Journal of Human Evolution 126 (2019) 112e123 Contents lists available at ScienceDirect Journal of Human Evolution journal homepage: www.elsevier.com/locate/jhevol The endocast of StW 573 (“Little Foot”) and hominin brain evolution lie Beaudet a, b, *, Ronald J. Clarke c, Edwin J. de Jager b, Laurent Bruxelles a, d, e, Ame Kristian J. Carlson c, f, Robin Crompton g, h, Frikkie de Beer i, Jelle Dhaene j, Jason L. Heaton c, k, Kudakwashe Jakata c, Tea Jashashvili c, l, m, Kathleen Kuman a, Juliet McClymont n, Travis R. Pickering c, o, Dominic Stratford a a School of Geography, Archaeology and Environmental Studies, University of the Witwatersrand, Private Bag 3, Johannesburg, WITS 2050, South Africa Department of Anatomy, University of Pretoria, PO Box 2034, Pretoria 0001, South Africa Evolutionary Studies Institute, University of the Witwatersrand, Private Bag 3, Johannesburg, WITS 2050, South Africa d French National Institute for Preventive Archaeological Researches (INRAP), Nîmes, France e French Institute of South Africa (IFAS), USR 3336 CNRS, Johannesburg 2001, South Africa f Department of Integrative Anatomical Sciences, Keck School of Medicine, University of Southern California, Los Angeles, CA 90033, United States g Department of Musculoskeletal Biology, Institute of Ageing and Chronic Disease, University of Liverpool, Liverpool L7 8TX, United Kingdom h Department of Rheumatology, Aintree University NHS Trust, Liverpool L9 7AL, UK i South African Nuclear Energy Corporation SOC Ltd. (Necsa), Pelindaba, North West Province, South Africa j UGCT Department of Physics and Astronomy, Ghent University, Proeftuinstraat 86/N12, B-9000 Gent, Belgium k Department of Biology, Birmingham-Southern College, Birmingham, AL, 35254, United States l Molecular Imaging Center, Department of Radiology, Keck School of Medicine, University of Southern California, Los Angeles, CA 90033, United States m Department of Geology and Paleontology, Georgian National Museum, Tbilisi 0105, Georgia n School of Health Sciences, Aldro Building, University of Brighton, Eastbourne, United Kingdom o Department of Anthropology, University of Wisconsin, Madison, WI, 53706, USA b c a r t i c l e i n f o a b s t r a c t Article history: Received 7 August 2018 Accepted 12 November 2018 One of the most crucial debates in human paleoneurology concerns the timing and mode of the emergence of the derived cerebral features in the hominin fossil record. Given its exceptional degree of preservation and geological age (i.e., 3.67 Ma), StW 573 (‘Little Foot’) has the potential to shed new light on hominin brain evolution. Here we present the first detailed comparative description of the external neuroanatomy of StW 573. The endocast was virtually reconstructed and compared to ten southern African hominin specimens from Makapansgat, Malapa, Sterkfontein and Swartkrans attributed to Australopithecus and Paranthropus. We apply an automatic method for the detection of sulcal and vascular imprints. The endocranial surface of StW 573 is crushed and plastically deformed in a number of locations. The uncorrected and therefore minimum cranial capacity estimate is 408 cm3 and plots at the lower end of Australopithecus variation. The endocast of StW 573 approximates the rostrocaudally elongated and dorsoventrally flattened endocranial shape seen in Australopithecus and displays a distinct left occipital petalia. StW 573 and the comparative early hominin specimens share a similar sulcal pattern in the inferior region of the frontal lobes that also resembles the pattern observed in extant chimpanzees. The presumed lunate sulcus in StW 573 is located above the sigmoid sinus, as in extant chimpanzees, while it is more caudally positioned in SK 1585 and StW 505. The middle branch of the middle meningeal vessels derives from the anterior branch, as in MH 1, MLD 37/38, StW 578. Overall, the cortical anatomy of StW 573 displays a less derived condition compared to the late Pliocene/early Pleistocene southern African hominins (e.g., StW 505, SK 1585). © 2018 Elsevier Ltd. All rights reserved. Keywords: Australopithecus sulcal pattern Middle meningeal vessels Pliocene Sterkfontein * Corresponding author. E-mail address: beaudet.amelie@gmail.com (A. Beaudet). https://doi.org/10.1016/j.jhevol.2018.11.009 0047-2484/© 2018 Elsevier Ltd. All rights reserved. A. Beaudet et al. / Journal of Human Evolution 126 (2019) 112e123 1. Introduction One of the most critical questions in human paleoneurology concerns the timing of the emergence of derived cerebral features within the hominin1 clade. The assessment of a reliable chronological framework for the evolution of the human brain is essential for discussing the process of cerebral changes. In particular, investigations of the palaeontological record, comparative neuroanatomy of extant mammals, and quantitative genetic analyses have converged on two distinct models, i.e., mosaic-like, and concerted evolution models (Finlay and Darlington, 1995; Barton and Harvey, 2000; de Winter and Oxnard, 2001; Holloway, 2001; Oxnard, 2004; Falk, 2009; Hager et al., 2012; Smaers and Soligo,  mez-Robles et al., 2014). While the mosaic-like pattern 2013; Go of evolution suggests independent evolutionary trajectories of cerebral structures (e.g., Barton and Harvey, 2000; de Winter and Oxnard, 2001; Holloway, 2001; Oxnard, 2004; Smaers and Soligo, 2013), a concerted pattern of evolution involves a global reorganization of the brain (e.g., Finlay and Darlington, 1995; Falk, 2009). As the earliest representatives of Homo have been suggested to display neuroanatomical structural features commonly regarded as typical of extant humans (e.g., Tobias, 1987; Falk, 1983a; Schoenemann, 2013 [in spite of later changes in morphology of the human endocast, Neubauer et al., 2018]), evidence for such changes in the brain and associated evolutionary patterns must be sought earlier in the hominin fossil record. In particular, three cerebral regions have been the focus of major interest for addressing the question of timing and process of evolutionary changes in the early hominin cerebrum (see review in de Sousa and Cunha [2012]). Because of its roles in executive functions and language (e.g., Kringelbach and Rolls, 2004; Keller et al., 2009), the prefrontal cortex has been extensively investigated in human paleoneurology. Indeed, the inferior frontal gyrus in Australopithecus has long been suggested to be similar to that seen in extant chimpanzees because of the presence of a frontoorbital sulcus (see review in Falk, 2014). However, the possibility that early structural changes in this region may be perceptible in the 1.9 Ma Australopithecus sediba holotype adds further complexity to our understanding of the evolution of the frontal lobes (Carlson et al., 2011; Falk, 2014; Falk et al., 2018). Furthermore, the morphology of the occipital lobes in Australopithecus has been proposed to reflect potential topographical reduction of the visual cortex, which is likely to indicate an expansion of the parietal association cortex. This hypothesis