Neurocranial shape variation among Tarascan populations: Evidence for varying degrees in artificially modified crania in pre-Hispanic West Mexico (1200-1400 AD).

INTRODUCTION: Along the Mesoamerican western margin, the Zacapu basin has yielded a large number of human remains demonstrating usage of artificial cranial modification (ACM). However, at the onset of the Middle Postclassic (1200-1400 AD) only few individuals still exhibit clear signs of ACM. Some authors have suggested that, rather than disappearing entirely, ACM may have become less visible anatomically, making it difficult to identify based on simple visual analyses. Here, we used 3D geometric morphometric methods to investigate the extent to which ACM persisted during the Postclassic in this region. MATERIALS AND METHODS: We measured the 3D vault's shape changes in a sample of surface-scanned human crania: 55 individuals from the Postclassic Zacapu basin and a control group of 31 individuals from a Huichol Mexican Indian sample and a French medieval series from La Granède. We used a principal component analysis to explore the shape variation within the sample and employed the neighbor joining method to identify morphological groups. Finally, we quantified each individual's asymmetry. RESULTS: We identified three groups displaying shape features diverging from those of the control group. The first group is characterized by marked fronto-obelionic ACM, whereas the other two show mild forms of ACM. The individuals in all three groups display moderate to high degrees of asymmetry compared to the control group. DISCUSSION: The marked fronto-obelionic modification is clear evidence of a specific ACM technique. The two types of mild ACM most likely result from different techniques but their moderate degree of modification brings into question the intentions behind their production.

Early dispersal of modern humans in Europe and implications for Neanderthal behaviour.

The appearance of anatomically modern humans in Europe and the nature of the transition from the Middle to Upper Palaeolithic are matters of intense debate. Most researchers accept that before the arrival of anatomically modern humans, Neanderthals had adopted several 'transitional' technocomplexes. Two of these, the Uluzzian of southern Europe and the Châtelperronian of western Europe, are key to current interpretations regarding the timing of arrival of anatomically modern humans in the region and their potential interaction with Neanderthal populations. They are also central to current debates regarding the cognitive abilities of Neanderthals and the reasons behind their extinction. However, the actual fossil evidence associated with these assemblages is scant and fragmentary, and recent work has questioned the attribution of the Châtelperronian to Neanderthals on the basis of taphonomic mixing and lithic analysis. Here we reanalyse the deciduous molars from the Grotta del Cavallo (southern Italy), associated with the Uluzzian and originally classified as Neanderthal. Using two independent morphometric methods based on microtomographic data, we show that the Cavallo specimens can be attributed to anatomically modern humans. The secure context of the teeth provides crucial evidence that the makers of the Uluzzian technocomplex were therefore not Neanderthals. In addition, new chronometric data for the Uluzzian layers of Grotta del Cavallo obtained from associated shell beads and included within a Bayesian age model show that the teeth must date to ~45,000-43,000 calendar years before present. The Cavallo human remains are therefore the oldest known European anatomically modern humans, confirming a rapid dispersal of modern humans across the continent before the Aurignacian and the disappearance of Neanderthals.

Ontogeny of modern human longitudinal body and transverse shoulder proportions.

OBJECTIVES: Whereas variation of modern human adult body size and shape has been widely studied in the context of ecogeographical clines, little is known about the differential growth patterns of transverse and longitudinal dimensions among human populations. Our study explored the ontogenetic variation of those body proportions in modern humans. METHODS: We compared results from four different approaches to study cross-sectional skeletal samples of Africans (n = 43), Amerindians (n = 69) and Europeans (n = 40) from 0 to 14 years of age. Clavicle, humerus, and femur intermetaphyseal lengths, and femoral distal metaphyseal breadth, were measured. Average ontogenetic trajectories were computed in order to compare the growth patterns of the three groups. RESULTS: Our findings demonstrated that the three geographical groups shared similar absolute and relative patterns of change with age for the four dimensions considered. Although interpopulation differences existed in transverse to longitudinal as well as in interlimb proportions, those differences did not seem to remain constant throughout ontogeny, similar to what has been shown for intralimb proportions. Growth rates of transverse shoulder proportions differed between populations from different regions after 10 years, whereas those for longitudinal proportions were very similar. CONCLUSIONS: The ontogeny of transverse shoulder proportions is more complex than what is observed for bi-iliac breadth, suggesting that transverse shoulder to limb proportions are not solely influenced by ecogeographical conditions. Our analysis demonstrates that methodologies that incorporate critical dimensions of body form could shed new light on human adaptation in both paleontological and neontological contexts.

