Understanding the complexities of cranial base development, function, and architecture is important for testing hypotheses about many aspects of craniofacial variation and evolution. We summarize key aspects of cranial base growth and development in primates that are useful for formulating and testing hypotheses about the roles of the chondrocranium and basicranium in cranial growth, integration, and function in primate and human evolution. We review interspecific, experimental, and ontogenetic evidence for interactions between the cranial base and brain, and between the cranial base and the face. These interactions indicate that the cranial base plays a key role in craniofacial growth, helping to integrate, spatially and functionally, different patterns of growth in various adjoining regions of the skull such as components of the brain, the eyes, the nasal cavity, the oral cavity, and the pharynx. Brain size relative to cranial base length appears to be the dominant influence on many aspects of basicranial variation, especially the angle of the cranial base in the midsagittal plane, but other factors such as facial size, facial orientation, and posture may also be important. Major changes in cranial base shape appear to have played crucial roles in the evolution of early primates, the origin of anthropoids, and the origin of Homo sapiens.

Numerous hypotheses explaining interspecific differences in the degree of basicranial flexion have been presented. Several authors have argued that an increase in relative brain size results in a spatial packing problem that is resolved by flexing the basicranium. Others attribute differences in the degree of basicranial flexion to different postural behaviors, suggesting that more orthograde animals require a ventrally flexed pre-sella basicranium in order to maintain the eyes in a correct forward-facing orientation. Less specific claims are made for a relationship between the degree of basicranial flexion and facial orientation. In order to evaluate these hypotheses, the degree of basicranial flexion (cranial base angle), palate orientation, and orbital axis orientation were measured from lateral radiographs of 68 primate species and combined with linear and volumetric measures as well as data on the size of the neocortex and telencephalon. Bivariate correlation and partial correlation analyses at several taxonomic levels revealed that, within haplorhines, the cranial base angle decreases with increasing neurocranial volume relative to basicranial length and is positively correlated with angles of facial kyphosis and orbital axis orientation. Strepsirhines show no significant correlations between the cranial base angle and any of the variables examined. It is argued that prior orbital approximation in the ancestral haplorhine integrated the medial orbital walls and pre-sella basicranium into a single structural network such that changes in the orientation of one necessarily affect the other. Gould's ("Ontogeny and Phylogeny." Cambridge: Belknap Press, 1977) hypothesis, that the highly flexed basicranium of Homo may be due to a combination of a large brain and a relatively short basicranium, is corroborated.

The influence of elastic properties on finite-element analysis was investigated using a finite-element model of a Macaca fascicularis skull. Four finite-element analyses were performed in which the model was assigned different sets of elastic properties. In analysis 1, elastic properties were modeled isotropically using published data obtained from human limb bones. Analyses 2-4 used data obtained from skulls of a closely allied species, M. mulatta, but varied as to how those data were incorporated into the model. In analysis 2, the model was assigned a single set of isotropic elastic properties. In analysis 3, each region within the model was assigned its own set of isotropic elastic properties. Finally, in analysis 4, each region received its own set of orthotropic elastic properties. Although a qualitative assessment indicates that the locations of strain concentrations across the model are broadly similar in all analyses, a quantitative assessment of strain indicates some differences between the analyses. When strain data from the finite-element analyses were compared to strain data derived from in vivo experiments, it was found that the model deformed most realistically using the orthotropic elastic properties employed in analysis 4. Results suggest that finite-element analyses can be adversely affected when elastic properties are modeled imprecisely, and that modelers should attempt to obtain elastic properties data about the species and skeletal elements that are the subjects of their analyses.

The African Plio-Pleistocene hominins known as australopiths evolved a distinctive craniofacial morphology that traditionally has been viewed as a dietary adaptation for feeding on either small, hard objects or on large volumes of food. A historically influential interpretation of this morphology hypothesizes that loads applied to the premolars during feeding had a profound influence on the evolution of australopith craniofacial form. Here, we test this hypothesis using finite element analysis in conjunction with comparative, imaging, and experimental methods. We find that the facial skeleton of the Australopithecus type species, A. africanus, is well suited to withstand premolar loads. However, we suggest that the mastication of either small objects or large volumes of food is unlikely to fully explain the evolution of facial form in this species. Rather, key aspects of australopith craniofacial morphology are more likely to be related to the ingestion and initial preparation of large, mechanically protected food objects like large nuts and seeds. These foods may have broadened the diet of these hominins, possibly by being critical resources that australopiths relied on during periods when their preferred dietary items were in short supply. Our analysis reconciles apparent discrepancies between dietary reconstructions based on biomechanics, tooth morphology, and dental microwear.evolution ͉ face ͉ finite element analysis ͉ hominin ͉ diet

