A shared pattern of midfacial bone modelling in hominids suggests deep evolutionary roots for human facial morphogenesis.

Midfacial morphology varies between hominoids, in particular between great apes and humans for which the face is small and retracted. The underlying developmental processes for these morphological differences are still largely unknown. Here, we investigate the cellular mechanism of maxillary development (bone modelling, BM), and how potential changes in this process may have shaped facial evolution. We analysed cross-sectional developmental series of gibbons, orangutans, gorillas, chimpanzees and present-day humans (n = 183). Individuals were organized into five age groups according to their dental development. To visualize each species's BM pattern and corresponding morphology during ontogeny, maps based on microscopic data were mapped onto species-specific age group average shapes obtained using geometric morphometrics. The amount of bone resorption was quantified and compared between species. Great apes share a highly similar BM pattern, whereas gibbons have a distinctive resorption pattern. This suggests a change in cellular activity on the hominid branch. Humans possess most of the great ape pattern, but bone resorption is high in the canine area from birth on, suggesting a key role of canine reduction in facial evolution. We also observed that humans have high levels of bone resorption during childhood, a feature not shared with other apes.

Author Correction: New fossils from Jebel Irhoud, Morocco and the pan-African origin of Homo sapiens.

In the originally published version of this Letter, the x axis in Fig. 3a should have been: 'PC1: 26%' rather than 'PC1: 46%', and the y axis should have been: 'PC2: 16%' rather than 'PC2: 29%'. We also noticed an error in the numbering of the fossils from Qafzeh: Qafzeh 27 should be removed, and Qafzeh 26 is actually Qafzeh 25, following Tillier (2014)1 and Schuh et al. (2017)2 and personal communication with B. Vandermeersch and M. D. Garralda. The correct enumeration of Qafzeh samples in the 'Mandibular metric data' section of the Methods is therefore: 'Qafzeh (9, 25)' rather than 'Qafzeh (9, 26, 27)'. Owing to the removal of Qafzeh 27, the convex hull of early modern humans changes slightly in Extended Data Fig. 1c. The sample sizes in Extended Data Fig. 1c should have read: Middle Pleistocene archaic Homo n = 19 (instead of 11), Neanderthals n = 40 (instead of 41), early modern humans n = 12 (instead of 7), and recent modern humans n = 46 (instead of 48). In Extended Data Table 2, the mean and standard deviation of corpus height and breadth at mental foramen for early modern humans should have been: x̅ = 33.15, σ = 3.26 for height (rather than x̅ = 34.23, σ = 4.57); and x̅ = 16.25, σ = 1.28 for breadth (rather than x̅ = 16.04, σ = 1.75). Accordingly, n = 12 (rather than n = 13) for both breadth and height. These errors have been corrected in the Letter online (the original Extended Data Fig. 1 is shown in Supplementary Information to this Amendment). These changes do not alter any inferences drawn from the data.

New fossils from Jebel Irhoud, Morocco and the pan-African origin of Homo sapiens.

Fossil evidence points to an African origin of Homo sapiens from a group called either H. heidelbergensis or H. rhodesiensis. However, the exact place and time of emergence of H. sapiens remain obscure because the fossil record is scarce and the chronological age of many key specimens remains uncertain. In particular, it is unclear whether the present day 'modern' morphology rapidly emerged approximately 200 thousand years ago (ka) among earlier representatives of H. sapiens or evolved gradually over the last 400 thousand years. Here we report newly discovered human fossils from Jebel Irhoud, Morocco, and interpret the affinities of the hominins from this site with other archaic and recent human groups. We identified a mosaic of features including facial, mandibular and dental morphology that aligns the Jebel Irhoud material with early or recent anatomically modern humans and more primitive neurocranial and endocranial morphology. In combination with an age of 315 ± 34 thousand years (as determined by thermoluminescence dating), this evidence makes Jebel Irhoud the oldest and richest African Middle Stone Age hominin site that documents early stages of the H. sapiens clade in which key features of modern morphology were established. Furthermore, it shows that the evolutionary processes behind the emergence of H. sapiens involved the whole African continent.

How did modern morphology evolve in the human mandible? The relationship between static adult allometry and mandibular variability in Homo sapiens.

