal arch has yet to be answered, but this study suggested that torsion of the second metatarsal might possibly be used as an indicator for the early diagnosis of flat foot and associated foot pathologies. Clin. Anat. 30:1058-1063, 2017. © 2017 Wiley Periodicals, Inc.
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Affiliation(s)
| | - Naomichi Ogihara
- Department of Mechanical Engineering, Keio University, Yokohama, Japan
| | - Tetsuro Kokubo
- Department of Orthopedic Surgery, Tachikawa Hospital, Tokyo, Japan
| | - Morio Matsumoto
- Department of Orthopedic Surgery, Keio University, Tokyo, Japan
| | - Masaya Nakamura
- Department of Orthopedic Surgery, Keio University, Tokyo, Japan
| | - Takeo Nagura
- Department of Clinical Biomechanics, Keio University, Tokyo, Japan
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Tardieu C, Hasegawa K, Haeusler M. How Did the Pelvis and Vertebral Column Become a Functional Unit during the Transition from Occasional to Permanent Bipedalism? Anat Rec (Hoboken) 2017; 300:912-931. [DOI: 10.1002/ar.23577] [Citation(s) in RCA: 22] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/16/2016] [Revised: 01/17/2017] [Accepted: 01/27/2017] [Indexed: 01/31/2023]
Affiliation(s)
- Christine Tardieu
- UMR 7179 “Mécanismes adaptatifs: des organismes aux communautés”, USM 301-Département E.G.B; Muséum National d'Histoire Naturelle; Paris France
| | | | - Martin Haeusler
- Institute of Evolutionary Medicine, University of Zürich; Zürich Switzerland
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Ruff CB, Burgess ML, Ketcham RA, Kappelman J. Limb Bone Structural Proportions and Locomotor Behavior in A.L. 288-1 ("Lucy"). PLoS One 2016; 11:e0166095. [PMID: 27902687 PMCID: PMC5130205 DOI: 10.1371/journal.pone.0166095] [Citation(s) in RCA: 52] [Impact Index Per Article: 6.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/25/2016] [Accepted: 10/21/2016] [Indexed: 11/19/2022] Open
Abstract
While there is broad agreement that early hominins practiced some form of terrestrial bipedality, there is also evidence that arboreal behavior remained a part of the locomotor repertoire in some taxa, and that bipedal locomotion may not have been identical to that of modern humans. It has been difficult to evaluate such evidence, however, because of the possibility that early hominins retained primitive traits (such as relatively long upper limbs) of little contemporaneous adaptive significance. Here we examine bone structural properties of the femur and humerus in the Australopithecus afarensis A.L. 288-1 ("Lucy", 3.2 Myr) that are known to be developmentally plastic, and compare them with other early hominins, modern humans, and modern chimpanzees. Cross-sectional images were obtained from micro-CT scans of the original specimens and used to derive section properties of the diaphyses, as well as superior and inferior cortical thicknesses of the femoral neck. A.L. 288-1 shows femoral/humeral diaphyseal strength proportions that are intermediate between those of modern humans and chimpanzees, indicating more mechanical loading of the forelimb than in modern humans, and by implication, a significant arboreal locomotor component. Several features of the proximal femur in A.L. 288-1 and other australopiths, including relative femoral head size, distribution of cortical bone in the femoral neck, and cross-sectional shape of the proximal shaft, support the inference of a bipedal gait pattern that differed slightly from that of modern humans, involving more lateral deviation of the body center of mass over the support limb, which would have entailed increased cost of terrestrial locomotion. There is also evidence consistent with increased muscular strength among australopiths in both the forelimb and hind limb, possibly reflecting metabolic trade-offs between muscle and brain development during hominin evolution. Together these findings imply significant differences in both locomotor behavior and ecology between australopiths and later Homo.