The purpose of this study is to test various hypotheses about balancing-side jaw muscle recruitment patterns during mastication, with a major focus on testing the hypothesis that symphyseal fusion in anthropoids is due mainly to vertically- and/or transversely-directed jaw muscle forces. Furthermore, as the balancing-side deep masseter has been shown to play an important role in wishboning of the macaque mandibular symphysis, we test the hypothesis that primates possessing a highly mobile mandibular symphysis do not exhibit the balancing-side deep masseter firing pattern that causes wishboning of the anthropoid mandible. Finally, we also test the hypothesis that balancing-side muscle recruitment patterns are importantly related to allometric constraints associated with the evolution of increasing body size. Electromyographic (EMG) activity of the left and right superficial and deep masseters were recorded and analyzed in baboons, macaques, owl monkeys, and thick-tailed galagos. The masseter was chosen for analysis because in the frontal projection its superficial portion exerts force primarily in the vertical (dorsoventral) direction, whereas its deep portion has a relatively larger component of force in the transverse direction. The symphyseal fusion-muscle recruitment hypothesis predicts that unlike anthropoids, galagos develop bite force with relatively little contribution from their balancing-side jaw muscles. Thus, compared to galagos, anthropoids recruit a larger percentage of force from their balancing-side muscles. If true, this means that during forceful mastication, galagos should have working-side/balancing-side (W/B) EMG ratios that are relatively large, whereas anthropoids should have W/B ratios that are relatively small. The EMG data indicate that galagos do indeed have the largest average W/B ratios for both the superficial and deep masseters (2.2 and 4.4, respectively). Among the anthropoids, the average W/B ratios for the superficial and deep masseters are 1.9 and 1.0 for baboons, 1.4 and 1.0 for macaques, and both values are 1.4 for owl monkeys. Of these ratios, however, the only significant difference between thick-tailed galagos and anthropoids are those associated with the deep masseter. Furthermore, the analysis of masseter firing patterns indicates that whereas baboons, macaques and owl monkeys exhibit the deep masseter firing pattern associated with wishboning of the macaque mandibular symphysis, galagos do not exhibit this firing pattern. The allometric constraint-muscle recruitment hypothesis predicts that larger primates must recruit relatively larger amounts of balancing-side muscle force so as to develop equivalent amounts of bite force. Operationally this means that during forceful mastication, the W/B EMG ratios for the superficial and deep masseters should be negatively correlated with body size. Our analysis clearly refutes this hypothesis. As already noted, the average W/B ratios for both the superficial and deep masseter are largest in thick-tailed galagos, and not, as predicted...

Many aspects of an animal's ecology are associated with activity pattern, the time of day when that animal is awake and active. There are two major activity patterns: diurnal, active during the day in a light-rich, or photopic, environment, and nocturnal, active after sunset in a light-limited, or scotopic, environment. Birds are also cathemeral, or equally likely to be awake at any time of day, or crepuscular, awake and active at dawn and dusk. Each of these activity patterns is associated with different levels of ambient light. This study examines how the morphology (size and shape) of the eye varies according to these different light environments for birds in a phylogenetic context. Activity pattern has a significant influence on eye shape and size in birds. Birds that are adapted for scotopic vision have eye shapes that are optimized for visual sensitivity, with larger corneal diameters relative to axial lengths. Birds that are adapted for photopic vision have eye shapes that are optimized for visual acuity, with larger axial lengths relative to corneal diameters. Birds adapted for scotopic vision also exhibit absolutely larger corneal diameters and axial lengths than do photopic birds. The results indicate that the light level under which the bird functions has a more significant influence on eye shape than phylogeny.

Comparative work among nonhominid primates has demonstrated that the basicranium becomes more flexed with increasing brain size relative to basicranial length and as the upper and lower face become more ventrally deflected (Ross and Ravosa [1993] Am. J. Phys. Anthropol. 91:305-324). In order to determine whether modern humans and fossil hominids follow these trends, the cranial base angle (measure of basicranial flexion), angle of facial kyphosis, and angle of orbital axis orientation were measured from computed tomography (CT) scans of fossil hominids (Sts 5, MLD 37/38, OH9, Kabwe) and lateral radiographs of 99 extant humans. Brain size relative to basicranial length was calculated from measures of neurocranial volume and basicranial length taken from original skulls, radiographs, CT scans, and the literature. Results of bivariate correlation analyses revealed that among modern humans basicranial flexion and brain size/basicranial length are not significantly correlated, nor are the angles of orbital axis orientation and facial kyphosis. However, basicranial flexion and orbit orientation are significantly positively correlated among the humans sampled, as are basicranial flexion and the angle of facial kyphosis. Relative to the comparative sample from Ross and Ravosa (1993), all hominids have more flexed basicrania than other primates: Archaic Homo sapiens, Homo erectus, and Australopithecus africanus do not differ significantly from Modern Homo sapiens in their degree of basicranial flexion, although they differ widely in their relative brain size. Comparison of the hominid values with those predicted by the nonhominid reduced major-axis equations reveal that, for their brain size/basicranial length, Archaic and Modern Homo sapiens have less flexed basicrania than predicted. H. erectus and A. africanus have the degree of basicranial flexion predicted by the nonhominid reduced major-axis equation. Modern humans have more ventrally deflected orbits than all other primates and, for their degree of basicranial flexion, have more ventrally deflected orbits than predicted by the regression equations for hominoids. All hominoids have more ventrally deflected orbital axes relative to their palate orientation than other primates. It is argued that hominids do not strictly obey the trend for basicranial flexion to increase with increasing relative brain size because of constraints on the amount of flexion that do not allow it to decrease much below 90 degrees. Therefore, if basicranial flexion is a mechanism for accommodating an expanding brain among non-hominid primates, other mechanisms must be at work among hominids.

Recent fossil discoveries have greatly increased our knowledge of the morphology and diversity of early Anthropoidea, the suborder to which humans belong. Phylogenetic analysis of Recent and fossil taxa supports the hypotheses that a haplorhine-strepsirrhine dichotomy existed at least at the time of the earliest record of fossil primates (earliest Eocene) and that eosimiids (middle Eocene, China) are primitive anthropoids. Functional analysis suggests that stem haplorhines were small, nocturnal, arboreal, visually oriented insectivore-frugivores with a scurrying-leaping locomotion. A change from nocturnality to diurnality was the fundamental adaptive shift that occurred at the base of the tarsier-eosimiid-anthropoid clade. Stem anthropoids remained small diurnal arborealists but adopted locomotor patterns with more arboreal quadrupedalism and less leaping. A shift to a more herbivorous diet occurred in several anthropoid lineages.