Article
To read the full-text of this research, you can request a copy directly from the authors.

Abstract

Although modern humans are considered to be morphologically distinct from other living primates because of our large brains, dexterous hands, and bipedal gait, all of these features are found among extinct hominins. The chin, however, appears to be a uniquely modern human trait. Probably because of the chin’s exclusivity, many evolutionary scenarios have been proposed to explain its origins. To date, researchers have developed adaptive hypotheses relating chins to speech, mastication, and sexual selection; still others see it as a structural artifact tangentially related to complex processes involving evolutionary retraction of the midfacial skeleton. Consensus has remained elusive, partly because hypotheses purporting to explain how this feature developed uniquely in modern humans are all fraught with theoretical and/or empirical shortcomings. Here we review a century’s worth of chin hypotheses and discuss future research avenues that may provide greater insight into this human peculiarity.

No full-text available

Request Full-text Paper PDF

To read the full-text of this research,
you can request a copy directly from the authors.

... The definition of the 'chin' remains slippery as there are disagreements over its essential characteristics (Pampush and Daegling, 2016a). Some argue that for a mandible to be in possession of a chin, there must be a raised 'falsum' ('⊥') produced through an interaction among tubercles, ridges, and depressions on the anterior surface of the symphysis (e.g., Schwartz and Tattersall, 2000). ...
... The bony projection (with and without a full falsum) contributes to the modern human mandible's unique shape, as it typically causes the lower border of the anterior symphysis to protrude beyond the anterior dentition. This unusual bony arrangement has captured the attention of biological anthropologists for over a century, and has been the focus of a considerable amount of research effort (for review, see Pampush and Daegling, 2016a). The reasons for this are threefold. ...
... Wolpoff et al., 1981;Lam et al., 1996;Wolpoff, 1996;Schwartz and Tattersall, 2000). Second, apart from some claims that a few Neanderthal specimens possess chins (see above), there is general agreement that anatomically modern humans (Homo sapiens) are the only hominin species (living or extinct) to consistently exhibit 'true' chins (e.g., Robinson, 1914;DuBrul and Sicher, 1954;Daegling, 1993a;Dobson and Trinkaus, 2002;Pampush and Daegling, 2016a). Thus, there has been a persistent effort to understand this uniquely modern human feature, partly in anticipation of the insight it might provide into the evolution of our species (Daegling, 1993a;Pampush and Daegling, 2016a). ...
Article
The rate of change in primate mandibular symphyseal angles was modeled with particular aim of locating a rate-shift within the hominin clade. Prior work noted that the human symphyseal angle must have experienced a rapid rate of change in order to assume the modern human form, suggestive of the non-random work of natural selection. This study indicates that the rate of symphyseal evolution rose dramatically between Australopithecus anamensis and Australopithecus afarensis and continued throughout the diversification of the hominin clade. Noting the timing of this event, we speculate as to what ecological factors could have been at play in driving this rearrangement of the anterior mandible, contributing to the eventual appearance of the human chin.
... This anthropological fascination with the human chin is not new, and presence of the trait has been used since the beginning of the field to debate the inclusion of specimens from our taxa, leading Arthur Keith and others to use the mandible and chin as an exclusionary feature in human phylogenetic debates revolving around the Piltdown hoax in the early 20 th century (Hrdlička and Pearson 1911;Robinson 1913;Keith 1916;Wallis 1917;Keith 1928). This "membership" within our species has not faded from contemporary debates, either, as the chin has played a role in debates regarding hominin speech, diet, and overall morphology and variation (Lieberman and Crelin 1971;Carlisle and Siegel 1974;Schwartz and Tattersall 2000;Antón 2002;Lieberman 2011;Pampush and Daegling 2016). ...
... The ability to evaluate shape changes are especially appealing for those interested in evolutionary comparisons both between and within species, both past and present (Viòarsdóttir, O'Higgins, Paul, and Stringer 2002;Viòarsdóttir and O'Higgins 2003). In particular, studies of mandibular form shed light on form and functional changes in the past (Thayer and Dobson 2010;Pampush and Daegling 2016). Shape studies add a powerful lens with which to evaluate studies of paleoanthropological populations that have traditionally been analyzed using linear measurements alone (see Kesterke and Ahern 2007). ...
... Contemporary mandibular research still exists primarily in the dental and orthodontic literature, but its applicability to broader fields is readily apparent. Many studies have evaluated some of the proposed hypotheses regarding mandibular and chin formation in primate populations (Hylander 1979;Hylander 1984;Lieberman 2011), but the fact that the human chin is still a popular topic of both academic and popular debate highlights the enigma of its formation and function (Pampush and Daegling 2016). This project utilizes methods that may be used as a model for shape differences existing between different populations, be they extant primate or paleoanthropological, that have been proposed over the last decade (Schwartz and Tattersall 2000;Ichim, Swain, and Kieser 2006;Thayer and Dobson 2010;Garvin and Ruff 2012). ...