Infant growth patterns of the mandible in modern humans: a closer exploration of the developmental interactions between the symphyseal bone, the teeth, and the suprahyoid and tongue muscle insertion sites.

The ontogenetic development of the mental region still poses a number of unresolved questions in human growth, development and phylogeny. In our study we examine the hypotheses of DuBrul & Sicher (1954) (The Adaptive Chin. Springfield, IL: Charles) and Enlow (1990) (Facial Growth, 3rd edn. Philadelphia, PA: Saunders) to explain the presence of a prominent mental region in anatomically modern humans. In particular, we test whether the prominence of the mental region and the positioning of the teeth are both correlated with the developmental relocation of the tongue and the suprahyoid muscles inserting at the lingual side of the symphysis. Furthermore, we test whether the development of the mental region is associated with the development of the back of the vocal tract. Using geometric morphometric methods, we measured the 3D mandibular and tooth surfaces in a cross-sectional sample of 36 CT-scanned living humans, incorporating the positions of the tongue and the geniohyoid and digastric muscle insertions. The specimens' ages range from birth to the complete emergence of the deciduous dentition. We used multivariate regression and two-block partial least squares (PLS) analysis to study the covariation among the mental region, the muscle insertions, and the teeth both across and within age stages. In order to confirm our results from the 3D cross-sectional sample, and to relate them to facial growth and the position of the cervical column and the hyoid bone, we used 46 lateral radiographs of eight children from the longitudinal Denver Growth Study. The 3D analysis demonstrates that the lingual side of the lower border of the symphysis develops downwards and forwards. These shape changes are significantly correlated with the relocation of muscle insertion sites and also with the vertical reorientation of the anterior teeth prior to emergence. The 2D analysis confirms the idea that as the mental region prominence develops, the space of the laryngopharynx becomes restricted due to upper mid-face retraction and the acquisition of upright body posture. In agreement with the hypotheses of DuBrul & Sicher (1954) and Enlow (1990), our results suggest that the presence of a prominent mental region responds to the space restriction at the back of the vocal tract, and to the packaging of the tongue and suprahyoid muscles in order to preserve the functionality of the laryngopharynx during respiration, feeding and speech.

Comparison of dental measurement systems for taxonomic assignment of Neanderthal and modern human lower second deciduous molars.

Traditional morphometric approaches for taxonomic assignment of Neanderthal and modern human dental remains are mainly characterized by caliper measurements of tooth crowns. Several studies have recently described differences in dental tissue proportions and enamel thickness between Neanderthal and modern human teeth. At least for the lower second deciduous molar (dm(2)), a three-dimensional lateral relative enamel thickness index has been proposed for separating the two taxa. This index has the advantage over other measurements of being applicable to worn teeth because it ignores the occlusal aspect of the crown. Nevertheless, a comparative evaluation of traditional crown dimensions and lateral dental tissue proportion measurements for taxonomic assignment of Neanderthal and modern human dm(2)s has not yet been performed. In this study, we compare various parameters gathered from the lateral aspects of the crown. These parameters include crown diameters, height of the lateral wall of the crown (lateral crown height = LCH), lateral enamel thickness, and dentine volume of the lateral wall, including the volume of the coronal pulp chamber (lateral dentine plus pulp volume = LDPV), in a 3D digital sample of Neanderthal and modern human dm(2)s to evaluate their utility in separating the two taxa. The LDPV and the LCH allow us to discriminate between Neanderthals and modern humans with 88.5% and 92.3% accuracy, respectively. Though our results confirm that Neanderthal dm(2)s have lower relative enamel thickness (RET) index compared with modern humans (p = 0.005), only 70% of the specimens were correctly classified on the basis of the RET index. We also emphasize that results of the lateral enamel thickness method depend on the magnitude of the interproximal wear. Accordingly, we suggest using the LCH or the LDPV to discriminate between Neanderthal and modern human dm(2)s. These parameters are more independent of interproximal wear and loss of lateral enamel.

Sexual dimorphism of the human mandible and its association with dental development.