is based on a presumed caudal position of the lunate sulcus in early hominins, which forms the anterolateral boundary of the primary visual cortex in apes (e.g., the Taung child, StW 505; Dart, 1925; Holloway, 1981, Holloway et al., 2004a). However, identification of the lunate sulcus in fossil hominins remains highly controversial (e.g., Falk, 1980; Holloway, 1981; Falk, 1983b; Holloway et al., 2004a; Falk, 2009). Finally, besides being involved in critical functions in the extant human brain (e.g., association of the precuneus with tasks integrating spatial, chronological and social relationships [Bruner et al., 2017]), the parietal region has played a fundamental role in the emergence of the characteristic globular shape of the modern human brain (Bruner et al., 2003a; Neubauer et al., 2018). However, because this region is rarely preserved in early hominins, little is known about early hominin parietal organization and morphology (Beaudet et al., 1 There is no universal agreement as to whether the separation of humans and their kin from great apes and their kin should be recognized at family level (Hominidae) or tribe level (Hominini) or subtribe level (Hominina). Contributors to this volume have agreed, while choosing their own term, to recognize that disagreement by inserting this footnote. 113 2018a). Despite an unknown degree of correspondence between the cerebral and endocranial sulcal patterns (Le Gros Clark et al., 1936, but see Dumoncel et al., 2018), the relevance of the frontoorbital and lunate sulci, as well as to a lesser extent the parietal sulci (see for example Tobias [1987] with regard to the inferior parietal lobule), exemplifies the pivotal role of sulcal patterns in detecting potential indications of cortical reorganization. Besides sulcal imprints, endocasts may preserve traces of the , 1997). Because of its vascular system (Saban, 1983; Grimaud-Herve role in the metabolism and thermoregulation of the brain, the endocranial vascular system might be considered a relevant proxy for analysis of fundamental changes in the hominin brain (see review in Bruner, 2017). Our knowledge of the endocranial vascular system in fossil hominin taxa relies mainly on imprints of the venous sinuses and of the middle meningeal vessels on the inner surface of the braincase, but also on the preservation of the diploic channels within diploic bone (Schepers, 1946; Saban, 1983; Falk and Conroy, 1983; Conroy et al., 1990; Falk, 1990; Tobias, 1991; Bruner et al., 2003b; Holloway et al., 2004b; Bruner et al., 2005; Bruner and Sherkat, 2008; Bruner et al., 2011; Rangel de L azaro et al., 2016; Beaudet et al., 2018b). Extracranial venous foramina represent an additional source of evidence for reconstructing the cranial blood flow in fossils (e.g., Braga and Boesch, 1997; Seymour et al., 2016). Comparison of the venous sinus systems and the middle meningeal vessel pattern in early hominins reveals substantial differences between Australopithecus and Paranthropus (Saban, 1983; Falk and Conroy, 1983; Conroy et al., 1990; Falk, 1990; Tobias, 1991). More specifically, an enlarged occipitalemarginal sinus is more commonly found in Paranthropus than in Australopithecus (except for the Hadar hominins) and the middle branch of the middle meningeal vessels is virtually absent in Australopithecus (Saban, 1983; Falk and Conroy, 1983; Conroy et al., 1990; Falk, 1990; Tobias, 1991). For now, there is no consensus over the physiological implications of the organization of the middle meningeal vessels in the fossil hominin record (Bruner et al., 2003b, 2011), even if the thermoregulation hypothesis has been largely favoured (e.g., Falk, 1990). Interpreting structural changes in the endocranial vascular system in the light of cerebral reorganization is therefore of interest for developing physiological hypotheses and investigating the potential coevolution of the brain and the vascular network. The early hominin fossil record is highly fragmentary and rarely preserves complete crania or natural endocasts (Holloway et al., 2004b). Consequently, our knowledge of early hominin brain evolution primarily derives from partial endocasts, usually documenting one of the three aforementioned key regions in the brain (e.g., frontal lobes in KNM-ER 1470, MH 1, and Sts 60; occipital lobes in AL 162-28, SK 1585 and StW 505; and the parietal lobe in Sts 1017 [Schepers,1946; Holloway, 1972; Falk, 1979; Holloway, 1983; Falk, 1983a; Holloway et al., 2004a; Carlson et al., 2011, Falk, 2014]) and traces of the vascular system (e.g., KNM-ER 1470, SK 1585, Sts 60, and Sts 1017 [Schepers,1946; Holloway,1972; Falk,1979; Saban,1983]). Considering these constraints, falsification of the hypotheses of a mosaic versus a concerted evolution of cortical areas, and direct comparison of the evolutionary trajectories of cortical folding and the vascular system might be hampered by the quality of the early hominin fossil record. In this regard, the ‘Little Foot’ cranium represents a unique opportunity to provide additional evidence of Pliocene hominin neuroanatomy and to discuss potential evolutionary scenarios. Indeed, besides its exceptional degree of preservation and completeness (Clarke and Tobias, 1995; Clarke, 1998; Clarke and Kuman, Submitted for publication), the Australopithecus specimen StW 573 (‘Little Foot’), discovered in 1994 and 1997 in Member 2 of the Sterkfontein Formation, is remarkable for its geological age of 3.67 Ma (Bruxelles et al., 2014; Granger et al., 2015; Kramers and Dirks, 2017a,b; Stratford et al., 2017; Bruxelles et al., Submitted 114 A. Beaudet et al. / Journal of Human Evolution 126 (2019) 112e123 for publication). Therefore, our study aims to: (i) provide the first description of the StW 573 endocast (including general morphology, structural asymmetries and cranial capacity), (ii) detect, identify and comparatively describe the sulcal and vascular pattern and (iii) discuss the significance of our findings for the evolution of the hominin brain. Table 2 Cranial capacity (ECV, cm3) of StW 573 compared with ECVs of Australopithecus and Paranthropus published in Du et al. (2018).a Specimen/taxa 2. Materials and methods 2.1. Comparative material Information on fossil and extant specimens examined in the present study is summarized in Table 1. We included ten fossil hominins currently housed in the Evolutionary Studies Institute at the University of the Witwatersrand in Johannesburg and the Ditsong National Museum of Natural History in Pretoria (South Africa; Table 1). These are from Makapansgat (n ¼ 1), Malapa (n ¼ 1), Sterkfontein (n ¼ 7) and Swartkrans (n ¼ 1). We use endocranial volumes (ECV) of Australopithecus (afarensis, africanus, sediba and garhi) and Paranthropus (aethiopicus, robustus and boisei) published in Du et al. (2018) (Table 2). As we need to take into consideration the hypothesis of a third Australopithecus species in southern Africa, the Australopithecus africanus group is split into A. africanus (MLD 37/38, Sts 19/58, Sts 5, Sts 60 and Taung) and Australopithecus prometheus (MLD 