Thesis
Full-text available
An estimated 3% of U.S. pregnancies are affected by maternal thyroid dysfunction, with between one and three of every 1,000 pregnancies being complicated by overactive maternal thyroid levels. Overactive maternal thyroid hormones have been linked to neurological impairment and craniofacial development dysmorphogenesis, affecting both endochondral and intramembranous bone. Using both Euclidean Distance Matrix Analysis (EDMA) and geometric morphometric approach, this study evaluates the role of in utero thyroxine overexposure on the growth of offspring mandibles in a sample of 241 mice. Principle component analysis (PCA) and canonical variate analysis (CVA) utilized 16 unilateral mandibular landmarks obtained from 3D microCT to assess shape changes between unexposed controls (n=63) and exposed mice (n=178). By evaluating shape changes in the mandible between different age groups (15, 20, and 25 days postnatal) and different dosage levels (low, medium, and high), this study found that maternal thyroxine alters offspring mandibular shape in both age- and dosage-dependent manners, particularly within the high dosage individuals in the oldest age group. The EDMA results demonstrate marked shape changes throughout the mandible, with the gonial angle and alveolus undergoing significant (p <0.10) changes. Geometric morphometric analysis revealed that group differences in overall shape were significant (p <0.001 for both PCA and CVA) and showed major changes in regions of the mandible associated with muscle attachment (coronoid process, gonial angle) and regions of growth largely governed by articulation with the cranial base (condyle) and occlusion (alveolus). These results compliment recent studies demonstrating that maternal thyroxine levels can alter the cranial base and cranial vault of offspring, contributing to a better understanding of both normal and abnormal mandibular development and facilitating a fuller understanding of evolutionary and medical implications of craniofacial growth and development.
... Additionally, unique features of the human masticatory apparatus such as the chin have received extensive discussion (e.g., Waterman 1916;DuBrul and Sicher 1954;Riesenfeld 1969;Wolff 1984;Daegling 1993;Dobson and Trinkaus 2002;Pampush and Daegling 2016). These discussions consider a range of issues from the adaptive nature of the human chin to the functional consequences for mastication and feeding (see Pampush and Daegling 2016 for review). ...
... Additionally, unique features of the human masticatory apparatus such as the chin have received extensive discussion (e.g., Waterman 1916;DuBrul and Sicher 1954;Riesenfeld 1969;Wolff 1984;Daegling 1993;Dobson and Trinkaus 2002;Pampush and Daegling 2016). These discussions consider a range of issues from the adaptive nature of the human chin to the functional consequences for mastication and feeding (see Pampush and Daegling 2016 for review). We take a closer look at how the human masticatory apparatus compares to other primates by contrasting several measures describing masticatory apparatus functional capacities. ...
Chapter
Feeding and diet played key roles in human evolution. It is well known that modern humans have a small masticatory apparatus for their body size among primates. However, identifying gracility does not necessarily tell us about the relative functional capacities of the human masticatory system beyond the obvious size-related consequences. We consider the functional consequences of gracilization and functional relationships within the human masticatory apparatus using nonhuman primates for comparison. Human jaws are short for their size, particularly the anterior portion, among primates. When considered relative to masticatory apparatus size, the shortened jaw compares more similarly to other apes. Because jaw length acts as a load arm, humans have improved leverage for biting, but smaller relative gapes. Human biting ability is not particularly improved by their favorable leverage because humans have relatively small muscles and because of a size-related decrease in bite force across primates. Humans have relatively reduced load resistance abilities in the jaw compared to other apes, but abilities that are still intermediate among primates. Human postcanine teeth are small for their size, but average-sized for their masticatory apparatus. Finally, an initial look at jaw-muscle activation patterns during chewing suggests that humans recruit their jaw muscles like similar-sized anthropoids. We conclude that any performance deficits in the human masticatory apparatus are primarily related to gracilization. Humans possess a relative masticatory apparatus configuration that compares similarly to many other primates suggesting the evolution of humans has not unraveled the basic functional relationships within the masticatory apparatus that characterize most primates.
... It has been argued that spandrels are a hypothesis of last resort, result from lack of imagination, or that such hypotheses are simply unfalsifiable. Pampush and Daegling (2016), for example, treat multiple spandrel hypotheses for the origin of the human chin as being "resistant to tests," despite themselves providing in the same paper either strong arguments against several spandrel hypotheses (not all of which would here be classified as spandrels, but which are nonadaptive) or showing that these hypotheses have not been well demonstrated with affirmative evidence in their favor. We agree with Lloyd (2013) that nonadaptive explanations, including spandrel hypotheses, should be tested alongside adaptive hypotheses on equal footing. ...
... Nonfunctional features that result from genetic drift are not spandrels (in contrast to the use in Pampush and Daegling 2016). Trade-offs are a form of constraint, but not necessarily spandrels (Aiello and Wheeler 1995;Pampush and Daegling 2016). To be diagnosed as a spandrel, a feature must originate as a direct sequel, consequence, or by-product of selection for another feature. ...
Article
Although generally considered rare in gastropods, septation has long been noted in turritellids, but functional hypotheses do not survive strong scrutiny. Here we outline a methodology for testing spandrel hypotheses and apply it to the problem of turritellid septa. We follow Gould in using “spandrel” as a term for all features that are nonadaptive sequelae of adaptive features of organisms, including those that are structurally necessary, those that are developmentally correlated, and nondeterministic by-products that are correlated to features under selection. In turritellids, septa are constructed in microstructural continuity with secondary internal thickening of the shell, are highly variable features infraspecifically, and are strongly associated with degree of shell thickening. We therefore conclude that rather than being themselves adaptive, turritellid septa are spandrels of shell thickening. Turritellid septa are composed of crossed lamellar aragonite, which appears to be constructed by mantle epithelium over the visceral mass. Septation was also found in 22 of 24 gastropod families examined from a broad phylogenetic distribution. Septa thus appear to be a widespread feature of caenogastropods, in strong contrast to previous assertions that septa are less common in modern or high-spired shells.
