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As the interface between the mandible and cranium, the mandibular ramus is functionally significant and its morphology has been suggested to be informative for taxonomic and phylogenetic analyses. In primates, and particularly in great apes and humans, ramus morphology is highly variable, especially in the shape of the coronoid process and the relationship of the ramus to the alveolar margin. Here we compare ramus shape variation through ontogeny in Homo neanderthalensis to that of modern and fossil Homo sapiens using geometric morphometric analyses of two-dimensional semilandmarks and univariate measurements of ramus angulation and relative coronoid and condyle height. Results suggest that ramus, especially coronoid, morphology varies within and among subadult and adult modern human populations, with the Alaskan Inuit being particularly distinct. We also identify significant differences in overall anterosuperior ramus and coronoid shapes between H. sapiens and H. neanderthalensis , both in adults and throughout ontogeny. These shape differences are subtle, however, and we therefore suggest caution when using ramus morphology to diagnose group membership for individual specimens of these taxa. Furthermore, we argue that these morphologies are unlikely to be representative of differences in masticatory biomechanics and/or paramasticatory behaviors between Neanderthals and modern humans, as has been suggested by previous authors. Assessments of ontogenetic patterns of shape change reveal that the typical Neanderthal ramus morphology is established early in ontogeny, and there is little evidence for divergent postnatal ontogenetic allometric trajectories between Neanderthals and modern humans as a whole. This analysis informs our understanding of intraspecific patterns of mandibular shape variation and ontogeny in H. sapiens and can shed further light on overall developmental and life history differences between H. sapiens and H. neanderthalensis . [ABSTRACT FROM AUTHOR]

Previous studies showed that there is variation in ontogenetic trajectories of human limb dimensions and proportions. However, little is known about the evolutionary significance of this variation. This study used a global sample of modern human immature long bone measurements and a multivariate linear mixed-effects model to study 1) whether the variation in ontogenetic trajectories of limb dimensions is consistent with ecogeographic predictions and 2) the effects of different evolutionary forces on the variation in ontogenetic trajectories. We found that genetic relatedness arising from neutral (nonselective) evolution, allometric variation associated with the change in size, and directional effects from climate all contributed to the variation in ontogenetic trajectories of all major long bone dimensions in modern humans. After accounting for the effects of neutral evolution and holding other effects considered in the current study constant, extreme temperatures have weak, positive associations with diaphyseal length and breadth measurements, while mean temperature shows negative associations with diaphyseal dimensions. The association with extreme temperatures fits the expectations of ecogeographic rules, while the association with mean temperature may explain the observed among-group variation in intralimb indices. The association with climate is present throughout ontogeny, suggesting an explanation of adaptation by natural selection as the most likely cause. On the other hand, genetic relatedness among groups, as structured by neutral evolutionary factors, is an important consideration when interpreting skeletal morphology, even for nonadult individuals. [ABSTRACT FROM AUTHOR]

A central challenge in human origins research is to understand how evolution has shaped modern human life history. As fossilized remains of our ancestors provide the only direct evidence for life history evolution, efforts to reconstruct life history in paleontological contexts have focused on hard tissues, particularly on dental development. However, among investigators of other vertebrate groups, there is a long tradition of examining primary bone microstructure to decipher growth rates and maturational timing, based on an empirical relationship between the microanatomy of primary bone and the rate at which it is deposited. We examined ontogenetic variation in primary bone microstructure at the midshaft femur of Chlorocebus aethiops , Hylobates lar , and Pan troglodytes to test whether tissue type proportions vary in accordance with predictions based on body mass growth patterns described previously. In all taxa, younger age classes were characterized by significantly higher percent areas of fibro-lamellar and/or parallel-fibered tissues, while older age classes showed significantly higher proportions of lamellar bone. In prior experimental studies, fibro-lamellar and parallel-fibered tissue types have been associated with faster depositional rates than lamellar bone. Principal components analysis revealed differences among taxa in the timing of this transition, and in the particular tissue types observed among individuals of similar dental emergence status. Among M1 and M2 age classes, higher proportions of parallel-fibered and fibro-lamellar tissues were observed in those taxa characterized by reportedly faster body mass growth rates. Further, persistence of fibro-lamellar tissue throughout DECID, M1 and M2 age classes in chimpanzees contrasts with the pattern reported previously for modern humans. Despite the necessary limitations of our cross-sectional study design and the secondary remodeling of bone in primates, large areas of primary bone remain intact and represent a valuable and independent source of information about the evolution of growth and development in the fossil record. [ABSTRACT FROM AUTHOR]