1, Sts 71 and StW 505 [Clarke, 2008]). For our extant comparative sample, we selected one adult human and one adult common chimpanzee individual curated in the Pretoria Bone Collection  et al., 2005) at the University of Pretoria (South Africa) and in (L'Abbe the Royal Museum for Central Africa in Tervuren (Belgium) respectively, and we also referred to available atlases and publications documenting variation in the extant human and chimpanzee brain (namely Connolly, 1950; Ono et al., 1990; Falk et al., 2018). ECV StW 573 Australopithecus prometheus (n ¼ 3)b Mean Range Australopithecus afarensis (n ¼ 6) Mean Range Australopithecus africanus (n ¼ 5) Mean Range Australopithecus garhi (n ¼ 1) Australopithecus sediba (n ¼ 1) Paranthropus aethiopicus (n ¼ 3) Mean Range Paranthropus robustus (n ¼ 3)c Mean Range Paranthropus boisei (n ¼ 8) Mean Range Date (Ma) 408 3.67 503 442e558 2.01e2.85 2.95e3.24 428 372e550 2.01e3.03 455 414e508 450 420 2.45e2.50 1.95 1.7e2.41 443 410e491 1.6e2.36 486 465e500c 1.41e1.93 492 430e545 a Date ranges are from Du et al. (2018) except for Australopithecus prometheus: for Sterkfontein Member 2 and StW 573, we use Granger et al., 2015, and for Member 4 Pickering and Kramers (2010) minimum age of 2.01. We note, however, that the stratigraphic association of the flowstones used to date the upper limit of Member 4 is more complex than proposed in Pickering and Kramers (2010) and older dates for Member 4 should not be discarded; for example, the faunal age estimate for that member is between 2.4 to 2.8 Ma (Vrba, 1985). b Following Clarke (2008). c The ECV values for TM 1517, SK 48 and SK 46 listed by Du et al. (2018) are not used here because RJC notes that these were only rough visual estimates published in brief by Schepers (1946) and Broom and Robinson (1952) based on partial and deformed specimens. We therefore use only the P. robustus specimens published by Holloway et al. (2004a,b). Table 1 Comparative sample used for analyzing and comparing the endocranial sulcal and vascular imprints. Specimen/sample Site/provenance Taxonomic attribution Sex StW 505 Sterkfontein Member 4 Australopithecus africanus/prometheus M StW 578 Jacovec Cavern Australopithecus sp. ? Sts 5 Sterkfontein Member 4 Australopithecus africanus F/M Sts 25 Sterkfontein Member 4 Australopithecus sp. ? Sts 60 Sterkfontein Member 4 Australopithecus africanus M Sts 1017 Sts 1960b SK 1585 MH 1 Sterkfontein Member 4 Sterkfontein Member 4 Dump Swartkrans Malapa ? Australopithecus africanus Paranthropus robustus Australopithecus sediba ? ? ? M MLD 37/38 Makapansgat Member 4 South Africa Congo Australopithecus africanus F Homo sapiens Pan troglodytes F M Extant human (n ¼ 1) Extant chimpanzee (n ¼ 1) a b c d e f g h Evolutionary Studies Institute; Ditsong National Museum of Natural History, Pretoria; Pretoria Bone Collection, University of Pretoria; Royal Museum for Central Africa. Palaeosciences Centre, University of the Witwatersrand, Johannesburg; South African Nuclear Energy Corporation, Pretoria (Hoffman and de Beer, 2012); European Synchrotron Radiation Facility, Grenoble; Centre for X-ray Tomography of Ghent University, Gent (Masschaele et al., 2013). References Lockwood and Tobias (1999, 2002); Clarke (2008) Partridge et al. (2003); Clarke (2013) Broom (1947); Broom and Robinson (1950); Broom et al. (1950); Clarke (2008); Grine et al. (2012) Brain (1981); Wolpoff and Lee (2006); Grine (2013) Broom and Schepers (1946) Falk (1979) de Ruiter (2004) Holloway (1972) Berger et al. (2010); Carlson et al. (2011) Dart (1959, 1962); Conroy et al. (1990)  et al. (2005) L'Abbe Stored at Imaging facility ESIa Pal. Centree 90.8 ESIa Pal. Centree 66.6 e 75.0 b Voxel size* (isotropic; mm) DNMNH Pal. Centre DNMNHb Necsaf 64.9 DNMNHb Necsaf 59.7 DNMNHb DNMNHb DNMNHb ESIa Necsaf Necsaf Necsaf ESRFg 42.8 63.5 65.5 91.42 ESIa Pal. Centree 80.8 PBCc RMCAd Necsaf UGCTh 114.1 85.0 A. Beaudet et al. / Journal of Human Evolution 126 (2019) 112e123 Sterkfontein StW 505 is an incomplete cranium preserving the splanchnocranium and most of the left side of the neurocranium (Lockwood and Tobias,1999) and is variably attributed to A. africanus (Lockwood and Tobias, 1999, 2002) or A. prometheus (Clarke, 2008). All of these studies regard StW 505 as an adult male specimen. This specimen was recovered in Member 4 of the Sterkfontein Formation during excavation of the type locality breccia where most of the Australopithecus remains were recovered from 1936 by R. Broom and later by J. Robinson (Lockwood and Tobias, 1999). The endocast preserves the left hemisphere and the right frontal lobe (Falk et al., 2000; Holloway et al., 2004a,b; Neubauer et al., 2012). StW 578 is a calotte assigned to Australopithecus sp. and discovered by R.J. Clarke and M. Makgothokgo in situ in the Jacovec Cavern (Partridge et al., 2003; Beaudet et al., 2018b). The endocast preserves the right parietal lobe, the posterior portion of the left parietal lobe and part of the frontal area. Sts 5 (‘Mrs Ples’) is the most complete and well-preserved cranium attributed to A. africanus (Broom, 1947; Broom et al., 1950). Sts 5 has been variably described as an adult female (Broom, 1947; Broom and Robinson, 1950; Broom et al., 1950; Clarke, 2008; Grine et al., 2012) or an adolescent male (Thackeray et al., 2002; see review in Grine, 2013). Sts 5 was found in Member 4 of the Sterkfontein Formation (Broom, 1947). The braincase lacks a strip of bone running across the frontal and left side that was physically filled using reconstruction material (see Clarke, 1990; Robinson, 1997 for comments on later damage). Sts 25 is an isolated neurocranium filled by calcified sediment found in Member 4 of the Sterkfontein Formation (Brain, 1981; Holloway et al., 2004b). There is no definite taxonomic diagnosis of Sts 25 (review in Beaudet et al., 2018b; Brain, 1981; Wolpoff and Lee, 2006). Sts 25 has been suggested to be an adult (Brain, 1981; Grine, 2013) or a young individual (Wolpoff and Lee, 2006). Sts 25 preserves the caudal part of the right frontal lobe, a portion of the right parietal lobe, most of the left parietal and occipital lobes. Sts 60 is a natural partial endocast found in Member 4 of the Sterkfontein Formation (Broom and Schepers, 1946). Sts 60 is associated with TM 1511, the holotype of Plesianthropus transvaalensis, which is now considered a junior synonym of A. africanus (Clarke, 1994). TM 1511 is an adult male (Clarke, 2008). The endocast preserves most of the left side while the right side is represented only by the frontal lobe and a limited part of the parietal lobe. Sts 1017 is a portion of a natural partial endocast from Member 4 of the Sterkfontein Formation initially identified as a cercopithecoid and subsequently as a hominin (Falk, 1979). The species and sex are unknown. Sts 1017 provides a portion of the right parietal, temporal and frontal lobes (Falk, 1979). Sts 1960b is a natural partial endocast embedded in a block of breccia from Sterkfontein Member 4 attributed to A. africanus based on comparisons with Sts 60 (de Ruiter, 2004). The visible surface of the endocast consists mostly of the right frontal lobe and a portion of the superior part of the left frontal lobe. Swartkrans SK 1585 is a natural partial endocast from the lime miners' hillside rubble dump at Swartkrans (Brain, 1970) and consists of the right hemisphere and a very small portion of the left occipital lobe (Holloway, 1972; Falk, 1980; Falk et al., 2000; Holloway et al., 2004b). So far, this is the only natural endocast known for Paranthropus robustus (Holloway et al., 2004b). This specimen has been tentatively identified as a P. robustus by Holloway (1972). Malapa The partial cranium of the holotype juvenile male from A. sediba (MH 1) was found in the 1.977 Ma deposits of Malapa (Berger et al., 2010). The endocast of MH 1 is missing the entire right hemisphere posterior to the coronal suture, posterior portions of the left occipital and temporal lobes, and the cerebellum (Carlson et al., 2011). 