... It has been argued that spandrels are a hypothesis of last resort, result from lack of imagination, or that such hypotheses are simply unfalsifiable. Pampush and Daegling (2016), for example, treat multiple spandrel hypotheses for the origin of the human chin as being "resistant to tests," despite themselves providing in the same paper either strong arguments against several spandrel hypotheses (not all of which would here be classified as spandrels, but which are nonadaptive) or showing that these hypotheses have not been well demonstrated with affirmative evidence in their favor. We agree with Lloyd (2013) that nonadaptive explanations, including spandrel hypotheses, should be tested alongside adaptive hypotheses on equal footing. ...
... Nonfunctional features that result from genetic drift are not spandrels (in contrast to the use in Pampush and Daegling 2016). Trade-offs are a form of constraint, but not necessarily spandrels (Aiello and Wheeler 1995;Pampush and Daegling 2016). To be diagnosed as a spandrel, a feature must originate as a direct sequel, consequence, or by-product of selection for another feature. ...
Conference Paper
Although septa are generally considered absent or rare in gastropods, septation has long been noted in some groups, including turritellids, and some Paleozoic forms. Yet the overall occurrence of septa in gastropods has apparently never been systematically analyzed. We examined the phylogenetic distribution of septation in gastropods and find that it is in fact quite broad. We then examined the pattern of septal insertion and shell microstructures present in several turritellid species to better understand the development of these structures. A variety of functional hypotheses have been proposed for septa in turritellids, including prevention of parasitic infestation after apical loss, protection against apex-damaging predation, and deposition as a response to excess calcium. None of these hypotheses survive strong scrutiny. Turritellid septa are thin and curved, inserted in varying number and position within a single species, and are also inserted far from the apex. They form in continuity with secondarily deposited crossed-lamellar shell material. We therefore propose that these dome-like structures are not themselves adaptive, but rather are associated with adaptive shell thickening; septa are therefore constructional artefacts (spandrels), rather than products of natural selection (adaptations). This is supported by the association of septa with more extensive shell thickening both among multiple species and also among multiple individuals of a single species.
... AU17-Lower lip raiser (Chin raiser). 1) Comparative muscular basis: In humans, the chin raiser is produced by the action of the mentalis m. that pushes the lower lip upwards, protrudes the chin boss and wrinkles the chin. The bony chin boss is an anatomical feature unique to humans and its function is still highly debated [70]. Therefore, even though the muscular basis is the same, AU17 in other species is sometimes designated as Lower lip raiser (e.g. ...
Article
Full-text available
Facial expressions are complex and subtle signals, central for communication and emotion in social mammals. Traditionally, facial expressions have been classified as a whole, disregarding small but relevant differences in displays. Even with the same morphological configuration different information can be conveyed depending on the species. Due to a hardwired processing of faces in the human brain, humans are quick to attribute emotion, but have difficulty in registering facial movement units. The well-known human FACS (Facial Action Coding System) is the gold standard for objectively measuring facial expressions, and can be adapted through anatomical investigation and functional homologies for cross-species systematic comparisons. Here we aimed at developing a FACS for Japanese macaques, following established FACS methodology: first, we considered the species’ muscular facial plan; second, we ascertained functional homologies with other primate species; and finally, we categorised each independent facial movement into Action Units (AUs). Due to similarities in the rhesus and Japanese macaques’ facial musculature, the MaqFACS (previously developed for rhesus macaques) was used as a basis to extend the FACS tool to Japanese macaques, while highlighting the morphological and appearance changes differences between the two species. We documented 19 AUs, 15 Action Descriptors (ADs) and 3 Ear Action Units (EAUs) in Japanese macaques, with all movements of MaqFACS found in Japanese macaques. New movements were also observed, indicating a slightly larger repertoire than in rhesus or Barbary macaques. Our work reported here of the MaqFACS extension for Japanese macaques, when used together with the MaqFACS, comprises a valuable objective tool for the systematic and standardised analysis of facial expressions in Japanese macaques. The MaqFACS extension for Japanese macaques will now allow the investigation of the evolution of communication and emotion in primates, as well as contribute to improving the welfare of individuals, particularly in captivity and laboratory settings.
... As such, midfacial reduction may have arisen simply as a result of loss of pre-existing selective pressures to maintain a large midface, with cultural adaptations to climate, feeding and lifestyle being possible factors in reducing these pressures and so allowing drift and other neutral processes to impact on midfacial form. Alternatively, it can be argued that the large midface, chinless jaws and enlarged brow ridges of MP hominins make up a suite of features adapted to masticatory or paramasticatory uses 59,60,65,66 . While brow ridges have been considered to have arisen as a structural consequence of fitting a large face under the frontal bone (spatial hypothesis) 67 , it has also been argued, but is less likely, that they play a role in resisting loading of the jaws (masticatory loading hypothesis) 68 . ...
Article
Full-text available
The face is the most distinctive feature used to identify others. Modern humans have a short, retracted face beneath a large globular braincase that is distinctively different from that of our closest living relatives. The face is a skeletal complex formed by 14 individual bones that houses parts of the digestive, respiratory, visual and olfactory systems. A key to understanding the origin and evolution of the human face is analysis of the faces of extinct taxa in the hominin clade over the last 6 million years. Yet, as new fossils are recovered and the number of hominin species grows, the question of how and when the modern human face originated remains unclear. By examining key features of the facial skeleton, here we evaluate the evolutionary history of the modern human face in the context of its development, morphology and function, and suggest that its appearance is the result of a combination of biomechanical, physiological and social influences.