Abstract: The occipital bone of Neandertals contains an association of morphological features that is considered characteristic of this fossil human population. One of the possible autapomorphic traits of Neandertals is the presence of a suprainiac fossa, a horizontal oval-shaped depression located on the occipital plane. The question of whether suprainiac depressions observed on Neandertals and in other human samples are homologous has been widely discussed. The present study provides a detailed anatomical description of the fossa and of the underlying internal bone composition for Neandertals of various geographical and chronological origins, and of different developmental stages. The suprainiac fossa is systematically present on Neandertals from the earliest developmental stages as well as at the beginning of the differentiation of the Neandertal lineage. A similar pattern for the structural composition of the bone is found among all analyzed Neandertals (i.e., the suprainiac fossa corresponds to a thinning of the diploic layer with no substantial remodeling nor variation in the external table thickness in this area). On the contrary, the occipital depressions present on a sample of anatomically modern humans from Europe and Africa correspond to a resorptive area of the external cranial surface only. These depressions also differ in shape, disposition, and aspect on the external cranial surface from those fossae on Neandertals. We demonstrate that the depressions described in anatomically modern humans, as well as other hominin species, are not homologous to the suprainiac fossa of Neandertals. Consequently, we confirm the autapomorphic status of the Neandertal suprainiac fossa. [Copyright &y& Elsevier]

Abstract: Little ontogenetic data exist to indicate whether muscular organization of neonates reflects adult locomotion (e.g., leaping) or infant activities like clinging or the initial quadrupedal phase of locomotion that typifies most infant primates. In the present study, five species of primates with contrasting modes of locomotion were examined. Twenty-eight preserved neonatal and adult cadavers were studied by careful dissection of the hip, thigh, and leg muscles. Wet weights were taken of limb muscles after removal, and the muscles were combined into major functional groups (e.g., flexors, extensors) of each limb segment. Results demonstrate that the distribution of muscle mass within the thigh and within the leg are similar between neonates and adults for all species, with major groups varying by 5% or less in all but two age comparisons. Crural indices of the neonates are nearly identical to those of the adults, but leg/thigh muscle mass ratios were higher in the neonates. Species vary greatly in the percentage of adult limb segment muscle mass present in neonates, with Tarsius syrichta having the greatest percentage for all segments and two lemurids showing the least. These results primarily track differences in relative body mass at birth rather than developmental differences. The adaptive distribution of muscle, as discussed previously for adult prosimians, appears to be established at birth. Neonates of leaping species already have much larger quadriceps muscles than quadrupeds. Differences between large- and small-bodied leapers (e.g., pronounced superficial plantarflexor masses in tarsiers and pronounced deep plantarflexor masses in sifakas) also are present in neonates. Ratios of muscle mass over body mass are smaller in all neonates than in their adult counterparts, suggesting that the neonates are relatively poorly muscled, and that muscle mass must increase with positive allometry during growth. [Copyright &y& Elsevier]

Abstract: Ontogenetic changes in the human femur associated with the acquisition of bipedal locomotion, especially the development of the bicondylar angle, have been well documented. The purpose of this study is to quantify changes in the three-dimensional structure of trabecular bone in the human proximal femur in relation to changing functional and external loading patterns with age. High-resolution X-ray computed tomography scan data were collected for 15 juvenile femoral specimens ranging in age from prenatal to approximately nine years of age. Serial slices were collected for the entire proximal femur of each individual with voxel resolutions ranging from 0.017 to 0.046mm depending on the size of the specimen. Spherical volumes of interest were defined within the proximal femur, and the bone volume fraction, trabecular thickness, trabecular number, and fabric anisotropy were calculated in three dimensions. Bone volume fraction, trabecular number, and degree of anisotropy decrease between the age of 6 months and 12 months, with the lowest values for these parameters occurring in individuals near 12 months of age. By age 2–3 years, the bone volume, thickness, and degree of anisotropy increase slightly, and regions in the femoral neck become more anisotropic corresponding to the thickening of the inferior cortical bone of the neck. These results suggest that trabecular structure in the proximal femur reflects the shift in external loading patterns associated with the initiation of unassisted walking in infants. [Copyright &y& Elsevier]