115 Makapansgat MLD 37/38 is a partial cranium attributed to an adult female A. africanus from Makapansgat (Dart, 1959, 1962). The endocast lacks most of the frontal area rostral to bregma (to the exception of the caudal portion of the right inferior frontal area) as well as the rostral part of the left temporal lobe (Conroy et al., 1990; Neubauer et al., 2004, 2012). 2.2. Scanning protocol and endocast reconstruction The skull of StW 573 was scanned at the microfocus X-ray tomography facility of the Palaeosciences Centre at the University of the Witwatersrand, in Johannesburg (South Africa), at a spatial resolution of 88 mm (isotropic voxel size). All of the comparative specimens investigated in this study have been similarly imaged by microfocus X-ray tomography using the various systems available at the Centre for X-ray Tomography of Ghent University (UGCT) in Ghent (Belgium; Masschaele et al., 2013), the Palaeosciences Centre at the University of the Witwatersrand in Johannesburg (South Africa) and the South African Nuclear Energy Corporation in Pelindaba (South Africa; Hoffman and de Beer, 2012, Table 1). The sediments filling the endocranial cavity in StW 573 and the comparative fossil specimens were digitally separated from the bony remains by combining a region-based segmentation approach and manual corrections via Avizo v9.0 software (Visualization Sciences Group Inc.; Meyer and Beucher, 1990; Roerdink and Meijster, 2001). The virtual endocranial volumes were extracted from the extant human and chimpanzee individuals by using Endex software (Subsol et al., 2010; http://liris.cnrs.fr/gilles.gesquiere/wiki/ doku.php?id¼endex). The cranial capacity of StW 573 was computed using Avizo v9.0. 2.3. Detection and identification of sulcal and vascular imprints We used an automatic method for the detection of cortical relief in endocasts based on the algorithm presented by Yoshizawa et al. (2007, 2008) for the detection of topographical variation in threedimensional (3D) meshes (see Beaudet et al., 2016 and Beaudet and Gilissen, 2018 for further technical details). Sulcal and vascular (i.e., middle meningeal vessels) imprints were detected via a geometrybased method using curvature lines computed from the surface. We manually corrected detection by removing non-anatomical features, mostly related to diagenetic processes, through a customized script written in MATLAB R2013a (Mathworks, https://www.mathworks. com/products/matlab.html). For reference for identification, we use human and chimpanzee brains (Connolly, 1950; Ono et al., 1990; Falk et al., 2018) and endocast (de Jager et al., 2018) atlases. We followed the nomenclature detailed in Connolly (1950) and Falk et al. (2018) and summarized in Table 3 for describing the sulcal patterns. With respect to the middle meningeal vessels (MMV), we referred to anterior (i.e., bregmatic), middle (i.e., obelic) and posterior (i.e., lambdatic) branches (Schepers, 1946; Saban, 1983; Falk, 1993; Bruner et al., 2003b, 2005; Bruner and Sherkat, 2008). Additionally, we applied the scoring system introduced by Adachi (1928) for the branching pattern of the middle branch of the MMV that includes type I (i.e., the middle ramus branches from the anterior ramus), type II (the middle ramus branches from the posterior ramus) and type III (the middle ramus branches from both rami) as illustrated in Falk (1993, Figure 3). 3. Results 3.1. General description Preservation: Preservation of the cranium of StW 573 is described in Clarke and Kuman, (Submitted for publication). A 116 A. Beaudet et al. / Journal of Human Evolution 126 (2019) 112e123 Table 3 Abbreviations of features identified on endocasts. Abbreviation Sulci ar c h hr fi fm fo fs ip L lc pci pcm pcs pl pti r S sca scp tm ts W Middle meningeal vessels ab fb mb pb Definition Ascending ramus of lateral fissure Central sulcus Horizontal ramus of inferior precentral sulcus Horizontal ramus of lateral fissure Inferior frontal sulcus Middle frontal sulcus Fronto-orbital sulcus Superior frontal sulcus Intraparietal sulcus Lunate sulcus Calcarine sulcus Inferior precentral sulcus Middle precentral sulcus Superior precentral sulcus Prelunate sulcus Inferior postcentral sulcus Rectus sulcus Lateral fissure Subcentral anterior sulcus Subcentral posterior sulcus Middle temporal sulcus Superior temporal sulcus Fronto-marginal sulcus Anterior branch Frontal branch Middle branch Posterior branch virtual rendering of the endocast is shown in Figure 1. A fracture runs along the coronal suture and the frontal region has collapsed. At the left side, a transverse break has induced a slight gap in the middle part of the frontal lobe. The orbital surfaces, the frontal poles, the inferior part of the frontal lobes and the left temporal pole are not preserved. The superior parts of the frontal and parietal lobes are damaged and distorted. An oblique crack runs from the superior part of the caudal portion of the left parietal lobe to the right sigmoid sinus, with some slight displacement. The left occipital and cerebellar lobes, and the portion of the occipital region in-between the two cerebella are damaged. The region of the basicranium surrounding the foramen magnum is crushed. Morphology: The general shape of the unreconstructed endocast of StW 573 is compared with a number of southern African Plio-Pleistocene hominins in Figure 2. The endocast of StW 573 approximates the rostrocaudally elongated and dorsoventrally flattened endocranial shape seen in Australopithecus (Fig. 2; Neubauer et al., 2012; Beaudet, 2018a). Even though the frontal pole is missing, the frontal lobes seem to be more acutely angled than in other Australopithecus, and we may hypothesize that the beak-shaped rostral orbital area that is found in extant great apes and Paranthropus may have been present in StW 573 (Fig. 2; Falk et al., 2000). However, the fracture along the sagittal plane may be responsible for the narrowing of the frontal area (Clarke and Kuman, Submitted for publication). On the other hand, unlike Paranthropus, the nearly complete right temporal lobe is anteriorly extended and laterally projected, as in