Key to understanding human origins are early Homo sapiens fossils from Jebel Irhoud, as well as from the early Late Pleistocene sites Tabun, Border Cave, Klasies River Mouth, Skhul, and Qafzeh. While their upper facial shape falls within the recent human range of variation, their mandibles display a mosaic morphology. Here we quantify how mandibular shape covaries with mandible size and how static allometry differs between Neanderthals, early H. sapiens, and modern humans from the Upper Paleolithic/Later Stone Age and Holocene (= later H. sapiens). We use 3D (semi)landmark geometric morphometric methods to visualize allometric trends and to explore how gracilization affects the expression of diagnostic shape features. Early H. sapiens were highly variable in mandible size, exhibiting a unique allometric trajectory that explains aspects of their 'archaic' appearance. At the same time, early H. sapiens share a suite of diagnostic features with later H. sapiens that are not related to mandibular sizes, such as an incipient chin and an anteroposteriorly decreasing corpus height. The mandibular morphology, often referred to as 'modern', can partly be explained by gracilization owing to size reduction. Despite distinct static allometric shape changes in each group studied, bicondylar and bigonial breadth represent important structural constraints for the expression of shape features in most Middle to Late Pleistocene hominin mandibles.

Quantifying maxillary development in chimpanzees and humans: An analysis of prognathism and orthognathism at the morphological and microscopic scales.

Facial orientation (projection and degree of prognathism) and form in hominins is highly variable, likely related to evolutionary modifications of the microscopic process of bone modeling (the simultaneous cellular activities of bone formation and resorption) during ontogeny. However, in anteriorly projected faces such as those of early hominins, little is known about the link between bone modeling and facial developmental patterns. Similarly, these aspects have been infrequently investigated in extant great apes. In this study, quantitative methods were applied to a cross-sectional ontogenetic sample of 33 chimpanzees (Pan troglodytes verus) and 59 modern humans (Homo sapiens) to compare the development of maxillary prognathism to orthognathism at both microscopic and macroscopic (or morphological) scales using surface histology and geometric morphometric techniques. Chimpanzees express on average lower amounts of bone resorption than humans on the maxillary periosteum throughout ontogeny; however, the premaxilla is consistently resorbed from early stages on. The presence of bone resorption in the chimpanzee premaxilla, such as that seen in some early hominins, suggests a more ape-like pattern of maxillary bone modeling in these specimens. However, this shows that similarities in bone modeling patterns can lead to variations in shape, suggesting that other aspects of facial growth (such as modifications of rates and timings of development, as well as sutural growth) also played a crucial role in facial evolution.

Reconstructed Homo habilis type OH 7 suggests deep-rooted species diversity in early Homo.

Besides Homo erectus (sensu lato), the eastern African fossil record of early Homo has been interpreted as representing either a single variable species, Homo habilis, or two species. In the latter case, however, there is no consensus over the respective groupings, and which of the two includes OH 7, the 1.8-million-year-old H. habilis holotype. This partial skull and hand from Olduvai Gorge remains pivotal to evaluating the early evolution of the Homo lineage, and by priority names one or other of the two taxa. However, the distorted preservation of the diagnostically important OH 7 mandible has hindered attempts to compare this specimen with other fossils. Here we present a virtual reconstruction of the OH 7 mandible, and compare it to other early Homo fossils. The reconstructed mandible is remarkably primitive, with a long and narrow dental arcade more similar to Australopithecus afarensis than to the derived parabolic arcades of Homo sapiens or H. erectus. We find that this shape variability is not consistent with a single species of early Homo. Importantly, the jaw morphology of OH 7 is incompatible with fossils assigned to Homo rudolfensis and with the A.L. 666-1 Homo maxilla. The latter is morphologically more derived than OH 7 but 500,000 years older, suggesting that the H. habilis lineage originated before 2.3 million years ago, thus marking deep-rooted species diversity in the genus Homo. We also reconstructed the parietal bones of OH 7 and estimated its endocranial volume. At between 729 and 824 ml it is larger than any previously published value, and emphasizes the near-complete overlap in brain size among species of early Homo. Our results clarify the H. habilis hypodigm, but raise questions about its phylogenetic relationships. Differences between species of early Homo appear to be characterized more by gnathic diversity than by differences in brain size, which was highly variable within all taxa.

Levantine cranium from Manot Cave (Israel) foreshadows the first European modern humans.