... Males, with their higher androgen levels, would thus be expected to have smaller chins. Instead, men have larger chins.100 Second, other species of primates do not universally follow the causal relationship between population density and social tolerance postulated by Cieri and colleagues.101 ...
Article
“Self‐domestication” has been invoked to understand important aspects of human evolution, integrating physiological, behavioral, and morphological information in a novel way. It proposes that selection for reduced aggression on animals undergoing domestication provides a model for selection favoring prosocial behaviors in humans and for a set of seemingly independent features, which arose as a result of developmental correlation. We review the history of the idea and examine patterns of domestication. A lack of empirical studies on evolutionary rates and variation thwarts meaningful comparison with domestication. The neural crest hypothesis for domestication has great explanatory power but it is difficult to test. We suggest a scenario in which the morphological byproducts of domestication can act as an honest signal of reduced xenophobia. Future studies should test if alternative explanations for the features deemed to result from self‐domestication are mutually exclusive and generate data to test predictions of these hypotheses.
... Taking in consideration the differences in both dietary and food processing habits between the australopiths and Homo, the vectors of the rates should be divergent, which we found was not the case. Intriguingly, sexual selection cannot explain the very high rates we observed in Homo sapiens and Homo neanderthalensis that are the species showing the lowest level of sexual dimorphism among primates, and the ostensibly divergent shape in Homo sapiens mandible is not shared by the Neanderthals 36,39 . ...
Article
Full-text available
Members of the hominins - namely the so-called 'australopiths' and the species of the genus Homo - are known to possess short and deep mandibles and relatively small incisors and canines. It is commonly assumed that this suite of traits evolved in early members of the clade in response to changing environmental conditions and increased consumption of though food items. With the emergence of Homo, the functional meaning of mandible shape variation is thought to have been weakened by technological advancements and (later) by the control over fire. In contrast to this expectation, we found that mandible shape evolution in hominins is exceptionally rapid as compared to any other primate clade, and that the direction and rate of shape change (from the ape ancestor) are no different between the australopiths and Homo. We deem several factors including the loss of honing complex, canine reduction, and the acquisition of different diets may have concurred in producing such surprisingly high evolutionary rates. This study reveals the evolution of mandibular shape in hominins has strong morpho-functional and ecological significance attached.
... Its presence has been associated with multiple factors (Horowitz and Thompson, 1964;Wolff, 1984;Ichim et al., 2007;Thayer and Dobson, 2010;Pampush, 2015) and with mechanical demands during biting and mastication (Daegling, 1993;Dobson and Trinkaus, 2002;Groning et al., 2011). However, recent studies question the extent to which the modern human symphysis and chin are optimized to resist masticatory system loads and suggest that, rather than being adaptive, it may be a by-product (Holton et al., 2014(Holton et al., , 2015Pampush and Daegling, 2016) of midfacial reduction. In this scenario, midfacial reduction results in posterior retraction of the alveolus relative to the basal part of the mandible due to occlusal interlocking and this is accommodated by resorptive activity in the alveolar region with depositional activity in the basal (Enlow and Hans, 1996), a pattern of remodelling that may well covary with the presence of a chin among recent hominins (Lacruz et al., 2013(Lacruz et al., , 2015. ...
Article
Modern humans have smaller faces relative to Middle and Late Pleistocene members of the genus Homo. While facial reduction and differences in shape have been shown to increase biting efficiency in Homo sapiens relative to these hominins, facial size reduction has also been said to decrease our ability to resist masticatory loads. This study compares crania of Homo heidelbergensis and H. sapiens with respect to mechanical advantages of masticatory muscles, force production efficiency, strains experienced by the cranium and modes of deformation during simulated biting. Analyses utilize X-ray computed tomography (CT) scan-based 3D models of a recent modern human and two H. heidelbergensis. While having muscles of similar cross-sectional area to H. heidelbergensis, our results confirm that the modern human masticatory system is more efficient at converting muscle forces into bite forces. Thus, it can produce higher bite forces than Broken Hill for equal muscle input forces. This difference is the result of alterations in relative in and out-lever arm lengths associated with well-known differences in midfacial prognathism. Apparently at odds with this increased efficiency is the finding that the modern human cranium deforms more, resulting in greater strain magnitudes than Broken Hill when biting at the equivalent tooth. Hence, the facial reduction that characterizes modern humans may not have evolved as a result of selection for force production efficiency. These findings provide further evidence for a degree of uncoupling between form and function in the masticatory system of modern humans. This may reflect the impact of food preparation technologies. These data also support previous suggestions that differences in bite force production efficiency can be considered a spandrel, primarily driven by the midfacial reduction in H. sapiens that occurred for other reasons. Midfacial reduction plausibly resulted in a number of other significant changes in morphology, such as the development of a chin, which has itself been the subject of debate as to whether or not it represents a mechanical adaptation or a spandrel.
... This is, however, not always the case. Whereas many morphological structures or shapes within organisms are well known, their functions are often not that clear, e.g. the function of the particular shape of the chin in humans, which puzzles anthropologists because it differs from the unpronounced shape in other primates for no obvious reason (see Pampush and Daegling 2016). This could be referred to by the 42-problem (after Douglas Adams' The Hitchhiker's Guide to the Galaxy; Vincent 2014:275): The solution is known, but what was the initial 'question'? ...
