The atlas of StW 573 and the late emergence of human-like head mobility and brain metabolism

Functional morphology of the atlas reflects multiple aspects of an organism’s biology. More specifically, its shape indicates patterns of head mobility, while the size of its vascular foramina reflects blood flow to the brain. Anatomy and function of the early hominin atlas, and thus, its evolutionary history, are poorly documented because of a paucity of fossilized material. Meticulous excavation, cleaning and high-resolution micro-CT scanning of the StW 573 (‘Little Foot’) skull has revealed the most complete early hominin atlas yet found, having been cemented by breccia in its displaced and flipped over position on the cranial base anterolateral to the foramen magnum. Description and landmark-free morphometric analyses of the StW 573 atlas, along with other less complete hominin atlases from Sterkfontein (StW 679) and Hadar (AL 333-83), confirm the presence of an arboreal component in the positional repertoire of Australopithecus. Finally, assessment of the cross-sectional areas of the transverse foramina of the atlas and the left carotid canal in StW 573 further suggests there may have been lower metabolic costs for cerebral tissues in this hominin than have been attributed to extant humans and may support the idea that blood perfusion of these tissues increased over the course of hominin evolution.

more expanded posteriorly (i.e., relative to the position of the inferior articular facets) than in any of the extant great apes (Figs. [2][3][4]. The anterior tubercle is more developed in StW 573 than in Homo but, is less prominent than in Gorilla and Pongo, and most closely approximates the condition in Pan (Figs. 2 and 4). The transverse processes in StW 573 and StW 679 are more posteriorly oriented than in Homo relative to the position of the articular facets, and they are positioned similarly to those in Pan, Gorilla and Pongo (Figs. [2][3][4]. Moreover, the transverse processes appear comparatively longer than those in Homo and Pongo, and approximate the relative length of those in Pan and Gorilla. The transverse foramina of StW 573 are situated inferior to the superior articular facets, as in Pan and Pongo, and unlike the more laterally positioned facets in Homo and Gorilla (Figs. [2][3][4]. The retro-glenoid tubercles project cranio-dorsally in StW 573, unlike the retro-glenoid tubercle of StW 679 (this region is not well-preserved in AL 333-83; Figs. 2-5 and Supplementary Fig. S1). In posterior view, the posterior arch in StW 573 is relatively large supero-inferiorly and particularly smooth as compared to the non-human hominoid specimens with no prominent posterior tubercle (Figs. 2 and 4). Both StW 573 and StW 679 lack prominent tubercles for attachment of the transverse ligament, while damage to AL 333-83 prevents comparison (Fig. 2). In this respect the condition exhibited by StW 573 and StW 679 is similar to that of non-human hominoids. There are no ponticulus posticus in StW 573 (Fig. 1), or in extant Homo, while they are present in Pan and Pongo (Fig. 2). However, a ponticulus lateralis is present on the left transverse process in StW 573 (and absent/missing from the right side), and in Pongo on both sides (Fig. 2).

Dimensions.
Dimensions of the atlas of StW 573 are reported in Table 1 and compared with those of Homo, Pan, Gorilla and Pongo (see Methods; Supplementary Fig. S2). Most dimensions of the StW 573 atlas fall within the ranges of those expressed by Pan (i.e., AATh, MDvD, MTrD, M 11, PaTh, STrD, 1L, 1R), and Pongo to a lesser extent (i.e., MTrD, M 11, STrD, 2L), and are smaller or at the lower end of the Homo range. In contrast, the left and right articular facets (2R and 2L) are relatively large (i.e., in the range of Homo or larger than any of the comparative measurements). The estimated diameter along the major axis (1L) of the left articular facets of StW 679 is smaller than in any of the comparative groups except for Pan, while 2L falls within the ranges of those exhibited by Homo, Pan and Gorilla. The areas of the articular facets in StW 573 are within the ranges of all comparative samples with the exception of Pan, while the area of the left facet in StW 679 only fits Pan's estimates. Anatomical ratios for the length and the width of the left articular facet in StW 573 and StW 679 fall within or close to the lower end of the Homo range, while the ratio of the right facet in StW 573 is more comfortably within the range Vertebral foramen and transverse foramina. We measure areas of the vertebral foramen and foramina of the transverse processes in StW 573 and comparative specimens ( Table 2; Supplementary Table S1 and Fig. S2). Absolute areas of the StW 573 vertebral foramen and right and left transverse foramina fall within the range of those for Pan. Within the limits of our sample, standardized areas of the vertebral foramen and transverse foramina (i.e., areas divided by the product of length and width of the atlas; see Methods) in StW 573 fall within the ranges of all extant comparative groups. The absolute area of the left transverse foramen in StW 679 is smaller than those of Homo, Gorilla and Pongo, but is similar to those of Pan and StW 573.