A key event in human evolution is the expansion of modern humans of African origin across Eurasia between 60 and 40 thousand years (kyr) before present (bp), replacing all other forms of hominins. Owing to the scarcity of human fossils from this period, these ancestors of all present-day non-African modern populations remain largely enigmatic. Here we describe a partial calvaria, recently discovered at Manot Cave (Western Galilee, Israel) and dated to 54.7 ± 5.5 kyr bp (arithmetic mean ± 2 standard deviations) by uranium-thorium dating, that sheds light on this crucial event. The overall shape and discrete morphological features of the Manot 1 calvaria demonstrate that this partial skull is unequivocally modern. It is similar in shape to recent African skulls as well as to European skulls from the Upper Palaeolithic period, but different from most other early anatomically modern humans in the Levant. This suggests that the Manot people could be closely related to the first modern humans who later successfully colonized Europe. Thus, the anatomical features used to support the 'assimilation model' in Europe might not have been inherited from European Neanderthals, but rather from earlier Levantine populations. Moreover, at present, Manot 1 is the only modern human specimen to provide evidence that during the Middle to Upper Palaeolithic interface, both modern humans and Neanderthals contemporaneously inhabited the southern Levant, close in time to the likely interbreeding event with Neanderthals.

A morphometric comparison of the parietal lobe in modern humans and Neanderthals.

The modern human brain and braincase have a characteristic globular shape including parietal and cerebellar bulging. In contrast, Neanderthals, although having similar endocranial volume, displayed more elongated endocrania with flatter parietal and cerebellar regions. Based on endocranial imprints, we compare the parietal lobe morphology of modern humans and Neanderthals, as this brain region is central to several cognitive functions including tool use and visual imaging. In paleoneurology, shape analyses of endocasts are based either on anatomical landmarks that represent endocranial surface features homologous to cortical convolutions (impressions of brain gyri and sulci) or on dense meshes of semilandmarks that capture overall endocranial shape. Previous analyses using the former suggested that modern humans have relatively longer and taller parietal lobes than extinct human species, while the latter emphasized parietal bulging without a significant size difference of parietal regions. In the present study, we combine both anatomical landmarks and surface semilandmarks to investigate the morphological differences of the parietal lobes between modern humans and Neanderthals. Despite limitations by landmark uncertainty, our analyses were able to detect and confirm average different parietal shapes, with modern humans displaying taller and anteroposteriorly extended parietal lobes. We also show mean size differences, with modern humans displaying slightly larger surface areas on the dorsal posterior parietal region, and on a lateral region comprising the supramarginal gyrus, angular gyrus, and intraparietal sulcus. While we observed average differences in the parietal form between the two species, their ranges of distribution overlap, indicating the differences could be a matter of degree. Thus, further analyses on intraspecific variation in parietal lobe morphology within modern human brains should help understand the differences between globular and elongated endocrania. This is crucial because changes to the parietal cortex might affect associative and integrative functions between somatic and visual primary inputs.

Skull reconstruction of the late Miocene ape Rudapithecus hungaricus from Rudabánya, Hungary.

We report on a computer-based reconstruction of a well-preserved ape skull from late Miocene deposits in Rudabánya, Hungary. Based on micro-computed tomographic scans of the original Rudapithecus hungaricus partial cranium RUD 200 and the associated mandible RUD 212 we realign displaced bone fragments, and reconstruct the shape of the upper and lower jaws guided by occlusal fingerprint analysis of dental wear patterns. We apply geometric morphometric methods based on several hundred landmarks and sliding semilandmarks to estimate missing data, and create multiple reconstructions of the specimen. We then compare the reconstructed overall cranial shape, as well as the volume and shape of the endocast, with extant primates. Multiple reconstructions of RUD 200 yield an average endocranial volume of 234 cc (S.D.: 9 cc; range: 221-247 cc). RUD 200 is most similar to African apes in overall cranial shape, but in a statistical analysis of endocranial shape the specimen falls closest to extant hylobatids. Our data suggest that R. hungaricus from the late Miocene in Europe displays aspects of the overall cranial geometry typical of extant African great apes, but it does not show an evolutionary reorganization of the brain evident in Pan, Gorilla, and Pongo.

Ontogenetic changes of diploic channels in modern humans.