Abstract: The present research investigates the effects of hybridization between Macaca maurus and M. tonkeana on adult male form and patterns of growth allometry. Comparisons of adult hybrid mean phenotypic values with the adult averages of the parental species indicate a condition of heterosis for cranial vault length and crown–rump length. Negative heterosis is indicated for body mass. Regression parameters describing growth allometry are generated for four craniofacial measurement variables and one body measurement using both least squares and reduced major axis regression. Comparisons of hybrid and parental regression slopes and intercepts using analysis of covariance and t-tests suggest that there is a hybrid pattern of growth allometry characterized by an increase in regression slope values coupled with lower intercept values compared to those of the parental species and the parental averages for most regression parameters. Multivariate analyses of the adult and ontogenetic morphometric data indicate significant differences across species taxa in form and shape during development and adulthood. Our finding of significant differences between hybrids and their parental taxa in growth allometry and craniofacial form and shape during development challenges the assumption often made regarding the reproductive and taxonomic significance of observed ontogenetic divergence between Neandertals and modern humans. We propose that anthropological primatology, with its goal of developing nonhuman primate models for investigating human evolution, can provide a biologically relevant means by which to empirically estimate the taxonomic significance of morphological and ontogenetic divergence observed in the hominid fossil record. [Copyright &y& Elsevier]

Abstract: Understanding the evolutionary history of canine sexual dimorphism is important for interpreting the developmental biology, socioecology and phylogenetic position of primates. All current evidence for extant primates indicates that canine dimorphism is achieved through bimaturism rather than via differences in rates of crown formation time. Using incremental growth lines, we charted the ontogeny of canine formation within species of Eocene Cantius, the earliest known canine-dimorphic primate, to test whether canine dimorphism via bimaturism was developmentally canalized early in primate evolution. Our results show that canine dimorphism in Cantius is achieved primarily through different rates of crown formation in males and females, not bimaturism. This is the first demonstration of rate differences resulting in canine dimorphism in any primate and therefore suggests that canine dimorphism is not developmentally homologous across Primates. The most likely interpretation is that canine dimorphism has been selected for at least twice during the course of primate evolution. The power of this approach is its ability to identify underlying developmental processes behind patterns of morphological similarity, even in long-extinct primate species. [Copyright &y& Elsevier]

This study investigates the developmental bases of size and shape variation in papionin primates (Macaca, Cercocebus, Mandrillus, Lophocebus, and Papio). The analysis tests hypotheses predicting that heterochronic changes in ontogeny, particularly in the degree of overall size growth, can account for cranial diversity and “allometric scaling” in this clade. Large developmental samples of extant papionin crania are examined to test heterochronic hypotheses using bivariate allometric methods. Analyses indicate that the crania of larger papionins (Mandrillus and Papio) are generally peramorphic, surpassing size and shape ranges of smaller, and probably less-derived, macaques and mangabeys. At least two heterochronic processes, including acceleration and hypermorphosis, can account for this pattern. Ontogenetic changes include decoupling of growth and development among cranial regions, along with simple shifts in size. Allometric scaling has complex developmental bases. Size change itself is not sufficient to explain all developmental differences among papionins, but these changes are extremely important in comparisons within cranial regions such as the face. Results imply that Papio exhibits strongly derived patterns of brain growth that impact postnatal patterns of size and shape transformation. Consideration of these results in the context of recent socioecological analyses suggests that derived patterns of cranial growth in Papio may be a response to selection during the early periods of ontogeny, resulting in a distinctive life history pattern. [Copyright &y& Elsevier]