Carotid canal. As comparative samples, we consider extant specimens of humans and chimpanzees as well as southern African specimens of Australopithecus, Paranthropus and early Homo (see Methods and Supplementary  Table S2). To estimate brain perfusion, we measure cross-sectional areas of the carotid canal in the basicranium of StW 573 (left side) and the comparative material (Table 3; Supplementary Table S2 and Fig. S3). Absolute cross-sectional area of the StW 573 left carotid canal falls within the reported range for Australopithecus and is close to Paranthropus. Cross-sectional area of the carotid canal in StW 573, like that of the other fossil hominins included in this study, falls within the range of variation of Pan, but below the range exhibited by extant Homo (with no overlap between the ranges of variation of the two groups, Table 3).
Brain glucose utilization. We estimate brain glucose utilization (BGU) in StW 573 by using the equation provided by Boyer and Harrington 24 (see Methods; Supplementary Table S3). Brain cost in StW 573 is about three times lower than in extant Homo, and it fits more closely within the range observed in extant Pan. Using basal metabolic rates (BMR) provided by Boyer and Harrington 24 for Homo (1557 kcal/day) and Pan (1370 kcal/day), we computed that StW 573 would have used 6.0% or 7.5% respectively of its BMR to support the brain, while Homo and Pan actually use 27.0% and 9.7% of their respective BMR. Even if the lack of direct association between crania and axial skeletons prevents firm conclusions for the other Australopithecus specimens included in our study, our estimations for specimens from Sterkfontein Member 4 and Makapansgat Member 4 concur with the observations for StW 573 and suggest lower brain cost compared with extant humans. When considering the range of variation estimated for StW 573 (Table 4), the minimum and maximum values for BGU are below the range of Homo and more closely approximate the range of Pan as reported in Table S3.   www.nature.com/scientificreports www.nature.com/scientificreports/

Discussion
Our study of the nearly intact StW 573 atlas and of the partial specimen StW 679 offers a unique opportunity to provide new insights into kinematics of head-neck movements and the neurovascular system in Australopithecus. The overall dimensions, shape of the articular facets, position of the transverse processes, and degree of development of the anterior tubercle in StW 573 are similar to Pan, while shape of the vertebral foramen and position of the posterior arch are more similar to Homo. When compared to the two other available fossil hominins, StW 573 shares vertically oriented articular facets with AL 333-83, while orientation of the facets in StW 679 is more similar to Homo. Intriguingly, the posterior arch in StW 573 is larger supero-inferiorly and smoother as compared to the extant hominoids. Moreover, both StW 573 and StW 679 lack prominent tubercles for attachment of the transverse ligament. Again, the area of the vertebral foramen and transverse foramina in both StW 573 and StW 679, and the area of the carotid canal in StW 573, fall within ranges for Pan and are smaller than Homo. Overall, our study suggests substantial similarities in shape and size of StW 573 and StW 679, with those of extant non-human hominoids, particularly Pan.
The musculoskeletal system of the neck is of prime interest for reconstructing kinematics of head-neck movements. Functional signals from the atlas derive from two main sources of evidence, i.e., the articular facets and the muscle and ligament insertions. The superior facets are in contact with the occipital condyles and are responsible for facilitating neck inclination angles and head movement repertoires, specifically flexion-extension in the sagittal plane [6][7][8]11,12 . Together with the presence of cranio-dorsally projecting retro-glenoid tubercles 6,11 , greater surface curvature in the atlanto-occipital articulations has been suggested to increase joint angular excursion in the sagittal plane, thus contributing to a higher range of sagittal motion (rev. in 7 ). An increased range of sagittal motion might confer advantages for arboreal species because of the greater necessity of maintaining the visual field towards substrates arranged more generally in three dimensions 37  www.nature.com/scientificreports www.nature.com/scientificreports/ and suspensory activities both implicate visual inspection for substrates above and below 38 . Moreover, since the atlanto-axial joint plays a role in the axial rotatory movements (rev. in 8 ), the shape of the inferior articular facets might constrain movements in the transversal plane.
