Head posture and craniofacial morphology

The associations between craniofacial morphology and the posture of the head and the cervical column were examined in a sample of 120 Danish male students aged 22–30 years. Two head positions were recorded on lateral cephalometric radiographs, one determined by the subject's own feeling of a natural head balance (self balance position), and the other by the subject looking straight into a mirror (mirror position). Craniofacial morphology was described by 42 linear and angular variables, and postural relationships by 18 angular variables. A comprehensive set of correlations was found between craniofacial morphology and head posture. The correlations were similar for both head positions investigated. Of the postural variables, the position of the head in relation to the cervical column showed the largest set of correlations with craniofacial morphology. Extension of the head in relation to the cervical column was found in connection with large anterior and small posterior facial heights, small antero-posterior craniofacial dimensions, large inclination of the mandible to the anterior cranial base and to the nasal plane, facial retrognathism, a large cranial base angle, and a small nasopharyngeal space. The possible role of functional factors in mediating the relationship between morphology and posture was discussed.     

Positional behavior in the Hominoidea

Quantitative studies on the positional behavior of members of the Hominoidea are compared in order (1) to identify consistencies across the superfamily, (2) to contrast ape positional behavior with that of Old World monkeys (forest-livingPapio anubis were chosen for study to reduce body size effects), and (3) to identify distinctive behaviors in each of the ape taxa. Differences in the way behaviors were sampled in the various studies necessitated considering posture and locomotion separately. Unimanual arm-hanging and vertical climbing were the most distinctive shared postural and locomotor modes among the apes (the gorilla excepted), constituting ≥5.0% and ≥4.9% of all behavior in each species. Arm-hanging and brachiation (sensu stricto) frequencies were the highest by far in hylobatids. Hand-foot hanging, bipedal posture, and clambering, an orthograde suspensory locomotion assisted by the hindlimbs, were more common in orangutans than in any other hominoid. Sitting and walking were observed in the highest frequencies in the African apes but were no more common than in the baboon. Relatively high frequencies of brachiation (sensu stricto) were reported for all apes except chimpanzees and gorillas. Brachiation and arm-hanging were kinematically different in apes and baboons, involving complete humeral abduction only in the former, whereas vertical climbing appeared to be kinematically similar in apes and baboons. It is concluded that the morphological specializations of the apes may be adaptations to (1) the unique physical demands of arm-hanging and (2) less kinematically distinct, but still quantitatively significant, frequencies of vertical climbing.     

Expansion of the neocerebellum in Hominoidea

Technological and conceptual breakthroughs have led to more serious consideration of the cerebellum as an essential element in cognition. Recent studies show the lateral cerebellum, seat of the neocerebellum, to be most active in cognitive tasks. An examination of the relative volumes of the cerebellar hemispheres in anthropoids would reveal whether some groups show greater neocerebellar development through hemispheric expansion beyond expected allometry, implying a greater contribution of the lateral hemispheres to cognition.This study expands the existing data on primate brain and brain part volumes by incorporating data from both magnetic resonance scans and histological sections for a total sample size of 97 specimens, including 42 apes, 14 humans and 41 monkeys. The resulting volumes of whole brain, cerebellum, vermis, and hemisphere enable a reliable linear regression contrast between hominoids and monkeys, and demonstrate a striking increase in the lateral cerebellum in hominoids.The uniformity of the grade shift suggests that this increase took place in the common ancestor to the hominoids. The importance of the neocerebellum in visual-spatial skills, planning of complex movements, procedural learning, attention switching, and sensory discrimination in manipulation would facilitate the adaptation of these early hominoids to frugivory and suspensory feeding.     

Mosaic evolution in the origin of the Hominoidea.

The initial appearance of hominoids, or apes, and the selective pressures that led to their emergence are currently disputed. Central to the argument are the proconsulids, variously described as the earliest apes or as stem catarrhines, based on facial and postcranial data, respectively. The present paper reports on incongruence and parsimony analyses applied to a combined data set. The results demonstrate that proconsulids are cladistic hominoids, and that the apparent incongruence between the data sets is due to mosaic evolution; the earliest changes in Hominoidea occurred in the face. These results suggest that the initial divergence of hominoids involved selection for an ape-like face, and was not driven by an adaptive shift to below-branch locomotion.     