other Australopithecus specimens (Fig. 2; Falk et al., 2000). The occipital poles in StW 573 are more pointed than in the Australopithecus comparative specimens and closely resemble the occipital morphology seen in SK 1585 and the extant human individual (Fig. 2). However, as for the frontal lobes, this protrusion might be related to the crack that runs along the occipital region. The cerebellar lobes are projected posteriorly, as in other Australopithecus, while in Paranthropus they are placed under the cerebrum (Fig. 2; Holloway, 1972; Falk, 1985; Holloway and Yuan, 2004). Structural asymmetries: In terms of structural asymmetries, StW 573 displays a left occipital petalia (i.e., protrusion of the left occipital lobe beyond the right occipital lobe) that is also found in Sts 5 and SK 1585 as well as in most fossil hominins, extant great apes and humans (Fig. 2; Holloway et al., 2004b; Balzeau et al., 2012). It is noteworthy that in MLD 37/38 the right occipital lobe protrudes beyond the left occipital lobe (Fig. 2). Cranial capacity StW 573 has an unreconstructed cranial capacity estimate of 408 cm3. We compiled the cranial capacity estimates of Australopithecus (afarensis, africanus, sediba and garhi) and Paranthropus (aethiopicus, robustus and boisei) from Du et al. (2018) in Table 2. The estimate for StW 573 falls within the range Figure 1. Original skull (left), virtual skull (light grey) showing the endocast in semi-transparency (middle) and virtual endocast (dark blue, right) of StW 573 (A). Oblique view of the virtual endocast of StW 573 showing the main cerebral imprints (B). Standard views (anterior, right, posterior, left, superior and inferior) of the endocast of StW 573 (C). Photo of the original skull by M. Lotter and R.J. Clarke. Scale bars: 2 cm. A. Beaudet et al. / Journal of Human Evolution 126 (2019) 112e123 117 Figure 2. Virtual endocast of StW 573 and of six Australopithecus (StW 505, StW 578, Sts 5, Sts 60, MH 1, MLD 37/38), one Paranthropus (SK 1585), one extant human (EH) and one extant chimpanzee (EC) specimens in lateral right (top row), dorsal (middle row) and rostral (bottom row) views. The left side of StW 505, Sts 60 and MH 1 are shown as right. Images not to scale. of Australopithecus afarensis but is slightly lower than the range of southern African Australopithecus as well as the eastern and southern African Paranthropus. This value is probably an underestimation thereof, because of the frontal region that has collapsed and the plastic deformation that has affected the cranium (Clarke and Kuman, Sumbitted). After correction of cranial capacity, StW 573 has the potential to fit within the range of southern African Australopithecus and eastern African Paranthropus. In any case, the cranial capacity of StW 573 would plot at the lower end of the Australopithecus variation. Additional identifiable features: On the top of the right parietal lobe, there is a distinct granular fovea produced by the protrusion of the arachnoid (Fig. 1 and Supplementary Online Material [SOM]). The size of the fovea is less than 10 mm in diameter and is located close to the sagittal plane at the top of the parietal lobe. 3.2. Sulcal imprints Frontal lobes: A remnant of the superior frontal sulcus is identifiable on the rostral part of the right hemisphere (Fig. 3 and SOM). The sulcus rectus makes an incision in the orbital margin. Caudal to the sulcus rectus are a series of rostrocaudally-oriented furrows, which correspond to the middle frontal sulcus. In the inferior part of the frontal lobe, two grooves are perceptible that could not be clearly identified (but see Discussion). The vertical sulcus detected in the caudal portion of the frontal area is identified as the inferior precentral sulcus. On the most superior aspect of the left frontal lobe, a series of sulci course from the anterior border close to the frontal pole towards the vertical crack delimiting approximately the transition to the parietal lobe. We identify the most rostral sulcus among these as the sulcus rectus, the intermediate furrows as part of the middle frontal sulcus and the most caudal imprints as part of the horizontal ramus of the inferior precentral sulcus. These three sulci could have been continuous in the brain (Connolly, 1950; Falk et al., 2018). The vertical sulcus transecting the middle frontal sulcus may correspond to an element of the superior frontal sulcus (Ono et al., 1990; Falk et al., 2018). The fronto-orbital sulcus incises the lateral margin of the inferior frontal convolutions. Two sulci are detected superior to the fronto-orbital sulcus in a position analogous to the unidentified sulci detected in the right hemisphere. Given their respective position, we consider the following hypotheses: (i) the two furrows may correspond to two rami of the fronto-orbital sulcus, (ii) the posterior furrow may correspond to the inferior frontal sulcus (Connolly, 1950; Ono et al., 1990; Falk et al., 2018). An additional sulcus is detected close to the fronto-orbital sulcus, which could not be identified. The vertical groove posterior to the fronto-orbital Figure 3. Sulcal pattern in StW 573 and in five Australopithecus (StW 505, StW 578, Sts 5, Sts 25, Sts 60) specimens in lateral views. Only sulci mentioned in the text are labelled. The right view of StW 505, the left view of StW 578 and both views of Sts 25 show the inner surface(s) of the opposite side(s) of the endocast. Question marks indicate uncertain sulcal identifications. Abbreviations are detailed in Table 3. Labels are placed at one of the extremities of the corresponding sulci. Images not to scale. 118 A. Beaudet et al. / Journal of Human Evolution 126 (2019) 112e123 sulcus corresponds to the inferior precentral sulcus. The horizontal ramus of the inferior precentral sulcus may have been confluent with the inferior precentral sulcus. In StW 573, Sts 5, Sts 25, SK 1585, MH 1, MLD 37/38 and to a lesser extent in Sts 60 and Sts 1960b, the inferior region of the frontal lobes is similarly convoluted (Figs. 3, 4, and SOM). In all of these endocasts, the inferior region is incised by three vertical clefts (which may correspond to the inferior precentral sulcus, the frontoorbital sulcus and the inferior frontal sulcus) bordering distinct bulges (particularly prominent in Sts 5, MH 1 and MLD 37/38). The pattern detected differs from the one reported in the human brain, where the horizontal and ascending rami of the lateral fissure are visible, and approximates more closely the pattern documented in the chimpanzee brain, even if the configuration of the inferior frontal sulcus could be extremely variable in the latter (Fig. 4, Connolly, 1950; Falk et al., 2018). The middle frontal sulcus is detected in StW 573, StW 505, Sts 60, Sts 1960b, and probably Sts 5. This sulcus runs relatively straight from the frontal pole towards the parietal area and is connected (or at least topographically close to) the sulcus rectus, as in the chimpanzee brain (Falk et al., 2018). In both StW 573 and Sts 60 the caudal end of the middle frontal sulcus curves to