The present study investigates whether the human mandible is sexually dimorphic during early postnatal development and whether early dimorphic features persist during subsequent ontogeny. We also examine whether mandibular dimorphism is linked to dimorphism of dental development. Dense CT-derived mandibular meshes of 84 females and 75 males, ranging from birth to adulthood, were analyzed using geometric morphometric methods. On the basis of the specimen's chronological ages and mineralization stages of the deciduous and permanent teeth, we compute dental age as proxy for dental development by the additive conjoint measurement method. By birth, males have, on average, more advanced age-specific shapes than females. However, sex differences decrease quickly as females catch up via a different association between shape and size. This leads to an almost complete reduction of sexual dimorphism between the ages of 4 and 14. From puberty to adulthood, males are characterized by allometric shape changes while the shape of the female mandible continues to change even after size has ceased to increase. Dimorphism of dental maturation becomes visible only at puberty. Sexual dimorphism, concentrated at the ramus and the mental region during the earliest ontogenetic stages and again at adulthood, is not associated with the development of the teeth. At puberty there is a simultaneous peak in size increase, shape development, and dental maturation likely controlled by the surge of sex hormones with a dimorphic onset age. We argue that the infant and adult dimorphism of the mental region may be associated with the development of supralaryngeal structures.

Comparison of dental measurement systems for taxonomic assignment of first molars.

Morphometrics of the molar crown is based traditionally on diameter measurements but is nowadays more often based on 2D image analysis of crown outlines. An alternative approach involves measurements at the level of the cervical line. We compare the information content of the two options in a three-dimensional (3D) digital sample of lower and upper first molars (M(1) and M(1) ) of modern human and Neanderthal teeth. The cervical outline for each tooth was created by digitizing the cervical line and then sectioning the tooth with a best fit plane. The crown outline was projected onto this same plane. The curves were analyzed by direct extraction of diameters, diagonals, and area and also by principal component analysis either of the residuals obtained by regressing out these measurements from the radii (shape information) or directly by the radii (size and shape information). For M(1) , the crown and cervical outline radii allow us to discriminate between Neanderthals and modern humans with 90% and 95% accuracy, respectively. Fairly good discrimination between the groups (80-82.5%) was also obtained using cervical measurements. With respect to M(1) , general overlap of the two groups was obtained by both crown and cervical measurements; however, the two taxa were differentiable by crown outline residuals (90-97%). Accordingly, while crown diameters or crown radii should be used for taxonomic analysis of unworn or slightly worn M(1) s, the crown outline, after regressing out size information, could be promising for taxonomic assignment of lower M1s.

Fetal and infant growth patterns of the mandibular symphysis in modern humans and chimpanzees (Pan troglodytes).

Comparison of the early development of the mandibular symphysis between primates and modern humans is of particular interest in human palaeontology. Using geometric morphometric methods, we explored and compared the ontogenetic shape changes of 14 chimpanzee mandibles (Pan troglodytes) against 66 human CT-scanned mandibles over the age range from fetal life to the complete emergence of the deciduous dentition in a visualization incorporating the deciduous tooth arrangement. The results reveal that the symphysis is anteriorly inclined in the youngest chimpanzee fetuses but develops an increasingly vertical orientation up until birth. At the same time, the anterior teeth reorient before a vertical emergence, and a symphyseal tuber appears on the labial side. When the deciduous canine emerges, the symphysis inclines anteriorly again, exhibiting the adult characteristic slope. These two phases are characterized by a repositioning of the simian shelf. Unlike chimpanzees, the human symphysis remains vertical throughout fetal development. However, the combination of morphological changes observed in chimpanzee fetuses is similar to that of modern humans after birth, as the mental region projects forward. By elongating the alveolar process, the inclination of the chimpanzee symphysis could be a key event for emergence of the deciduous canine, as space is lacking at the alveolar ridge in a vertical symphysis once the deciduous incisors and molars have emerged. The repositioning of the simian shelf suggests that the suprahyoid muscles have a significant influence on the anterior growth of the symphysis. The anteroposterior positioning of the basal symphysis in both species may be related to hyoid bone position during ontogeny.

Three-Dimensional Geometric Morphometric Analysis of Fossil Canid Mandibles and Skulls.

Much of the fossil record for dogs consists of mandibles. However, can fossil canid mandibles be reliably identified as dogs or wolves? 3D geometric morphometric analysis correctly classifies 99.5% of the modern dog and wolf mandibles. However, only 4 of 26 Ust'-Polui fossil mandibles, a Russian Arctic site occupied from 250BCE to 150CE, were identified as dogs and none of the 20 Ivolgin mandibles, an Iron Age site in southern Russia, were identified as dogs. Three of the Ust'-Polui mandibles and 8 of the Ivolgin mandibles were identified as wolves. In contrast, all 12 Ivolgin skulls and 5 Ust'-Polui skulls were clearly identified as dogs. Only the classification of the UP6571 skull as a dog (Dog Posterior Probability = 1.0) was not supported by the typical probability. Other evidence indicates these canids were domesticated: they were located within human dwellings, remains at both sites have butchery marks indicating that they were consumed, and isotope analysis of canid and human remains from Ust'-Polui demonstrate that both were consuming freshwater protein; indicating that the humans were feeding the canids. Our results demonstrate that the mandible may not evolve as rapidly as the cranium and the mandible is not reliable for identifying early dog fossils.