OBJECTIVES: The diploic channels are bony passages of veins, running within frontal, parietal, and occipital bones. In this study, we investigate ontogenetic changes of these channels in a sample of nonadult and adult modern humans. MATERIALS AND METHODS: Using computed tomography scans of dried crania, we provide quantitative comparisons of lumen size, branch length, volume, and vascular asymmetries, and correlations with age, cranial size, and bone thickness. RESULTS: The vascular system displays progressive but nonlinear changes throughout ontogeny, becoming even more complex with adulthood. Vascular variables are significantly different in frontal, parietal, and occipital bones for most of the postnatal ontogeny. Diploic channels of the left and right sides are developed similarly. Vascular variables display a nonlinear association with age and cranial size in modern humans. Cranial bone thickness is shown to be a major determinant of lumen size, branch length, and volume. CONCLUSIONS: A previous radiographic survey suggested that diploic channels are more developed in adult modern humans than in nonadults. Recent advances in digital anatomy have been used in this study to investigate this craniovascular structure. The complexity of the channels increases during development, with a noticeable boost in adults. Taking into account the potential metabolic differences and constraints associated with modern human brain size and shape, the vascular differences found might be related to endocranial thermoregulation.

Intraspecific variability in human maxillary bone modeling patterns during ontogeny.

OBJECTIVES: This study compares the ontogenetic bone modeling patterns of the maxilla to the related morphological changes in three human populations to better understand how morphological variability within a species is established during ontogeny at both micro- and macroscopic levels. MATERIALS AND METHODS: The maxillary bones of an ontogenetic sample of 145 subadult and adult individuals from Greenland (Inuit), Western Europe (France, Germany, and Portugal), and South Africa (Khoekhoe and San) were analyzed. Bone formation and resorption were quantified using histological methods to visualize the bone modeling patterns. In parallel, semilandmark geometric morphometric techniques were used on 3D models of the same individuals to capture the morphological changes. Multivariate statistics were applied and shape differences between age groups were visualized through heat maps. RESULTS: The three populations show differences in the degree of shape change acquired during ontogeny, leading to divergences in the developmental trajectories. Only subtle population differences in the bone modeling patterns were found, which were maintained throughout ontogeny. Bone resorption in adults mirrors the pattern found in subadults, but is expressed at lower intensities. DISCUSSION: Our data demonstrate that maxillary morphological differences observed in three geographically distinct human populations are also reflected at the microscopic scale. However, we suggest that these differences are mostly driven by changes in rates and timings of the cellular activities, as only slight discrepancies in the location of bone resorption could be observed. The shared general bone modeling pattern is likely characteristic of all Homo sapiens, and can be observed throughout ontogeny.

Ontogeny of the human maxilla: a study of intra-population variability combining surface bone histology and geometric morphometrics.

Bone modeling is the process by which bone grows in size and models its shape via the cellular activities of the osteoblasts and osteoclasts that respectively form and remove bone. The patterns of expression of these two activities, visible on bone surfaces, are poorly understood during facial ontogeny in Homo sapiens; this is due mainly to small sample sizes and a lack of quantitative data. Furthermore, how microscopic activities are related to the development of morphological features, like the uniquely human-canine fossa, has been rarely explored. We developed novel techniques for quantifying and visualizing variability in bone modeling patterns and applied these methods to the human maxilla to better understand its development at the micro- and macroscopic levels. We used a cross-sectional ontogenetic series of 47 skulls of known calendar age, ranging from birth to 12 years, from a population of European ancestry. Surface histology was employed to record and quantify formation and resorption on the maxilla, and digital maps representing each individual's bone modeling patterns were created. Semilandmark geometric morphometric (GM) methods and multivariate statistics were used to analyze facial growth. Our results demonstrate that surface histology and GM methods give complementary results, and can be used as an integrative approach in ontogenetic studies. The bone modeling patterns specific to our sample are expressed early in ontogeny, and fairly constant through time. Bone resorption varies in the size of its fields, but not in location. Consequently, absence of bone resorption in extinct species with small sample sizes should be interpreted with caution. At the macroscopic level, maxillary growth is predominant in the top half of the bone where bone formation is mostly present. Our results suggest that maxillary growth in humans is highly constrained from early stages in ontogeny, and morphological changes are likely driven by changes in osteoblastic and osteoclastic rates of expression rather than differences in the bone modeling patterns (i.e. changes in location of formation and resorption). Finally, the results of the micro- and macroscopic analyses suggest that the development of the canine fossa results from a combination of bone resorption and bone growth in the surrounding region.