form the horizontal ramus of the precentral sulcus, which might also be observed in the human and chimpanzee brain (Connolly, 1950; Ono et al., 1990; Falk et al., 2018). Parietal lobes: In both hemispheres, the central sulcus in StW 573 runs along the main fracture approximately delimiting the frontal lobe (Fig. 3, and SOM). The inferior end of the central sulcus is slightly convex in both hemispheres. A remnant of a vertical sulcus is detected caudal to the central sulcus on both sides. Because of its close topographical proximity with the central sulcus, we suggest that this furrow may correspond to the inferior postcentral sulcus. In both hemispheres, a short vertical sulcus transects the lateral fissure near the inferior end of the presumed inferior postcentral sulcus. We identify this imprint as the subcentral posterior sulcus. The central sulcus in StW 573, Sts 60 and SK 1585 is relatively straight as compared to the hook-like inferior end of the central sulcus in Sts 1017 (Figs. 3-4, and SOM). In both StW 573 and Sts 1017, the posterior subcentral sulcus intersects the lateral fissure and it is topographically close to the postcentral sulcus, which is also found in the human and chimpanzee brain (Connolly, 1950; Ono et al., 1990; Falk et al., 2018). Temporal lobes: The lateral fissure as well as the superior temporal sulcus are clearly visible on the right temporal lobe of StW 573 (Fig. 3, SOM). Imprints of the superior temporal sulcus run caudally towards the occipital lobe. Fragmentary furrows in the inferior part of the right and left temporal lobes are tentatively identified as remnants of the middle temporal sulcus. In StW 573, Sts 5, Sts 60 and Sts 1017 the superior and middle temporal sulci are parallel, as in extant chimpanzees, while in SK 1585 (and maybe MLD 37/38) they intersect (Figs. 3-4, and SOM). In all of the fossil specimens considered in this study, the temporal sulci are relatively continuous, as in extant chimpanzees, whereas in extant humans the middle temporal sulcus is often subdivided (Connolly, 1950; Falk et al., 2018). Occipital lobes: Only one sulcus could be unambiguously detected in the occipital part of the endocast. This crescent-shaped sulcus is tentatively identified as the lateral edge of the lunate sulcus. However, as StW 573 lacks any clear imprints of other occipital sulci and the bordering parietal sulci, our identification has to be considered speculative. A faint sulcus was detected on the right occipital lobe of the StW 573 endocast, but is difficult to assign. 3.3. Vascular imprints Figure 4. Sulcal patterns in four Australopithecus (Sts 1017, Sts 1960b, MH 1, MLD 37/ 38), one Paranthropus (SK 1585), one extant human (EH) and one extant chimpanzee (EC) specimens in lateral views. Only sulci mentioned in the text are labelled. The right view of MH 1 shows the inner surface of the left side of the endocast. Question marks indicate uncertain sulcal identifications. Abbreviations are detailed in Table 3. Labels are placed at one of the extremities of the corresponding sulci. Images not to scale. Traces of the middle meningeal vessels (MMV) are clearly visible on both hemispheres of StW 573 (Fig. 5). On the right side, the anterior branch of the MMV extends from the temporo-orbital notch to the parietal lobe and bifurcates into two ramifications. The middle branch of the MMV is connected to the anterior branch and runs along the Sylvian fissure. The posterior branch derives from the branch connected to the sigmoid sinus in the inferior part of the temporal lobe (tentatively interpreted as the superior petrosal sinus) and is parallel to the anterior and middle branches. The meningeal vessel pattern on the left side is similar to the right A. Beaudet et al. / Journal of Human Evolution 126 (2019) 112e123 119 patterns that differ in the branching of the middle branch of the MMV. The first pattern includes MLD 37/38, the extant human endocast, and probably StW 578 and MH 1, depicting a middle branch deriving from the anterior branch of the MMV (type I in Falk [1993] adapted from the classification of Adachi [1928]). The second pattern includes SK 1585 and corresponds to a middle branch deriving from the posterior branch of the MMV (type II in Falk [1993] adapted from the classification of Adachi [1928]). The degree of ramification varies from a very simple pattern (e.g., StW 578, Sts 60) to a more complex organization (left hemisphere of Sts 1017, SK 1585, MLD 37/38, extant human and chimpanzee). In this comparative context, StW 573 is similar to MLD 37/38, StW 578, MH 1 and extant humans in terms of branching pattern (i.e., type I). 4. Discussion 4.1. General aspects Figure 5. Vascular patterns in StW 573 and in five Australopithecus (StW 578, Sts 60, Sts 1017, MH 1, MLD 37/38), one Paranthropus (SK 1585), one extant human (EH) and one extant chimpanzee (EC) specimens in lateral views. Only vessels mentioned in the text are labelled. Abbreviations are detailed in Table 3. Images not to scale. Despite a number of fractures and potential plastic deformation, the endocast of StW 573 represents one of the most complete and well-preserved early hominin endocasts currently available in the fossil record (Holloway et al., 2004b). In terms of overall morphology, the unreconstructed endocast is similar to the typical shape described in early hominins (Falk et al., 2000; Neubauer et al., 2012; Beaudet et al., 2018a) with some intriguing similarities with Paranthropus in the frontal and occipital lobes that need to be confirmed by virtual reconstruction of the endocast and quantitative morphometric analyses. The presence of occipital petalias in Australopithecus specimens has been reported previously (e.g., Sts 5, Holloway et al., 2004b). This pattern of structural asymmetry has been suggested to be shared by extant non-human great apes and fossil hominins and to have been inherited from the last common ancestor (Balzeau et al., 2012). In this regard, StW 573 confirms that occipital petalias were present in the hominin clade more than three million years ago. The StW 573 estimated unreconstructed cranial capacity plots at the lower end of the Australopithecus range, which is consistent with its old geological age and the hypothesis of a gradual increase in brain size within the hominin clade (but see Du et al., 2018). However, since the frontal region has collapsed and plastic deformation has probably affected the cranium (Clarke and Kuman, Submitted for publication), this volume is likely underestimated. Accordingly, the corrected volume will likely fit within the A. africanus and/or the A. prometheus range, and also within A. afarensis variation. Interestingly, StW 573 shows traces of an arachnoid granulation. In extant humans, arachnoid granulations are usually found adjacent to the superior sagittal sinus and are suggested to be involved in the drainage of the cerebrospinal fluid (Ikushima et al., 1999). Even if possible correlations between size and frequency of arachnoid granulations and age are disputed (e.g., Ikushima et al., 1999; Bayrak