Internal Tooth Structure and Burial Practices: Insights into the Neolithic Necropolis of Gurgy (France, 5100-4000 cal. BC).

Variations in the dental crown form are widely studied to interpret evolutionary changes in primates as well as to assess affinities among human archeological populations. Compared to external metrics of dental crown size and shape, variables including the internal structures such as enamel thickness, tissue proportions, and the three-dimensional shape of enamel-dentin junction (EDJ), have been described as powerful measurements to study taxonomy, phylogenetic relationships, dietary, and/or developmental patterns. In addition to providing good estimate of phenotypic distances within/across archeological samples, these internal tooth variables may help to understand phylogenetic, functional, and developmental underlying causes of variation. In this study, a high resolution microtomographic-based record of upper permanent second molars from 20 Neolithic individuals of the necropolis of Gurgy (France) was applied to evaluate the intrasite phenotypic variation in crown tissue proportions, thickness and distribution of enamel, and EDJ shape. The study aims to compare interindividual dental variations with burial practices and chronocultural parameters, and suggest underlying causes of these dental variations. From the non-invasive characterization of internal tooth structure, differences have been found between individuals buried in pits with alcove and those buried in pits with container and pits with wattling. Additionally, individuals from early and recent phases of the necropolis have been distinguished from those of the principal phase from their crown tissue proportions and EDJ shape. The results suggest that the internal tooth structure may be a reliable proxy to track groups sharing similar chronocultural and burial practices. In particular, from the EDJ shape analysis, individuals buried in an alcove shared a reduction of the distolingual dentin horn tip (corresponding to the hypocone). Environmental, developmental and/or functional underlying causes might be suggested for the origin of phenotypic differences shared by these individuals buried in alcoves.

3D morphometric analysis of fossil canid skulls contradicts the suggested domestication of dogs during the late Paleolithic.

Whether dogs were domesticated during the Pleistocene, when humans were hunter-gatherers, or during the Neolithic, when humans began to form permanent settlements and engage in agriculture, remains controversial. Recently discovered Paleolithic fossil skulls, Goyet dated 31,680 +/- 250 YBP and Eliseevichi MAE 447/5298 dated 13,905 +/- 55 YBP, were previously identified as dogs. However, new genetic studies contradict the identification of these specimens as dogs, questioning the validity of traditional measurements used to morphologically identify canid fossil skulls. We employ 3D geometric morphometric analyses to compare the cranial morphology of Goyet and Eliseevichi MAE to that of ancient and modern dogs and wolves. We demonstrate that these Paleolithic canids are definitively wolves and not dogs. Compared to mesaticephalic (wolf-like breeds) dog skulls, Goyet and Eliseevichi MAE, do not have cranial flexion and the dorsal surface of their muzzles has no concavity near the orbits. Morphologically, these early fossil canids resemble wolves, and thus no longer support the establishment of dog domestication in the Paleolithic.

Virtual reconstruction of very large skull defects featuring partly and completely missing midsagittal planes.

Despite the development of computer-based methods, cranial reconstruction of very large skull defects remains a challenge particularly if the damage affects the midsagittal region hampering the usage of mirror imaging techniques. This pilot study aims to deliver a new method that goes beyond mirror imaging, giving the possibility to reconstruct crania characterized by large missing areas, which might be useful in the fields of paleoanthropology, bioarcheology, and forensics. We test the accuracy of digital reconstructions in cases where two-thirds or more of a human cranium were missing. A three-dimensional (3D) virtual model of a human cranium was virtually damaged twice to compare two destruction-reconstruction scenarios. In the first case, a small fraction of the midsagittal region was still preserved, allowing the application of mirror imaging techniques. In the second case, the damage affected the complete midsagittal region, which demands a new approach to estimate the position of the midsagittal plane. Reconstructions were carried out using CT scans from a sample of modern humans (12 males and 13 females), to which 3D digital modeling techniques and geometric morphometric methods were applied. As expected, the second simulation showed a larger variability than the first one, which underlines the fact that the individual midsagittal plane is of course preferable in order to minimize the reconstruction error. However, in both simulations the Procrustes mean shape was an effective reference for the reconstruction of the entire cranium, producing models that showed a remarkably low error of about 3 mm, given the extent of missing data.