Endostructural morphology in hominoid mandibular third premolars: Discrete traits at the enamel-dentine junction.

The mandibular third premolar (P3) exhibits substantial differences in size and shape among hominoid taxa, and displays a number of discrete traits that have proven to be useful in studies of hominin taxonomy and phylogeny. Discrete traits at the enamel-dentine junction (EDJ) can be accurately assessed on moderately worn specimens, and often appear sharper than at the outer-enamel surface (OES). Here we use microtomography to image the P3 EDJ of a broad sample of extant apes, extinct hominins and modern humans (n = 100). We present typologies for three important premolar discrete traits at the EDJ (transverse crest, marginal ridge and buccal grooves), and score trait frequencies within our sample. We find that the transverse crest is variable in extant apes, while the majority of hominins display a transverse crest which runs directly between the two major premolar cusps. Some Neanderthals display a unique form in which the transverse crest fails to reach the protoconid. We find that mesial marginal ridge discontinuity is common in Australopithecus anamensis and Australopithecus afarensis while continuous marginal ridges largely characterize Australopithecus africanus and Paranthropus. Interrupted mesial and distal marginal ridges are again seen in Homo sapiens and Neanderthals. Premolar buccal grooves, previously identified at the OES as important for hominin systematics, are again found to show a number of taxon-specific patterns at the EDJ, including a clear difference between Australopithecus and Paranthropus specimens. However, their appearance may be dependent on the morphology of other parts of the crown such as the protoconid crest, and the presence of accessory dentine horns. Finally, we discuss rare variations in the form of dentine horns that underlie premolar cusps, and their potential homology to similar morphologies in other tooth positions.

Endostructural morphology in hominoid mandibular third premolars: Geometric morphometric analysis of dentine crown shape.

In apes, the mandibular third premolar (P3) is adapted for a role in honing the large upper canine. The role of honing was lost early in hominin evolution, releasing the tooth from this functional constraint and allowing it to respond to subsequent changes in masticatory demands. This led to substantial morphological changes, and as such the P3 has featured prominently in systematic analyses of the hominin clade. The application of microtomography has also demonstrated that examination of the enamel-dentine junction (EDJ) increases the taxonomic value of variations in crown morphology. Here we use geometric morphometric techniques to analyze the shape of the P3 EDJ in a broad sample of fossil hominins, modern humans, and extant apes (n = 111). We test the utility of P3 EDJ shape for distinguishing among hominoids, address the affinities of a number of hominin specimens of uncertain taxonomic attribution, and characterize the changes in P3 EDJ morphology across our sample, with particular reference to features relating to canine honing and premolar 'molarization'. We find that the morphology of the P3 EDJ is useful in taxonomic identification of individual specimens, with a classification accuracy of up to 88%. The P3 EDJ of canine-honing apes displays a tall protoconid, little metaconid development, and an asymmetrical crown shape. Plio-Pleistocene hominin taxa display derived masticatory adaptations at the EDJ, such as the molarized premolars of Australopithecus africanus and Paranthropus, which have well-developed marginal ridges, an enlarged talonid, and a large metaconid. Modern humans and Neanderthals display a tall dentine body and reduced metaconid development, a morphology shared with premolars from Mauer and the Cave of Hearths. Homo naledi displays a P3 EDJ morphology that is unique among our sample; it is quite unlike Middle Pleistocene and recent Homo samples and most closely resembles Australopithecus, Paranthropus and early Homo specimens.

Morphological trends in arcade shape and size in Middle Pleistocene Homo.