In concert with prominent retro-glenoid tubercles, more concave superior articular facets and more vertically oriented inferior articular facets in StW 573 (i.e., non-human hominoid-like) may contribute to an adaptive complex of features that suggests a kinematic signal corroborating arboreal behaviour. Such a complex would be consistent with a study of the vestibular apparatus of StW 573 that demonstrates that the vertical canals, which provide feedback on head movements in sagittal and coronal planes 39 , are more similar to those of extant Pan than to those of Homo 40 . Finally, modern human-like orientation of the superior and inferior articular facets and the absence of prominent retro-glenoid tubercles in StW 679 might reflect greater selection for arboreal activities in earlier Australopithecus compared to this later Sterkfontein Member 4 Australopithecus specimen.   Table 4. Estimation of the range of brain glucose utilization (BGU) in the fossil specimens. ACA: total arterial cross-sectional area (mm 2 ), BGU: brain glucose utilization; s.d.: standard deviation. For each Australopithecus individual besides StW 573, we use the estimate of cross-sectional area of the transverse foramen of StW 679 for a proxy value and combine these with cross-sectional areas of carotid canal measures provided in Table 3.
Scientific RepoRtS | (2020) 10:4285 | https://doi.org/10.1038/s41598-020-60837-2 www.nature.com/scientificreports www.nature.com/scientificreports/ This is highlighted by the more non-human, ape-like configuration exhibited in both StW 573 from Sterkfontein Member 2 and AL 333-83 from Hadar even if the identification of derived features in the 4.2 million-years-old partial atlases of A. anamensis from Assa Issie raises critical questions on evolutionary polarity of such traits 10 . Alternatively, the features of StW 679 may reflect the fact that it belongs to a different species, i.e., A. africanus 30 , although a larger comparative sample would be needed to interpret inter-individual differences.
The posterior vertebral arches of the cervical vertebrae, including the atlas, are connected to the head via nuchal muscles and the nuchal ligament. In StW 573, there are no enlarged areas of insertion for the attachment of muscles and ligaments on the posterior arch, and the surface is relatively smooth as compared to extant hominoids. This observation might support the hypothesis of the absence or reduction of the nuchal ligament in Australopithecus [13][14][15] . The relatively long length and dorsal orientation of the transverse processes in the StW 573 atlas indicate mechanical advantages similarly seen in the suboccipital muscles of Pan and Gorilla and in particular, in the muscles that move the pectoral girdle (i.e., atlanto-clavicularis, levator scapulae 4,7,14 ). Such features might be interpreted as conferring potential selective advantages for arboreal activities 4,41 . The analysis of the scapula and clavicle of StW 573 will add further insight into functional capacity of the pectoral girdle 42 . Because of the role of the long muscles of the neck in controlling and maintaining neck posture and in counteracting the lordosis increment related to the weight of the head, relatively minimal development of the anterior tubercle in StW 573 might indicate a low degree of cervical lordosis as compared to extant humans and later Australopithecus 9,43 . Finally, StW 573 and StW 679 do not present prominent tubercles for attachment of the transverse ligament, which is unlike the condition seen in extant Homo. Gómez-Olivencia et al. 44 suggested that 'small tubercles' might represent the derived condition in Neanderthal, while the prominent tubercles in Homo sapiens illustrate the primitive condition for Homo. However, the relatively high prevalence of small tubercles in Australopithecus might contradict this interpretation.