Growth changes in endocranial capacity in the Cercopithecoidea and Hominoidea

On the basis of examination of 2588 skulls representing 15 genera (56 species) from the Cercopithecoidea, together with 973 skulls representing six genera (10 species) of the Hominoidea, grouped according to stages of dental development, the pattern of postnatal growth change in endocranial volume has emerged as constant throughout Old World Primates. The brain attains 80% of its adult size by the time the deciduous dentition is fully established; 90% by the time the first permanent molars erupt; 95% during the period of eruption of the premolars and second molars; and some 98 % by the time the last permanent teeth are erupting. This constancy in timing of postnatal endocranial growth contrasts with a progressive variation in parameters associated with brain weight-body weight relationship.     

Head and shoulder posture affect scapular mechanics and muscle activity in overhead tasks

Forward head and rounded shoulder posture (FHRSP) is theorized to contribute to alterations in scapular kinematics and muscle activity leading to the development of shoulder pain. However, reported differences in scapular kinematics and muscle activity in those with forward head and rounded shoulder posture are confounded by the presence of shoulder pain. Therefore, the purpose of this study was to compare scapular kinematics and muscle activity in individuals free from shoulder pain, with and without FHRSP. Eighty volunteers were classified as having FHRSP or ideal posture. Scapular kinematics were collected concurrently with muscle activity from the upper and lower trapezius as well as the serratus anterior muscles during a loaded flexion and overhead reaching task using an electromagnetic tracking system and surface electromyography. Separate mixed model analyses of variance were used to compare three-dimensional scapular kinematics and muscle activity during the ascending phases of both tasks. Individuals with FHRSP displayed significantly greater scapular internal rotation with less serratus anterior activity, during both tasks as well as greater scapular upward rotation, anterior tilting during the flexion task when compared with the ideal posture group. These results provide support for the clinical hypothesis that FHRSP impacts shoulder mechanics independent of shoulder pain.     

Phenotypic integration of the cervical vertebrae in the Hominoidea (Primates)

Phenotypic integration and modularity represent important factors influencing evolutionary change. The mammalian cervical vertebral column is particularly interesting in regards to integration and modularity because it is highly constrained to seven elements, despite widely variable morphology. Previous research has found a common pattern of integration among quadrupedal mammals, but integration patterns also evolve in response to locomotor selective pressures like those associated with hominin bipedalism. Here, I test patterns of covariation in the cervical vertebrae of three hominoid primates (Hylobates, Pan, Homo) who engage in upright postures and locomotion. Patterns of integration in the hominoid cervical vertebrae correspond generally to those previously found in other mammals, suggesting that integration in this region is highly conserved, even among taxa that engage in novel positional behaviors. These integration patterns reflect underlying developmental as well as functional modules. The strong integration between vertebrae suggests that the functional morphology of the cervical vertebral column should be considered as a whole, rather than in individual vertebrae. Taxa that display highly derived morphologies in the cervical vertebrae are likely exploiting these integration patterns, rather than reorganizing them. Future work on vertebrates without cervical vertebral number constraints will further clarify the evolution of integration in this region.     

Structural allometry of the femur and tibia in Hominoidea and Macaca

Allometric scaling relationships between body weight, bone length, and cross-sectional dimensions of the lower limb bones which measure structural strength and rigidity (area, second moments of area) are investigated in Homo, Gorilla, Pan, Pongo, and Macaca. Cross-sectional dimensions are slightly positively allometric and highly correlated with body weight; within-bone proportional differences are largely a result of differences in relative bone length to body weight. Orangutans show the greatest deviation from general scaling relationships between lower limb bone structural strength and weight, probably due to habitual upper limb suspension. Formulas for the prediction of weight from cross-sectional dimensions are presented.     