OBJECTIVES: Middle Pleistocene fossil hominins, often summarized as Homo heidelbergensis sensu lato, are difficult to interpret due to a fragmentary fossil record and ambiguous combinations of primitive and derived characters. Here, we focus on one aspect of facial shape and analyze shape variation of the dental arcades of these fossils together with other Homo individuals. MATERIALS AND METHODS: Three-dimensional landmark data were collected on computed tomographic scans and surface scans of Middle Pleistocene fossil hominins (n = 8), Homo erectus s.l. (n = 4), Homo antecessor (n = 1), Homo neanderthalensis (n = 13), recent (n = 52) and fossil (n = 19) Homo sapiens. To increase sample size, we used multiple multivariate regression to reconstruct complementary arches for isolated mandibles, and explored size and shape differences among maxillary arcades. RESULTS: The shape of the dental arcade in H. erectus s.l. and H. antecessor differs markedly from both Neanderthals and H. sapiens. The latter two show subtle but consistent differences in arcade length and width. Shape variation among Middle Pleistocene fossil hominins does not exceed the amount of variation of other species, but includes individuals with more primitive and more derived morphology, all more similar to Neanderthals and H. sapiens than to H. erectus s.l. DISCUSSION: Although our results cannot reject the hypothesis that the Middle Pleistocene fossil hominins belong to a single species, their shape variation comprises a more primitive morph that represents a likely candidate for the shape of the last common ancestor of Neanderthals and H. sapiens, and a more derived morph resembling Neanderthals. The arcade shape difference between Neanderthals and H. sapiens might be related to different ways to withstand mechanical stress.

Structural effects of variation in the human clavicle.

OBJECTIVES: Purported evolutionary shifts in shoulder structure have been linked to changes in hominin behavior and adaptation. Researchers use clavicle morphology to infer these shifts. However, there is a lack of empirical data underlying such predictive relationships. This study investigates how clavicle morphology affects articulated shoulder girdle and upper thorax configuration in humans. MATERIALS AND METHODS: Landmarks and scalar measurements on the clavicle, scapula, and ribs 1-3 were collected from three-dimensional computed tomographic scans of living humans. Covariation between disarticulated and articulated morphology was assessed using partial least squares and regression analyses. RESULTS: We found support for hypotheses linking combined dimensions of the clavicle, ribs, and scapula to resting protraction. Individuals with relatively short clavicles tend to exhibit protracted and elevated resting positions of the scapula. It is more difficult to predict superoinferior configuration, which is only minimally affected by clavicle curvature. Instead, the superoinferior position of the scapula on the thorax is governed equally by clavicle orientation and rib declination. Shoulder breadth is determined primarily by clavicle length, but orientation has a comparable effect. Therefore, reliable reconstructions of shoulder breadth can be established using clavicle length, together with consideration of orientation. Relationships between clavicle and thorax morphology are weaker. DISCUSSION: Understanding the determinants of variation in human shoulder structure informs interpretation of skeletal remains. Our investigations describe how important aspects of shoulder structure can be inferred from disarticulated clavicles and we provide the attendant predictive equations. Future work on interspecific variation will improve skeletal reconstruction for more ancient hominins.

Morphology and function of Neandertal and modern human ear ossicles.

The diminutive middle ear ossicles (malleus, incus, stapes) housed in the tympanic cavity of the temporal bone play an important role in audition. The few known ossicles of Neandertals are distinctly different from those of anatomically modern humans (AMHs), despite the close relationship between both human species. Although not mutually exclusive, these differences may affect hearing capacity or could reflect covariation with the surrounding temporal bone. Until now, detailed comparisons were hampered by the small sample of Neandertal ossicles and the unavailability of methods combining analyses of ossicles with surrounding structures. Here, we present an analysis of the largest sample of Neandertal ossicles to date, including many previously unknown specimens, covering a wide geographic and temporal range. Microcomputed tomography scans and 3D geometric morphometrics were used to quantify shape and functional properties of the ossicles and the tympanic cavity and make comparisons with recent and extinct AMHs as well as African apes. We find striking morphological differences between ossicles of AMHs and Neandertals. Ossicles of both Neandertals and AMHs appear derived compared with the inferred ancestral morphology, albeit in different ways. Brain size increase evolved separately in AMHs and Neandertals, leading to differences in the tympanic cavity and, consequently, the shape and spatial configuration of the ossicles. Despite these different evolutionary trajectories, functional properties of the middle ear of AMHs and Neandertals are largely similar. The relevance of these functionally equivalent solutions is likely to conserve a similar auditory sensitivity level inherited from their last common ancestor.

Using the covariation of extant hominoid upper and lower jaws to predict dental arcades of extinct hominins.