The area of the vertebral foramen in StW 573 is absolutely and relatively similar in size to Pan. The vertebral foramen notably accommodates the spinal cord and other neurovascular structures, including the internal vertebral venous plexus 23 . According to Falk's hypothesis 45 , the shift to orthograde posture in hominin evolution would be correlated with changes in the vascular system resulting in blood draining preferentially to the vertebral plexus instead of into the internal jugular veins. Following this assumption, we might therefore expect changes in the size of the vertebral plexus in bipedal early hominins as compared to the ancestral pattern. However, StW 573 exhibits neither an absolutely nor relatively larger vertebral foramen than the comparative extant hominoids (nor extant Homo if we consider the relative values in Table 2, but see Meyer and Haeusler 46 for lumbar regions). Nevertheless, shape of the vertebral foramen in the atlas does differ between StW 573 and extant Homo when compared with non-human hominoids, and an antero-posterior elongation and lateral compression of the canal in StW 573 and Homo could be consistent with a rearrangement of neurovascular structures passing through the vertebral canal. However, the rest of the StW 573 vertebral column must be investigated to assess vertebral foramen shape throughout the column and explore potential functional implications.
Variation in the size of transverse foramina and the carotid canal may provide evidence for estimating blood flow to the brain, informing about brain metabolism 24 . Our study reveals relatively smaller cross-sectional areas of transverse foramina and the carotid canal in StW 573 compared with extant Homo. Given that the vertebral arteries are bigger in species with bigger brains and the cranial capacity in StW 573 is similar to the extant chimpanzee values 24,47 , these results may be expected. It is interesting to consider that brain perfusion in extant great apes is suggested to be higher than in Australopithecus 48 . Thus, future analyses would have to investigate selective pressures that could explain the increase of brain perfusion over the last three million years of hominin evolution and if similar evolution could be detected in other primate lineages. In addition, our measurements of the total encephalic arterial flow (i.e., vertebral arteries and internal carotid arteries) and calculation of the brain glucose utilization (BGU) in StW 573 support a low brain metabolism in Australopithecus compared to that in Homo. This could suggest a relatively recent emergence of the human-like metabolic pattern in the hominin lineage 20 , if this pattern in StW 573 is representative of Australopithecus. Low investment in brain metabolism could be tentatively explained by a low quality diet in Pliocene Australopithecus, and more specifically to a low proportion of high-quality animal-based products 21,25 ; but see Sponheimer and Thorpe 49 ). Alternatively, if we consider the hypothesis of less efficient bipedalism in Australopithecus as compared to extant Homo (e.g., 50,51 ), high metabolic energy may have been required by upright walking gaits if these were of an obligate nature in this fossil taxon (but see previous simulations suggesting human-like bipedal performance in Australopithecus 52-54 ). By extension, another possible explanation for the lower brain metabolism in StW 573 could be a higher investment in postural and locomotor activities that may reduce proportions of the metabolic rate that would have been available for allocation to the brain. The fact that body proportions may have not been adapted to maintain appropriate thermoregulation while walking bipedally in Australopithecus might support the latter scenario 55,56 .

Materials.
As comparative materials, we included a cast of the specimen AL 333-83 from the Hadar Formation recovered during the field seasons of 1974-1977 27 and that is currently housed at the Cleveland Museum of Natural History. AL 333-83 represents a partial atlas that preserves most of the left side, including the inferior and superior articular facets and a portion of the posterior arch, but it is missing the left transverse process 27 (Fig. 2). Additionally, for measuring dimensions of the carotid canal, we investigated 10 southern African fossil hominin crania from the sites of Makapansgat (Member 4), Sterkfontein (Member 4) and Swartkrans (Member 1) (Supplementary Tables S1, S2; for further details see Beaudet et al. 40,47,57 ).
Our comparative sample of extant specimens comprised 30 atlases of non-pathological adult Homo, Pan, Gorilla and Pongo sampling males and females, and 15 basicrania (humans and common chimpanzees only) (Supplementary Tables S1, S2).