Evolution of the ischial spine and of the pelvic floor in the Hominoidea

Study of the pelvis in 143 different mammals reveals that in quadrupeds the ischial spines are barely noticeable and are located posteriorly near the sacrum. In humans, the ischial spines are prominent and more anteriorly located. As a consequence of their position and size, the ischial spines in humans become an obstacle to parturition. Herein a theory is proposed to account for what appears to be an incongruous development and orientation of the ischial spines in humans. The pelvic diaphragm is a vertical pelvic "wall" in tailed mammals and is composed of muscles involved mostly with the motion of the tail. In humans, the muscles of the pelvic diaphragm have a very different anatomical orientation. They form a horizontal pelvic "floor," and their functions are first to support the abdominopelvic organs and resist intra-abdominal pressure that is exerted from above, and second, as levator ani, to control the anal sphincter. In humans the muscles and fascias of the pelvic diaphragm are inserted on the ischial spines either directly or indirectly through the sacrospinous ligament and the tendinous arch of the pelvic fascia. The result is a medial pull on the ischial spines to produce a more rigid and narrower pelvic floor. An inconstant ossification center for the ischial spines make them more prominent. The backward tilt of the sacrum placed the bispinal line in a diameter position. Pongids and even fossil hominids occupy an intermediate position between tailed mammals and Homo sapiens. The present form of the pelvis in Homo sapiens may be determined by a significant genetic component but may also be partly acquired during childhood and adolescence.     

Evolutionary and morphological significance of the deflecting wrinkle in the lower molars of the Hominoidea

The deflecting wrinkle is a well-known character state of the lower m2 and M1 of the human dentition, but there is little information regarding its presence in great apes. The deflecting wrinkle is more frequent on M2-3 in all extant pongid genera studied in this paper except Pan paniscus, in which M1 has the highest frequency (16.0%) and in which this wrinkle is absent on M3. In Gorilla, it is absent on M1, with only a low incidence on M2-3. Its greatest frequency in Pongo is always on M2 (20.2%), which is the greatest expression of the trait in the great apes. We interpret the presence of the deflecting wrinkle as an incidental effect and suggest that it represents a plesiomorphic character state in the Hominoidea.     

Facial patterns in Cercopithecoidea and Hominoidea: a geometric approach

The maxillofacial and orbital compartments of the primate skull contribute to the ontogenetic and phylogenetic variability of the viscerocranium and are of crucial evolutionary relevance. As the form of organisms changes depending on endo- and exogenous factors, metrical evaluation of specific adaptations and incorporation of the results into a biological framework could be helpful in identifying valid characters for separation of taxa (e.g. family, genus, and species) and in understanding divergence and convergence. During the last two decades a morphometric "revolution" heralded by Rohlf & Marcus (1993), Adams et al. (2004) and Oxnard (2004) brought about a synthesis of traditional quantitative-morphometrical with modern methods. This approach is called "Geometric Morphometrics (GM)" and constitutes the coremethod applied here. Based on standardized photographs (in Norma frontalis), landmarks (LM) were set and two-dimensional coordinates (X, Y) recorded for the facial cranium in selected representatives of the superfamilies Cercopithecoidea and Hominoidea. The comparison of two datasets by means of factor analysis and distance computation for the complete maxillofacial complex on the one hand, and circumorbital and orbital features on the other, indicate that morphological differences between super-families and genera are valid for separating them even in a heterogeneous sample like the one presented here. Including more landmarks and therewith capturing the morph in a more complex way optimizes separation within the sample.     

Man's place in Hominoidea as inferred from molecular clocks of DNA

Summary Divergence dates among primates were estimated by molecular clock analysis of DNA sequence data. A molecular clock of -globin pseudogene was calibrated by setting the date of divergence between Catarrhini and Platyrrhini at 38 million years (Myr) ago. The clock gave dates of 25.3±2.4, 11.9±1.7, 5.9±1.2, and 4.9±1.2 Myr ago (± refers to standard error) for the separation of rhesus monkey, orangutan, gorilla, and chimpanzee, respectively, from the line leading to humans. In placing confidence intervals of the estimates in a robust way, a bootstrap method was used. The 95% confidence intervals are 20.5–29.5, 9.0–14.8, 4.1–7.8, and 3.1–7.0 Myr ago for the separation of rhesus monkey, orangutan, gorilla, and chimpanzee, respectively. By a molecular clock dating of the Prosimii-Anthropoidea splitting, it was suggested that the evolutionary rate of the -globin gene was high early in primate evolution and subsequently decreased in the line of Anthropoidea. And, by a relative rate test using bootstrap sampling, the possibility of further decrease of the rate (more than 10%) in the line of Hominoidea compared with that of Cercopithecoidea was suggested. Therefore, the above dating of the splittings within Hominoidea may be biased slightly toward younger dates. On the other hand, mitochondrial DNA (mtDNA) seems to have evolved in mammals with a more uniform rate than the -globin gene. The ratio of the dates of orangutan splitting to chimpanzee splitting is larger for the mtDNA clock than that for the -globin clock, suggesting the possibilities of mt-DNA introgression among the early hominids and the early African apes, and/or of mtDNA polymorphism within the common ancestral species of orangutan and the African apes that obscures the date of the true species separation of orangutans.     