Article
Full-text available
Many successful examples of biomimetic products are available, and most research efforts in this emerging field are directed towards the development of specific applications. The theoretical and conceptual underpinnings of the knowledge transfer between biologists, engineers and architects are, however, poorly investigated. The present article addresses this gap. We use a ‘technomorphic’ approach, i.e. the application of conceptual tools derived from engineering design, to better understand the processes operating during a typical biomimetic research project. This helps to elucidate the formal connections between functions, working principles and constructions (in a broad sense)—because the ‘form-function-relationship’ is a recurring issue in biology and engineering. The presented schema also serves as a conceptual framework that can be implemented for future biomimetic projects. The concepts of ‘function’ and ‘working principle’ are identified as the core elements in the biomimetic knowledge transfer towards applications. This schema not only facilitates the development of a common language in the emerging science of biomimetics, but also promotes the interdisciplinary dialogue among its subdisciplines.
... Our study explores the hypothesis that during human fetal ontogeny the spacial changes at the back of the vocal tract are linked to the formation of the inverted-T relief. Recently, Pampush and Deagling (2016a) hypothesized that because the modern human symphysis, both lingually and labially, contains exceptionally thick tables of cortical bone, the modern human chin is not an adaptation to vocal tract constriction. In order for this hypothesis to be well-supported, it is thus expected that the anteroposterior breadth of the symphyseal basilar bone is statistically independent from spatial constriction of the vocal tract. ...
Article
Full-text available
The chin prominence is a hallmark of the modern human face and bears on its labial surface an inverted-T bony relief. Evolutionarily, whether the human chin is an adaptation for mastication or speech is debated but there is little compelling data supporting either claim. Furthermore, some suggest that the inverted-T relief is more important for phylogenetic inference than the chin prominence. However, there is no evidence for the developmental independence of the inverted-T relief and chin prominence. This debate requires empirical data on fetal development of the human chin. Using 3D imaging of the musculo-cervico-craniofacial skeleton of human fetuses and geometric morphometric methods, we discovered a developmental sequence leading to a chin prominence during early fetal development that is very similar to that which we previously observed in postnatal modern humans and in chimpanzee fetuses. Furthermore, we provide the evidence that the inverted-T relief is developmentally integrated with the chin prominence. The evolution of the human chin is constrained by cervico-craniofacial developmental that maintain an unobstructed fetal airway. Finally, the inverted T-relief should be neither treated independently from the chin prominence in phylogenetic analysis, nor is it a relevant taxonomic trait that defines the symphysis of modern humans.
Article
The primate skull is a complex bony structure that serves a variety of functions, including feeding, respiration, and communication. Features that distinguish the primate skull from that of most other mammals include orbital convergence and orbital frontation, the presence of a postorbital bar, and an increase in cranial base flexion along with a decrease in the cranial base angle. Within the Primate order, there is remarkable diversity in skull size and shape and a variety of hypotheses have been advanced to explain this diversity, including evolutionary changes in activity pattern, brain size, feeding behavior, and diet. In addition to traditional comparative studies of skull morphology, many of these hypotheses are now being tested experimentally, on primate as well as non‐primate animal models, using more sophisticated methods of data acquisition and analysis.
Article
Full-text available
The past 200,000 years of human cultural evolution have witnessed the persistent establishment of behaviors involving innovation, planning depth, and abstract and symbolic thought, or what has been called “behavioral modernity.” Demographic models based on increased human population density from the late Pleistocene onward have been increasingly invoked to understand the emergence of behavioral modernity. However, high levels of social tolerance, as seen among living humans, are a necessary prerequisite to life at higher population densities and to the kinds of cooperative cultural behaviors essential to these demographic models. Here we provide data on craniofacial feminization (reduction in average brow ridge projection and shortening of the upper facial skeleton) in Homo sapiens from the Middle Pleistocene to recent times. We argue that temporal changes in human craniofacial morphology reflect reductions in average androgen reactivity (lower levels of adult circulating testosterone or reduced androgen receptor densities), which in turn reflect the evolution of enhanced social tolerance since the Middle Pleistocene.
Article
Full-text available
Mandibular condylar cartilage is the center of greatest growth in the craniofacial complex, and is associated with maxillofacial skeleton morphogenesis and temporomandibular joint function. The condylar process grows in a wide range of directions from anterosuperior to posterior, resulting in highly diverse mandibular growth and morphology. Condylar growth direction is closely related to mandibular displacement direction and vertical jaw deviations (i.e., high or low angle). Condylar cartilage, which is ontogenetically designated secondary cartilage, differs from other primary cartilage (e.g., articular cartilage and growth plate of a long bone cranial base cartilage, nasal septal cartilage) in the following ways. (1) Condylar cartilage is a heterogeneous tissue containing fibroblasts, osteochondral progenitor cells, and chondrocytes. (2) Type I collagen, which is derived from progenitor cells, and cartilage-characteristic type II collagen are colocalized in the cartilaginous cell layer. Colocalization of both collagen types may be an adaptation to the complex biomechanical environments of condylar cartilage. (3) Peripheral condylar cartilage contains chondroid bone, a specialized calcified tissue with morphological properties intermediate between those of bone and cartilage. This hybrid tissue may play an important role in regulating different rates of bone formation in intramembranous and endochondral ossification, allowing for highly diverse growth directions and condylar and maxillofacial morphology.
Article
Full-text available
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 - the 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.