Scientific RepoRtS |
(2020) 10:4285 | https://doi.org/10.1038/s41598-020-60837-2 www.nature.com/scientificreports www.nature.com/scientificreports/ Virtual reconstruction of the atlas. The skull of StW 573 and the StW 679 atlas were 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 µm and 19 µm, respectively (isotropic voxel size). All comparative specimens investigated in this study were imaged by X-ray tomography using various systems 58,59 (Supplementary Tables S1, S2). Most of the comparative extant specimens have been downloaded from MorphoSource (https://www.morphosource.org/) and from the Digital Morphology Museum KUPRI (http:// dmm.pri.kyoto-u.ac.jp/dmm/WebGallery/dicom/researcherTop.html; Supplementary Tables S1, S2). The cast of AL 333-83 has been rendered by using photogrammetry. Four of the extant human specimens have been imaged by using a Next Engine scanner.
The atlas of StW 573 was virtually extracted from the skull using Avizo v9.0 (Visualization Sciences Group Inc.) combining the watershed tool and manual corrections 60,61 . We reconstructed the missing portions of the left transverse process by mirroring the intact right transverse process (Fig. 2).
Surface-based comparisons. We investigated morphology of the atlas by using a size-independent and landmark-free registration method based on smooth and invertible surface deformation 36,62 . As a pre-processing step, surfaces were automatically aligned in position, orientation and scale with respect to one surface randomly selected using the Iterative Closest Point (ICP) algorithm 63 . From this set of pre-aligned surfaces, an automatic non-rigid registration process was performed on the extant specimens only (i.e., Homo, Pan, Gorilla, and Pongo) via the deformation of a template using the software Deformetrica 36,62 (available online at http://www.deformetrica.org/). A global mean shape and the deformation fields from the global mean shape to each extant specimen were computed from the set of aligned surfaces. Then, the global mean shape was subsequently deformed to StW 573, StW 679 and AL 333-83. Moreover, taxon mean shapes were generated for each extant group and deformed to StW 573.
Because StW 679 and AL 333-83 are incomplete, we performed a second analysis focusing on the left articular facets following the protocol published in Dumoncel et al. 35 and Beaudet et al. 36 . Non-common regions of the atlases (i.e., regions not preserved in StW 679 and AL 333-83) were automatically eliminated from the sample so that all of the specimens contain only comparable information that can be used for the new registration process. This step has been automatically performed by using the deformation of the global mean shape to the extant and fossil specimens of the first analysis (see Fig. 1 in Beaudet et al. 36 ). Subsequently, partial atlases were re-aligned using the ICP algorithm and the same reference as we used for the first analysis. In this manner, we computed a second analysis, including a second global mean shape and corresponding taxon mean shapes, from the set of partial atlases of using extant specimens only. The global mean shape and the taxon mean shapes were subsequently deformed to the partial atlases of StW 573, StW 679 and AL 333-83, providing deformation fields from the global mean shape to each fossil specimen.
Deformation fields integrating local orientation and the amplitude of deformations from the global mean shape to each specimen were statistically analysed by a principal component analysis (PCA) using the package ade4 for R 64 (Fig. 3). The fossil specimens were subsequently projected into shape space. Thus, we computed two separate PCAs, i.e., a first one with the complete atlases (excluding StW 679 and AL 333-83, Fig. 3a) and a second one with the partial atlases (including the two partial fossil specimens and StW 573, Fig. 3b). Cumulative displacement variations are rendered and visualized using a pseudo-colour scale ranging from dark blue (lowest values) to red (highest values; Fig. 3). In addition to colour maps, we used vectors representing local maxima of the displacements. We computed a backtransform morphospace, i.e., representative shapes computed from the deformation fields are superimposed to the PCA to show how shape varies across the morphospace (see Olsen 65 for an example based on 2D landmarks; Fig. 3). Colour scale represents variation from the global mean shape computed for the entire extant sample.
Linear measurements. We used standard measurements reported in Gómez-Olivencia et al. 44 and illustrated in Figure S2A for measuring dimensions of the atlas of StW 573 and those in the comparative extant sample. Because StW 679 and AL 333-83 are incomplete and could not be virtually reconstructed due to the absence of the anterior and posterior vertebral arches, we could not estimate on them any of the metric variables used in this study, except for the size of the left superior articular facet in StW 679. We used the product of the diameter in the major axis (1L/R) and the orthogonal diameter (2L/R) of each facet as an estimation of the area of the articular surface. Moreover, we divided the diameter in the major axis by the orthogonal diameter to assess a ratio between the length and breadth of each facet.