Mechanisms of reference posture correction in the system of upright posture control

It was earlier shown that ultraslow tilts of the support under quiet standing conditions evoke an unusual response reflecting the operation of compensatory mechanisms: postural sway is a superposition of postural oscillations typical of quiet standing and greater, slower inclinations of the body caused by the tilt. This may be explained by the presence of two hierarchical levels of upright posture control: real-time control compensates for small deviations of the body from the reference posture prescribed by presetting control. Mathematical simulation methods have been used to study the mechanisms of reference posture control. The results are compared with available experimental data. It is demonstrated that the reference posture can be corrected according to the gravitational vertical with the use of a kinesthetic reference alone. It is hypothesized that, when correcting the reference posture, the nervous system “assumes” the support to be immobile. The afferent input from sole pressure receptors is an important factor in reference posture correction. The advantages of the putative two-level control over control based on an explicit internal model are discussed.     

Buccal dental microwear variability in extant African Hominoidea: taxonomy versus ecology

Buccal microwear patterns on teeth are good indicators of the abrasiveness of foodstuffs and have been used to trace the dietary habits of fossil species, including primates and hominids. However, few studies have addressed the variability of this microwear. The abrasiveness of dietary components depends not only on the hardness of the particles ingested, but also on the presence of dust and other exogenous elements introduced during food processing. These elements are responsible for the microwear typology observed on the enamel surfaces of primate teeth. Here we analyzed the variability of buccal microwear patterns in African Great Apes (Gorilla gorilla and Pan troglodytes), using tooth molds obtained from the original specimens held in several osteological collections. Our results suggest that ecological adaptations at subspecies or population level account for differences in microwear patterns, which are attributed to habitat and ecological conditions within populations rather than differences between species. The findings from studies on the variability of buccal dental microwear in extant species will contribute to a better understanding of extinct hominids’ diet and ecology.     

Age at first molar emergence in early Miocene Afropithecus turkanensis and life-history evolution in the Hominoidea

Among primates, age at first molar emergence is correlated with a variety of life history traits. Age at first molar emergence can therefore be used to broadly infer the life histories of fossil primate species. One method of determining age at first molar emergence is to determine the age at death of fossil individuals that were in the process of erupting their first molars. This was done for an infant partial mandible of Afropithecus turkanensis (KNM-MO 26) from the approximately 17.5 Ma site of Moruorot in Kenya. A range of estimates of age at death was calculated for this individual using the permanent lateral incisor germ preserved in its crypt, by combining the number and periodicity of lateral enamel perikymata with estimates of the duration of cuspal enamel formation and the duration of the postnatal delay in the inception of crown mineralization. Perikymata periodicity was determined using daily cross striations between adjacent Retzius lines in thin sections of two A. turkanensis molars from the nearby site of Kalodirr. Based on the position of the KNM-MO 26 M(1)in relation to the mandibular alveolar margin, it had not yet undergone gingival emergence. The projected time to gingival emergence was estimated based on radiographic studies of M(1)eruption in extant baboons and chimpanzees.The estimates of age at M(1)emergence in KNM-MO 26 range from 28.2 to 43.5 months, using minimum and average values from extant great apes and humans for the estimated growth parameters. Even the absolute minimum value is well outside the ranges of extant large Old World monkeys for which there are data (12.5 to <25 months), but is within the range of chimpanzees (25.7 to 48.0 months). It is inferred, therefore, that A. turkanensis had a life history profile broadly like that of Pan. This is additional evidence to that provided by Sivapithecus parvada (Function, Phylogeny, and Fossils: Miocene Hominoid Evolution and Adaptations, 1997, 173) that the prolonged life histories characteristic of extant apes were achieved early in the evolutionary history of the group. However, it is unclear at present whether life-history prolongation in apes represents the primitive catarrhine pace of life history extended through phyletic increase in body mass, or whether it is derived with respect to a primitive, size-adjusted life history that was broadly intermediate between those of extant hominoids and cercopithecoids. Life history evolution in primates as a whole may have occurred largely through a series of grade-shifts, with the establishment of fundamental life-history profiles early in the histories of major higher taxa. These may have included shifts that were largely body mass dependent, as well as those that occurred in the absence of significant changes in body mass.