et al., 2009), the presence of this trace in StW 573 may be explained by the advanced age of the specimen (i.e., the teeth are heavily worn and the absence of internal signs of fused sutures [Clarke and Kuman, Submitted for publication]) and related erosion/calcification of bone. 4.2. Sulcal pattern counterpart, except ramification of the anterior and middle branches is missing. Moreover, the anterior and middle branches are more reticulated than in the right hemisphere. Unfortunately, the occipital part is damaged and nothing can be said about the configuration of the transverse-sigmoid system and, in particular, about the presence/absence of an enlarged occipitalemarginal sinus (Falk and Conroy, 1983; Conroy et al., 1990; Tobias, 1991). Within the fossil hominin specimens, the middle meningeal branch is absent from Sts 60 and probably Sts 1017. We identify two The inferior frontal circumvolutions and occipital sulci in early hominins have played a central role in the debate dealing with the timing and mode of brain evolutionary history (e.g., Falk, 1980; Holloway, 1981; Holloway et al., 2004a; Falk, 2014). In particular, the human-like configuration of the inferior frontal gyri (i.e., presence of the horizontal and ascending branches of the lateral fissure) has long been acknowledged to emerge concomitantly with the emergence of genus Homo (Falk, 1983a; Tobias, 1987). However, 120 A. Beaudet et al. / Journal of Human Evolution 126 (2019) 112e123 the combination of a chimpanzee-like sulcal pattern (i.e., presence of a fronto-orbital sulcus) with evidence of shape reorganization in the A. sediba endocast (MH 1) adds further complexity to the controversial evolutionary history of this cortical region (Carlson et al., 2011; Falk, 2014). The description of the endocast of StW 573 and our comparative sample have the potential to provide additional evidence of variation of the configuration of the early hominin inferior frontal area. In our study, StW 573 and the Australopithecus and Paranthropus specimens show nearly systematically the same chimpanzee-like sulcal pattern combining three vertical furrows, which encompass prominent bulges. However, the degree of protrusion of the inferior frontal bulges is particularly high in Sts 5, Sts 25, SK 1585 and MH 1 (but not to the same degree), especially as compared to StW 573, which might suggest early shape reorganization prior to changes in the sulcal pattern in this region (Carlson et al., 2011; but see; Falk et al., 2018). If our identification of the lunate sulcus in StW 573 is correct, then it would be placed nearly above the sigmoid sinus, as in extant chimpanzees, while in StW 505, SK 1585 and in extant humans the lunate sulcus is found more caudally (Figs. 3, 4). Thus, its rostral position would suggest an ancestral organization of the visual cortex in Pliocene southern African hominins. In this regard, our results cannot falsify either of the two competing hypotheses, arguing for a human-like caudal (Dart, 1925; Holloway, 1972, 1981; Holloway et al., 2004a) or chimpanzee-like rostral (Falk, 1980, 1983b, 2009) position of the lunate sulcus in the geologically younger southern African hominin endocasts (e.g., Taung, StW 505, SK 1585). On the contrary, if we consider the condition reported for contemporaneous eastern African hominins, the StW 573 endocast might provide crucial evidence for discussing evolutionary scenarios. Indeed, for southern African hominins, the A. afarensis specimen AL 162-28 has been suggested to show either a humanlike (Holloway, 1983; Holloway and Kimbel, 1986) or chimpanzeelike (Falk, 1985) lunate sulcus. If AL 162-28 shows a human-like lunate sulcus, it would mean that the derived organization of this region occurred in eastern African Australopithecus, and that this population was probably ancestral to the southern African hominins displaying similar organisation, or that this trait emerged in different localities in Africa as a result of convergent evolution. On the other hand, if AL 162-28 is similar to StW 573 in terms of occipital organization, the origins of a derived occipital organization would postdate those two specimens. If “Little Foot” is significantly geologically younger than 3.67 Ma (Kramers and Dirks, 2017a,b; but see Stratford et al., 2017; Bruxelles et al., Submitted for publication), it might indicate a high degree of variation of the lunate sulcus within Pleistocene hominins. However, our identification of the lunate sulcus in StW 573 needs to be confirmed through the application of quantitative methods (e.g., Holloway, 1981; Beaudet, 2017). Moreover, variation of the lunate sulcus has been revealed to be particularly high in the extant human brain (e.g., in Allen et al., [2006], a continuous lunate sulcus could only be identified in 1.4% of the hemispheres observed), which may raise some concerns about the identification of this structure in fossil hominin endocasts. The parietal lobes contributed significantly to the emergence of globular shape of the modern human brain (Bruner et al., 2003a). This region is also of interest due to the association of particular neural districts with functions, such as the angular and supramarginal gyri for language (Tobias, 1987). In our sample, ramifications of the caudal portion of the superior temporal sulcus are perceptible in Sts 1017 and the inferior part of the parieto-occipital region are well-convoluted (e.g., StW 578 and StW 505). Moreover, the posterior subcentral sulcus is present in StW 573, a region not often preserved in early hominin endocasts (but see Sts 1017). This sulcus forms the caudal boundary of Brodmann's area 43, which is related to primary gustatory cortex, but also has implications for language and digit stimulation (Trans Cranial Technologies, 2012). Similarly, the superior temporal gyrus is involved in a number of critical functions (notably as part of Wernicke's area) (Trans Cranial Technologies, 2012). Interestingly, in all of the specimens preserving temporal sulci, the superior and middle temporal sulci are parallel, with the exception of the Paranthropus specimen SK 1585, and probably the Australopithecus specimen from Makapansgat, MLD 37/38, in which the two sulci intersect. This observation might be consistent with the differences reported in the morphology of the temporal lobe between Australopithecus and Paranthropus (Falk et al., 2000), and could be of interest considering the combination of Australopithecus- and Paranthropus-like traits previously pointed out in MLD 37/38 (Aguirre, 1970; Tobias, 1980). 