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Affiliation(s)
- Christopher B. Ruff
- Center for Functional Anatomy and Evolution, Johns Hopkins University School of Medicine, Baltimore, Maryland, United States of America
| | - M. Loring Burgess
- Center for Functional Anatomy and Evolution, Johns Hopkins University School of Medicine, Baltimore, Maryland, United States of America
| | - Richard A. Ketcham
- Department of Geological Sciences, The University of Texas Austin, Austin, Texas, United States of America
| | - John Kappelman
- Department of Geological Sciences, The University of Texas Austin, Austin, Texas, United States of America
- Department of Anthropology, The University of Texas Austin, Austin, Texas, United States of America
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Rearfoot posture of Australopithecus sediba and the evolution of the hominin longitudinal arch. Sci Rep 2015; 5:17677. [PMID: 26628197 PMCID: PMC4667273 DOI: 10.1038/srep17677] [Citation(s) in RCA: 26] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/10/2015] [Accepted: 11/02/2015] [Indexed: 11/11/2022] Open
Abstract
The longitudinal arch is one of the hallmarks of the human foot but its evolutionary history remains controversial due to the fragmentary nature of the fossil record. In modern humans, the presence of a longitudinal arch is reflected in the angular relationships among the major surfaces of the human talus and calcaneus complex, which is also known as the rearfoot. A complete talus and calcaneus of Australopithecus sediba provide the opportunity to evaluate rearfoot posture in an early hominin for the first time. Here I show that A. sediba is indistinguishable from extant African apes in the angular configuration of its rearfoot, which strongly suggests that it lacked a longitudinal arch. Inferences made from isolated fossils support the hypothesis that Australopithecus afarensis possessed an arched foot. However, tali attributed to temporally younger taxa like Australopithecus africanus and Homo floresiensis are more similar to those of A. sediba. The inferred absence of a longitudinal arch in A. sediba would be biomechanically consistent with prior suggestions of increased midtarsal mobility in this taxon. The morphological patterns in talus and calcaneus angular relationships among fossil hominins suggest that there was diversity in traits associated with the longitudinal arch in the Plio-Pleistocene.
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Fernández PJ, Almécija S, Patel BA, Orr CM, Tocheri MW, Jungers WL. Functional aspects of metatarsal head shape in humans, apes, and Old World monkeys. J Hum Evol 2015; 86:136-46. [DOI: 10.1016/j.jhevol.2015.06.005] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/07/2015] [Revised: 05/26/2015] [Accepted: 06/08/2015] [Indexed: 11/28/2022]
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16
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Fornai C, Bookstein FL, Weber GW. Variability of Australopithecus second maxillary molars from Sterkfontein Member 4. J Hum Evol 2015; 85:181-92. [DOI: 10.1016/j.jhevol.2015.05.013] [Citation(s) in RCA: 18] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/02/2014] [Revised: 05/21/2015] [Accepted: 05/22/2015] [Indexed: 11/15/2022]
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17
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Ruff CB, Higgins R. Femoral neck structure and function in early hominins. AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 2013; 150:512-25. [DOI: 10.1002/ajpa.22214] [Citation(s) in RCA: 36] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/25/2012] [Revised: 11/28/2012] [Indexed: 11/08/2022]
Affiliation(s)
- Christopher B. Ruff
- Center for Functional Anatomy and Evolution; Johns Hopkins University School of Medicine; Baltimore; MD; 21111
| | - Ryan Higgins
- Center for Functional Anatomy and Evolution; Johns Hopkins University School of Medicine; Baltimore; MD; 21111
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18
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Metatarsal torsion in monkeys, apes, humans and australopiths. J Hum Evol 2012; 64:93-108. [PMID: 23219163 DOI: 10.1016/j.jhevol.2012.10.008] [Citation(s) in RCA: 30] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/24/2012] [Revised: 10/11/2012] [Accepted: 10/12/2012] [Indexed: 11/23/2022]
Abstract
This paper presents an analysis of metatarsal torsion in apes, cercopithecoids and humans, compares australopiths with these species, and discusses their inferred foot morphology and function relative to prehensility, arboreality and the presence or absence of a longitudinal arch. Our results show that locomotor modes are reflected in metatarsal torsion values. Apes, which climb vertically with their foot inverted, have hallucal metatarsal heads that are turned toward the other toes and lateral toes that are inverted. Cercopithecoids, which tend to orient their feet in an axis more parallel to the line of motion, present signs of prehensility by having inverted 2nd metatarsals that oppose the hallux, while their two lateral-most metatarsals are strongly everted. Humans, with their rigid feet and longitudinal arches, have all toes that present their plantar surface toward the ground, resulting in hallucal and 2nd metatarsals that are relatively untwisted and the others that are strongly everted. Humans are different from all taxa only for the 2nd and 3rd metatarsal. It is hypothesized that the untwisted 2nd metatarsal reflects the lack of digit opposability of the medial foot and the strongly everted 3rd metatarsal reflects the longitudinal arch. Australopithecus afarensis was characterized by an everted lateral foot, the prerequisite for the development, but not necessarily an indicator, of a longitudinal arch. In Australopithecus africanus, torsion of fragmentary and complete 1st, 2nd, 3rd and 5th metatarsals suggest that the species did not have a foot with monkey- or ape-like prehensile capabilities and did not have a human-like longitudinal arch. In the Swartkrans remains, torsion is consistent with an unprehensile foot. The morphology of the fossils indicates that there was strong selection to orient the plantar surface of the toes facing the ground at the expense of a grasping foot and inversion ability.
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