Short faces, big tongues: developmental origin of the human chin.

During the course of human evolution, the retraction of the face underneath the braincase, and closer to the cervical column, has reduced the horizontal dimension of the vocal tract. By contrast, the relative size of the tongue has not been reduced, implying a rearrangement of the space at the back of the vocal tract to allow breathing and swallowing. This may have left a morphological signature such as a chin (mental prominence) that can potentially be interpreted in Homo. Long considered an autopomorphic trait of Homo sapiens, various extinct hominins show different forms of mental prominence. These features may be the evolutionary by-product of equivalent developmental constraints correlated with an enlarged tongue. In order to investigate developmental mechanisms related to this hypothesis, we compare modern 34 human infants against 8 chimpanzee fetuses, whom development of the mandibular symphysis passes through similar stages. The study sets out to test that the shared ontogenetic shape changes of the symphysis observed in both species are driven by the same factor--space restriction at the back of the vocal tract and the associated arrangement of the tongue and hyoid bone. We apply geometric morphometric methods to extensive three-dimensional anatomical landmarks and semilandmarks configuration, capturing the geometry of the cervico-craniofacial complex including the hyoid bone, tongue muscle and the mandible. We demonstrate that in both species, the forward displacement of the mental region derives from the arrangement of the tongue and hyoid bone, in order to cope with the relative horizontal narrowing of the oral cavity. Because humans and chimpanzees share this pattern of developmental integration, the different forms of mental prominence seen in some extinct hominids likely originate from equivalent ontogenetic constraints. Variations in this process could account for similar morphologies.

Differential growth and development of the upper and lower human thorax.

The difficulties in quantifying the 3D form and spatial relationships of the skeletal components of the ribcage present a barrier to studies of the growth of the thoracic skeleton. Thus, most studies to date have relied on traditional measurements such as distances and indices from single or few ribs. It is currently known that adult-like thoracic shape is achieved early, by the end of the second postnatal year, with the circular cross-section of the newborn thorax transforming into the ovoid shape of adults; and that the ribs become inclined such that their anterior borders come to lie inferior to their posterior. Here we present a study that revisits growth changes using geometric morphometrics applied to extensive landmark data taken from the ribcage. We digitized 402 (semi) landmarks on 3D reconstructions to assess growth changes in 27 computed tomography-scanned modern humans representing newborns to adults of both sexes. Our analyses show a curved ontogenetic trajectory, resulting from different ontogenetic growth allometries of upper and lower thoracic units. Adult thoracic morphology is achieved later than predicted, by diverse modifications in different anatomical regions during different ontogenetic stages. Besides a marked increase in antero-posterior dimensions, there is an increase in medio-lateral dimensions of the upper thorax, relative to the lower thorax. This transforms the pyramidal infant thorax into the barrel-shaped one of adults. Rib descent is produced by complex changes in 3D curvature. Developmental differences between upper and lower thoracic regions relate to differential timings and rates of maturation of the respiratory and digestive systems, the spine and the locomotor system. Our findings are relevant to understanding how changes in the relative rates of growth of these systems and structures impacted on the development and evolution of modern human body shape.

The association between dental mineralization and mandibular form: a study combining additive conjoint measurement and geometric morphometrics.

Studies have suggested that dental development substantially influences the variation of mandibular morphology and growth in primates. As a contribution to the methodology of such studies, we introduce a novel approach to quantifying the covariation between teeth and mandible. This was done showing fluctuations in the magnitude of this covariation within a sample of modern human mandibles at different postnatal ages. Dense CT- derived mandibular surface meshes of 73 females and 71 males, ranging in age from birth to adulthood, were processed by methods of geometric morphometrics. Each specimen's deciduous and permanent teeth were rated for mineralization stage. Form-space principal component analysis of the morphometric data was used to produce a single metric variable that best explains mandibular-form variation. This variable was then used to quantify the developing teeth, all together, through the use of the additive conjoint measurement method. This new metric variable corresponds to the dental prediction of the mandibular-form variation. Finally, we examine the covariation of the two over the full range of mineralization stages. We found a strikingly tight association between mandibular form and dental maturation up through the full emergence of the deciduous dentition (about age 2 years), followed by an equally striking decline in that association in later developmental stages, particularly for girls. The onset of the decline of the teeth-mandible relationship coincides with the onset time of the adult-like pattern of mastication and speech. The increasingly functional diversity may lead to more independence between dental development and mandibular growth than during the first two years.