4.3. Vascular pattern The endocast of StW 573 reveals a complex organization of the vascular system including ramifications of the anterior branch and the presence of a middle branch of the MMV. Saban (1983, 1984) described two patterns of the MMV within early hominins taxa, with Paranthropus (SK 1585, KNM-ER 407, KNM-ER 1813) displaying a middle branch that is virtually absent in Australopithecus (Taung, Sts 60, Omo 338 Y). Within our sample, we detected the presence of a middle branch of the MMV in Australopithecus specimens, including in StW 573. Accordingly, our study suggests that the middle meningeal vascular system in early hominins may have been more complex and variable than previously thought. Moreover, within the limit of our sample (i.e., SK 1585 being the unique representative of the Paranthropus genus), we found distinct patterns in terms of branching of the middle branch of the MMV between Australopithecus (type I) and Paranthropus (type II). Similarly, Schepers (1946) identified various patterns in the southern African early hominins depending on the dominance or codominance of the anterior or posterior branches. Nonetheless, as previously pointed out in great apes, the branching patterns of the middle meningeal vessels variy substantially within a species (Falk, 1993), and thus could be expected to vary as well within hominin taxa. Interestingly, the middle meningeal vessels supply both the diploic vessels in the cranial vault, and the dura mater (see review in Bruner and Sherkat, 2008). Even if the extent of the diploic channels housing diploic vessels has not been estimated yet in early hominins, a recent study revealed a large proportion of diploic bone in the cranial vault of southern African Australopithecus as compared to the relatively thin diploic layer in Paranthropus (Beaudet et al., 2018b). Together with the typical enlarged occipitalemarginal sinus (Falk and Conroy, 1983; Conroy et al., 1990), those features tend to support different cranial venous outflow in Paranthropus and, possibly, a different metabolic rate (Seymour et al., 2016). This might be consistent with the hypothesis of enlarged sinuses for Paranthropus and reliance of A. africanus on a different vascular system involving the veins (Falk, 1990). 5. Conclusion StW 573 provides a unique opportunity to investigate the neuroanatomy of a Pliocene Australopithecus specimen and offers fresh evidence for discussing the timing and mode of early hominin brain evolution. While the degree of cortical folding and reticulation of the middle meningeal vessels in early hominin adult endocasts was previously known mainly from partial endocasts (Schepers, 1946; Holloway, 1972; Falk, 1979; Holloway, 1983; Saban, 1984; Holloway et al., 2004a,b; Carlson et al., 2011; Falk, 2014), the nearly complete endocast of StW 573 preserves a large extent of the sulcal and vascular imprints. A. Beaudet et al. / Journal of Human Evolution 126 (2019) 112e123 Consistent with its geological age, StW 573 shows a minimum cranial capacity estimate (pending the results of ongoing work in producing a reconstructed endocast) that fits near the lower end of observed Australopithecus variation and exhibits an overall cortical folding pattern that is potentially less derived than in late Pliocene/ early Pleistocene southern African hominins (i.e., protrusion of the inferior frontal convolutions in particular compared to MH 1, caudal position of the presumed lunate sulcus compared to SK 1585 or StW 505). As previously predicted (Balzeau et al., 2012), the endocast of StW 573 suggests that the left occipital petalia is already present in Pliocene hominins. The exceptional preservation of StW 573 allows the identification of very fine neuroanatomical details, such as an arachnoid granulation, which opens new perspectives for our understanding of Australopithecus palaeobiology. Moreover, our description of StW 573, together with our revision of the hominin fossil record in southern Africa, suggests a complex organization of the vascular system involving various branching patterns of the middle meningeal arteries. As the 3.67 Ma StW 573 specimen differs from the late Pliocene/ early Pleistocene southern African hominin specimens in key cerebral areas (e.g., inferior frontal gyri, visual cortex), we may hypothesize that environmental and biological changes that occurred during the Plio-Pleistocene transition were probably selective pressures on the cortical reorganisation of early hominins. More specifically, a significant environmental shift took place during the Plio-Pleistocene transition along with faunal turnovers in mammal communities (Vrba, 1992; Bobe et al., 2002; deMenocal, 2004; Robinson et al., 2017). Accordingly, this transition may have involved substantial changes in primate ecological niches (e.g., Elton, 2001) and group size (e.g., Bettridge and Dunbar, 2012), which may in turn be responsible for critical reorganization in the brain (e.g., Aiello and Wheeler, 1995; Dunbar, 2009; Holloway et al., 2004b). The virtual reconstruction of the skull of StW 573 will be crucial for comparatively and quantitatively assessing the global and local endocranial morphology of this unique specimen and identifying potential early morphological changes within the hominin clade. Acknowledgements We are indebted to E. Gillisen and W. Wendelen (Tervuren), G.  (Pretoria), L. Kgasi, H. Fourie, S. Potze and M. Krüger and E. L'Abbe Tawane (Pretoria), and B. Zipfel (Johannesburg) for having granted access to fossil and comparative material under their care. We also thank L. Bam and J. Hoffman (Pelindaba), M. Dierick (Ghent) for Xray microtomographic acquisitions. We are grateful to the Ditsong National Museum of Natural History and the University of the Witwatersrand for loaning hominin crania in their collections. Permission for fossil access granted by H. Fourie, S. Potze and M. Tawane (Ditsong National Museum of Natural History) and B. Zipfel (Evolutionary Studies Institute). For technical and/or scientific discussion/collaboration we are grateful to: M. Carmen Arriaza  (Pretoria) and J.F. (Johannesburg), J. Dumoncel (Toulouse), A. Oettle Thackeray (Johannesburg). We thank the DST-NRF for sponsoring the Micro-XCT facility at Necsa, and the DST-NRF and Wits University for funding the microfocus X-ray CT facility in the ESI (www. wits.ac.za/microct). The support of the AESOPþ program, the Claude Leon Foundation, the DST-NRF Center of Excellence in Palaeosciences (CoE-Pal), The Palaeontological Scientific Trust (PAST) and the French Institute of South Africa towards this research is hereby acknowledged. Major funding for the Sterkfontein excavations and MicroCT scanning work have been provided by National Research Foundation grants to KK (#82591 and 82611) and to DS (#98808) and by PAST, without whose support this research could not have been able to continue. We thank Andrea 121 Leenen and Robert Blumenschine for their help in securing major corporate funding, including sustained support from Standard Bank and JP Morgan. Opinions expressed and conclusions arrived at are those of the authors and are not necessarily to be attributed to the Center of Excellence in Palaeosciences. We thank S. Elton as well as thre anonymous reviewers for their comments, which contributed to improve the original version of this manuscript. Ethical clearance for the use of extant human crania was obtained from the Main Research Ethics committee of the Faculty of Health Sciences, University of Pretoria in February 2016. 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