Article
Full-text available
Investigations of nonhuman primate mandibles have demonstrated that they are bent, twisted, and sheared during the power stroke of mastication. Inferences have been made regarding potential relationships between local stress patterns and the external morphology of the mandibular symphysis. This study reports the quantitative assessment of cross-sectional bone distribution patterns in the modem human symphysis by use of high-resolution microfocal X-ray computed tomography. Parameters that were examined include (1) bone substance area, (2) the ratio of bone substance to total cross-sectional area, and (3) cortical thicknesses along the perimeters of the symphyseal cross-section. The observed bone distribution was then compared with the hypothetical patterns of mechanical stress during mastication. Results showed that cortical bone was significantly thicker on the lingual than on the labial aspect of the symphysis at all superoinferior levels. The thickest cortical bone was observed on the lingual aspect of the symphysis immediately inferior to the mental spine, and labially at the mental protuberance. Bone area measurements were largest and second largest in the inferolingual and inferolabial quadrants of the symphyseal cross-section. These results show that bone is concentrated particularly at the lower lingual aspect of the symphysis, which is thought to experience high concentrations of tensile stress during mastication. Such a bone distribution pattern contributes to decreasing stress gradients in the mandibular symphysis, and therefore provides some support to the idea that bone distribution of the mandibular symphysis is in part determined by function.
Article
Full-text available
Patterns of stress were analyzed in the mandibular symphysis of Macaca fascicularis using rosette strain gages. During jaw opening, the mandibular symphysis is bent due to medial transverse bending of the mandibular corpora. Levels of stress and strain are relatively low at this time, and the source of this stress is the medially-directed component of force from the lateral pterygoid muscles. During the power stroke of mastication, the symphysis is maximally stressed. At this time the symphysis experiences dorsoventral shear and bending due to lateral transverse bending of the mandibular corpora, i.e., "wishboning." The dorsoventral shear is due to the vertical component of the balancingside adductor muscle force; the "wishboning" is due to the laterally-directed components of the bite and jaw adductor muscle forces. Unlike dorsoventral shear, "wishboning" results in considerable levels of stress and strain, particularly along the most lingual aspect of the symphysis. The most effective way to counter this stress is to increase the thickness of the symphysis in the labio-lingual direction. The stress analysis and an allometric analysis of mandibular dimensions in female cercopithecine (Old World) monkeys indicates that allometric changes in the symphysis are readily understood if the mandible is modelled as a curved beam. With increasing body size, symphyseal thickness in cercopithecines must increase in a positively allometric fashion so as to prevent the occurrence of dangerously high levels of stress along the most lingual aspect of the symphysis. This is because increasing body size is associated with three factors thathave important consequences within the context of the biomechanics of curved beams: (1) jaw length is positively allometric to body size, (2) mandibular-arch width is negatively allometric to body size, and (3) there is a tendency to use relatively greater amounts of balancing-side jaw muscle force with increased body size because of dietary changes and allometricconstraints on total jaw muscle force.
Article
Full-text available
In this article we examine the mandible of Riparo Mezzena a Middle Paleolithic rockshelter in the Monti Lessini (NE Italy, Verona) found in 1957 in association with Charentian Mousterian lithic assemblages. Mitochondrial DNA analysis performed on this jaw and on other cranial fragments found at the same stratigraphic level has led to the identification of the only genetically typed Neanderthal of the Italian peninsula and has confirmed through direct dating that it belongs to a late Neanderthal. Our aim here is to re-evaluate the taxonomic affinities of the Mezzena mandible in a wide comparative framework using both comparative morphology and geometric morphometrics. The comparative sample includes mid-Pleistocene fossils, Neanderthals and anatomically modern humans. This study of the Mezzena jaw shows that the chin region is similar to that of other late Neanderthals which display a much more modern morphology with an incipient mental trigone (e.g. Spy 1, La Ferrassie, Saint-Césaire). In our view, this change in morphology among late Neanderthals supports the hypothesis of anatomical change of late Neanderthals and the hypothesis of a certain degree of interbreeding with AMHs that, as the dating shows, was already present in the European territory. Our observations on the chin of the Mezzena mandible lead us to support a non abrupt phylogenetic transition for this period in Europe.
Article
Full-text available
Two sites of the Neandertal-associated Middle Paleolithic of Iberia, dated to as early as approximately 50,000 years ago, yielded perforated and pigment-stained marine shells. At Cueva de los Aviones, three umbo-perforated valves of Acanthocardia and Glycymeris were found alongside lumps of yellow and red colorants, and residues preserved inside a Spondylus shell consist of a red lepidocrocite base mixed with ground, dark red-to-black fragments of hematite and pyrite. A perforated Pecten shell, painted on its external, white side with an orange mix of goethite and hematite, was abandoned after breakage at Cueva Antón, 60 km inland. Comparable early modern human-associated material from Africa and the Near East is widely accepted as evidence for body ornamentation, implying behavioral modernity. The Iberian finds show that European Neandertals were no different from coeval Africans in this regard, countering genetic/cognitive explanations for the emergence of symbolism and strengthening demographic/social ones.