Cross-sectional areas of the vertebral foramen and transverse foramina. We measured crosssectional areas of the vertebral foramen (VFA) and the right (RFA) and left (LFA) transverse foramina ( Supplementary Fig. S2B). In order to do this, we first defined the best-fit plane to the atlas using the module 'Points to Fit' in Avizo v9.0. This plane was moved to pass between superior and inferior articular facets when measuring the VFA, and through the most lateral points of the right and left transverse processes when measuring the RFA and LFA. A 2D section was extracted for both positions. Cross-sectional areas of the vertebral foramen and of the foramina were measured on the respective 2D sections by segmenting the canal and foramina using the module 'Material statistics' in Avizo v9.0. For standardizing measurements, we divided areas by the product of the length (i.e., from the most lateral tip of the right transverse process to the most lateral tip of the left transverse process) and width of the atlas (i.e., the most anterior point of the anterior arch to the most posterior point of the posterior arch). Only RFA could be measured in StW 679, while AL 333-83 preserves neither transverse process (Fig. 2 www.nature.com/scientificreports www.nature.com/scientificreports/ Cross-sectional area of the carotid canal. To avoid any potential distortions related to the presence of bony eminences surrounding the external opening or to torsion of the canal, we measured cross-sectional areas (CSA) of the left carotid canal at mid-distance between the external opening of the canal and the bending of the canal observed when entering the petrous bone ( Supplementary Fig. S3). First, we placed landmarks on the aperture and defined a best-fit plane using the module 'Points to Fit' in Avizo v9.0 ( Supplementary Fig. S3). This position was considered to represent the external opening of the canal (plane C in Supplementary Fig. S3). This plane was moved until reaching the 'elbow' of the canal in the petrous bone, after which the position was noted as representing the 'elbow' of the canal (plane A in Supplementary Fig. S3). Subsequently, the plane was moved and positioned equidistant between the two previously identified positions (i.e., the opening and the 'elbow') before virtually extracting the corresponding 2D plane and subjecting it to further measurements (plane B in Supplementary Fig. S3). Cross-sectional area of the carotid canal was measured by segmenting the canal and using the module 'Material statistics' in Avizo v9.0. As the portion of the carotid canal between the opening and the 'elbow' is partially filled with sediments in SK 847, we extracted a plane parallel to the plane passing through the foramen and that samples the best-preserved part of the bony canal. Accordingly, this measurement could not be directly compared to the rest of the sample, but it could be used as an approximation of the early Homo condition.
Estimation of brain glucose utilization. We estimated brain glucose utilization (BGU) in StW 573 by using the equation provided by Boyer and Harrington 24 , i.e., ln(BGU) = 1.17*ln(ACA)−0.10, where ACA represents twice the sum of the respective averages of transverse foramen cross-sectional area and carotid canal cross-sectional area. Since the endocast of StW 573 is distorted 36 , we could not include the cranial capacity in the equation as recommended by the authors 24 . For extant comparative specimens, we used the mean, minimum and maximum values of CSA, RFA and LFA for computing the mean, minimum and maximum values of ACA (Table 4 and Supplementary Table S3). For fossil comparative specimens, we used cross-sectional area of the transverse foramen preserved in StW 679 as a proxy estimate for Australopithecus specimens. Because of the degree of variation in the measurement of the cross-sectional area of the transverse foramina of extant samples (e.g., up to 7% of differences between the right and left transverse foramina in Homo, Table 2), and because we could not account for inter-individual variation in transverse foramina in the Australopithecus sample, our computations of the ACA in the comparative Australopithecus specimens should be considered as initial provisional estimations. However, we tentatively estimated ranges of variations for fossil specimens by using the standard deviation of RFA and LFA of extant humans and extant chimpanzees and recomputing values for ACA and BGU (Table 4).

Data availability
Permission to access and use 3D surfaces of the StW 573 and StW 679 atlases might be granted by submitting a request to the curator of the Evolutionary Studies Institute (B. Zipfel) via MorphoSource (Sterkfontein project, https://www.morphosource.org/Detail/ProjectDetail/Show/project_id/632).