Upper and lower jaws are well represented in the fossil record of mammals and are frequently used to diagnose species. Some hominin species are only known by either their maxillary or mandibular morphology, and in this study, we explore the possibility of predicting their complementary dental arcade shape to aid the recognition of conspecific specimens in the fossil record. To this end, we apply multiple multivariate regression to analyze 3D landmark coordinates collected on associated upper and lower dental arcades of extant Homo, Pan, Gorilla, Pongo, and Hylobates. We first study the extant patterns of variation in dental arcade shape and quantify how accurate predictions of complementary arcades are. Then we explore applications of this extant framework for interpreting the fossil record based on two fossil hominin specimens with associated upper and lower jaws, KNM-WT 15000 (Homo erectus sensu lato) and Sts 52 (Australopithecus africanus), as well as two non-associated specimens of Paranthropus boisei, the maxilla of OH 5 and the Peninj mandible. We find that the shape differences between the predictions and the original fossil specimens are in the range of variation within genera or species and therefore are consistent with their known affinity. Our approach can provide a reference against which intraspecific variation of extinct species can be assessed. We show that our method predicts arcade shapes reliably even if the target shape is not represented in the reference sample. We find that in extant hominoids, the amount of within-taxon variation in dental arcade shape often overlaps with the amount of between-taxon shape variation. This implies that whereas a large difference in dental arcade shape between two individuals typically suggests that they belong to different species or even genera, a small shape difference does not necessarily imply conspecificity.

Reconstruction, endocranial form and taxonomic affinity of the early Homo calvaria KNM-ER 42700.

When first described, the small calvaria KNM-ER 42700 from Ileret, Kenya, was considered a late juvenile or young adult and assigned to Homo erectus. However, this species attribution has subsequently been challenged because the specimen's neurocranial shape differs substantially from that of H. erectus adults. Here, (1) we describe the postmortem damage and deformation that could have influenced previous shape analyses, (2) present digital reconstructions based on computed tomographic scans correcting for these taphonomic defects, and (3) analyze the reconstructed endocranial shape and form, considering both static allometry among adults and ontogenetic allometry. To this end, we use geometric morphometrics to analyze the shape of digital endocasts based on landmarks and semilandmarks. Corroborating previous studies of the external surface, we find that the endocranial shape of KNM-ER 42700 falls outside the known adult variation of H. erectus. With an endocranial volume estimate between 721 and 744 ml, size cannot explain its atypical endocranial shape when static allometry within H. erectus is considered. However, the analysis of ontogenetic allometry suggests that it may be a H. erectus individual that is younger than previously thought and had not yet reached adult endocranial shape. Future work should therefore comprehensively review all cranial indicators of its developmental age, including closure of the spheno-occipital synchondrosis. An alternative hypothesis is that KNM-ER 42700 represents an as yet unidentified species of early Homo. Importantly, KNM-ER 42700 should not be included in the adult hypodigm of H. erectus.

Hominoid arcade shape: Pattern and magnitude of covariation.

The shape of the dental arcade and canine size distinguish extant humans from all apes. Humans are characterized by a parabolic arcade with short postcanine tooth rows and small canines, whereas apes have long, U-shaped arcades with large canines. The evolutionary and biomechanical mechanisms underlying arcade shape differences between and within groups are not well understood. It is unclear, for example, whether evolutionary changes in the covariation among modules comprising the upper and lower jaws are the cause and/or consequence of different arcade shapes. Here we use 3D geometric morphometric methods to explore to what extent the morphological differences in arcade shape between living hominoids are related to differences in covariation of upper and lower jaws, and the premaxilla and the maxilla. We show that all extant hominoids follow a very similar covariation pattern between upper and lower dental arcades, as well as between the premaxilla and the maxilla. We find comparably high magnitudes of covariation between the premaxilla and the maxilla in all groups. Between the upper and lower jaws, levels of covariation are similar in apes (Pan, Gorilla, Pongo, and Hylobates), but overall lower in extant humans. Our results demonstrate an independence of the pattern of arcade shape covariation from dental spatial arrangements. Importantly, we show that a shared hominoid pattern of covariation between premaxilla and maxilla together with the covariation of upper and lower jaw is consistent with major evolutionary arcade shape changes in hominoids. We suggest that with the reduction of canine and diastema size in hominins, the incisors move posteriorly and the tooth row becomes more parabolic. Our study provides a framework for addressing questions about fossil hominin dentognathic diversity, including inter- and intraspecific variation and associations of upper and lower jaw morphology.