Article
Full-text available
Among the unique traits of human mandibles is the finding of relatively greater utilization of cortical bone with respect to other hominoids. The functional significance of this trait is not plausibly linked to masticatory demands given the diminution of the masticatory musculature in human evolution and the behavioral universal of extraoral food preparation in recent humans. Similarly, the presence of more mandibular bone is not a correlated effect of systemic skeletal robusticity, since gracilization of the skeleton is a feature diagnostic of modern humans. The mandibular symphysis in modern humans is manifested as the chin, and it is here where cortical bone hypertrophy is most pronounced. The potential covariation between the expression of the chin and bone hypertrophy is explored in an attempt to clarify their respective biomechanical roles. Current developments in skeletal biomechanics implicate low magnitude, high frequency strains in bone hypertrophy. The physiology of speech production likely produces strains in mandibular bone of greater frequency and lesser magnitude than those associated with mastication. Consequently, language acquisition plausibly accounts for cortical hypertrophy in modern human mandibles. Its role in the evolution and development of the chin is less clear.
Chapter
Wood is formed by a secondary meristem called the vascular cambium as a result of a complex interaction between environment and programmed cell development that is sensitive to both genotype and microenvironment. Wood formation involves both cell division and cell differentiation including cell expansion, secondary wall formation, lignification, and programmed cell death. As a tree gets older, the type of wood that is formed changes. During the first 10 or so years of growth, juvenile wood is formed, which is characterized typically by fast growth rate, low basic density, low stiffness, short fibers, and more abundant reaction wood. Wood has two main physiological functions: to conduct water from roots to leaves and to support the branches and crown in large trees. The wood formed by angiosperm trees shows a much greater diversity of structure and hence a more complex process of wood formation than the wood formed by conifers.
Article
Some representative vocalizations of captive rhesus monkey, chimpanzee, and gorilla were recorded and analyzed by means of sound spectrograms and oscillograms. It was found that these animals' vocal mechanisms do not appear capable of producing human speech. The laryngeal output was breathy and irregular. A uniform cross section, schwalike configuration appeared to underlie all the vocalizations. These animals did not modify the shape of their supralaryngeal vocal tracts by means of tongue maneuvers during a vocalization.Formant transitions occurred in some vocalizations, but they appeared to have been generated by means of laryngeal and possibly velar or lip movements. The nonhuman primates lack a pharyngeal region like man's, where the cross‐sectional area continually changes during speech. The data suggest that speech cannot be viewed as an overlaid function that makes use of a vocal tract that has evolved solely for respiratory and deglutitious purposes; the skeletal evidence of human evolution shows a series of changes from the primate vocal tract that may have been, in part, for the purpose of generatingspeech. Articulate speech may not have been fully developed in some of man's ancestors. The study of the peripheral speech‐production apparatus of a fossil thus may be useful in the assessment of its phylogenetic grade.
Article
— The idea of genetic assimilation, that environmentally induced phenotypes may become genetically fixed and no longer require the original environmental stimulus, has had varied success through time in evolutionary biology research. Proposed by Waddington in the 1940s, it became an area of active empirical research mostly thanks to the efforts of its inventor and his collaborators. It was then attacked as of minor importance during the “hardening” of the neo-Darwinian synthesis and was relegated to a secondary role for decades. Recently, several papers have appeared, mostly independently of each other, to explore the likelihood of genetic assimilation as a biological phenomenon and its potential importance to our understanding of evolution. In this article we briefly trace the history of the concept and then discuss theoretical models that have newly employed genetic assimilation in a variety of contexts. We propose a typical scenario of evolution of genetic assimilation via an intermediate stage of phenotypic plasticity and present potential examples of the same. We also discuss a conceptual map of current and future lines of research aimed at exploring the actual relevance of genetic assimilation for evolutionary biology.
Article
An adaptationist programme has dominated evolutionary thought in England and the United States during the past 40 years. It is based on faith in the power of natural selection as an optimizing agent. It proceeds by breaking an oragnism into unitary 'traits' and proposing an adaptive story for each considered separately. Trade-offs among competing selective demands exert the only brake upon perfection; non-optimality is thereby rendered as a result of adaptation as well. We criticize this approach and attempt to reassert a competing notion (long popular in continental Europe) that organisms must be analysed as integrated wholes, with Baupläne so constrained by phyletic heritage, pathways of development and general architecture that the constraints themselves become more interesting and more important in delimiting pathways of change than the selective force that may mediate change when it occurs. We fault the adaptationist programme for its failure to distinguish current utility from reasons for origin (male tyrannosaurs may have used their diminutive front legs to titillate female partners, but this will not explain why they got so small); for its unwillingness to consider alternatives to adaptive stories; for its reliance upon plausibility alone as a criterion for accepting speculative tales; and for its failure to consider adequately such competing themes as random fixation of alleles, production of non-adaptive structures by developmental correlation with selected features (allometry, pleiotropy, material compensation, mechanically forced correlation), the separability of adaptation and selection, multiple adaptive peaks, and current utility as an epiphenomenon of non-adaptive structures. We support Darwin's own pluralistic approach to identifying the agents of evolutionary change.
Article
Objectives: This research theoretically models and empirically records symphyseal surface strain during in vitro human mandibular wishboning (lateral transverse bending) in order to test one aspect of the hypothesis that the chin is an adaptive response to masticatory stresses. From a perspective of optimality, three questions were tested: 1) Do human mandibles function as curved beams during wishboning? 2) Is the presence of a chin associated with lower than predicted curved beam effects? 3) Are there relatively low strain gradients on the lingual and labial sym- physeal surfaces respectively? Materials and Methods: Based on morphometric criteria, theoretical wishboning strains were calculated for five dentate adult human mandibles. The same mandibles were fitted with strain gauges and subjected to simulated wishboning loads. From the empirically-recorded strains, relative strains were calculated by dividing all strains by the absolute lowest strain in a given specimen. The theoretical and empirical results were compared in order to address the three related questions guiding this research. Results: Human mandibles behave as curved beams during wishboning (question 1). Empirical strain measures showed greater disparity both between and within the labial and lingual symphyseal surfaces than the theoretical models predictions (questions 2 and 3). Discussion: Human symphyseal form, with its distinctive chin, is unlikely to be adapted for countering wishbon- ing loads. Chins are associated with larger than expected strain gradients within and between symphyseal surfaces, which runs counter to the optimality criterion typically invoked in assessing trait performance for signs of adapta- tion. The implications are twofold: 1) wishboning may not, in fact, be a regular feature of human mastication or 2) wishboning may not pose the same structural risks in human jaws as this load does in other anthropoid primates.
Article
SUMMARY The purpose of this study was to describe condylar growth and mandibular remodelling changes associated with bionator therapy. Twenty-five patients (15 males and 10 females) between 6.9 and 11.2 years of age with Class II division 1 malocclusions were randomly allocated to either control (n = 11) or treatment (n = 14; bionator only) groups and followed longitudinally for approximately 1 year. Treatment consisted of a bionator only, constructed to clear the buccal dentition by 2 mm and to position the mandible into an edge-to-edge incisor relationship. Using metallic implants for superimposition, mandibular growth, displacement, and true rotation were evaluated cephalometrically. The results showed significant changes in the direction (more posterior) but not in the overall amount of condylar growth. The bionator appliance produced greater than expected posterior drift of landmarks in the condylar and gonial regions. Cranial base superimposition showed greater than expected anterior mandibular displacement, but little or no true mandibular forward rotation with bionator therapy. The bionator appliance alone produced changes in condylar growth direction and remodelling changes associated with mandibular rotation and displacement.
Article
The presence of a prominent chin in modern humans has been viewed by some researchers as an architectural adaptation to buttress the anterior corpus from bending stresses during mastication. In contrast, ontogenetic studies of mandibular symphyseal form suggest that a prominent chin results from the complex spatial interaction between the symphysis and surrounding soft tissue and skeletal anatomy during development. While variation in chin prominence is clearly influenced by differential growth and spatial constraints, it is unclear to what degree these developmental dynamics influence the mechanical properties of the symphysis. That is, do ontogenetic changes in symphyseal shape result in increased symphyseal bending resistance? We examined ontogenetic changes in the mechanical properties and shape of the symphysis using subjects from a longitudinal cephalometric growth study with ages ranging from 3 to 20+ years. We first examined whether ontogenetic changes in symphyseal shape were correlated with symphyseal vertical bending and wishboning resistance using multivariate regression. Secondly, we examined ontogenetic scaling of bending resistance relative to bending moment arm lengths. An ontogenetic increase in chin prominence was associated with decreased vertical bending resistance, while wishboning resistance was uncorrelated with ontogenetic development of the chin. Relative to bending moment arm lengths, vertical bending resistance scaled with significant negative allometry whereas wishboning resistance scaled isometrically. These results suggest a complex interaction between symphyseal ontogeny and bending resistance, and indicate that ontogenetic increases in chin projection do not provide greater bending resistance to the mandibular symphysis. © 2015 Anatomical Society.
Article
Changes in the technology of food preparation over the last few thousand years (especially cooking, softening, and grinding) are hypothesized to have contributed to smaller facial size in humans because of less growth in response to strains generated by chewing softer, more processed food. While there is considerable comparative evidence to support this idea, most experimental tests of this hypothesis have been on non-human primates or other very prognathic mammals (rodents, swine) raised on hard versus very soft (nearly liquid) diets. Here, we examine facial growth and in vivo strains generated in response to raw/dried foods versus cooked foods in a retrognathic mammal, the rock hyrax (Procavia capensis). The results indicate that the hyrax cranium resembles the non-human primate cranium in having a steep gradient of strains from the occlusal to orbital regions, but differs from most non-anthropoids in being primarily twisted; the hyrax mandible is bent both vertically and laterally. In general, higher strains, as much as two-fold at some sites, are generated by masticating raw versus cooked food. Hyraxes raised on cooked food had significantly less growth (approximately 10%) in the ventral (inferior) and posterior portions of the face, where strains are highest, resembling many of the differences evident between humans raised on highly processed versus less processed diets. The results support the hypothesis that food processing techniques have led to decreased facial growth in the mandibular and maxillary arches in recent human populations.
Article
Any new knowledge that goes beyond the stone tools and techniques used in the Palaeolithic and Mesolithic is most significant as it reveals the cultural and technical capabilities of the people living in these periods. In 1963, two pitch finds were discovered in a lignite open-mining pit in the northern foothills of the Harz Mountains, in a layer the geological age of which was dated as being older than 80,000 years. The great significance of these finds was therefore immediately apparent. One of the finds showed a fingerprint as well as the imprints of a flint stone tool and the structure of wood cells. This was indicative of the pitch piece having served as an adhesive to secure a wooden haft to a flint stone blade. Over 30 years later these finds were transferred to the Doerner Institut for investigation. The GC and GC/MS analyses revealed that, in both cases, birch pitches, well-known historical adhesives, had been used. These consist predominantly of pentacyclic triterpenoid components of the lupane type, with betulin forming the major component. The comparison with birch bark extracts showed that the biological peak profile (bio-marker) was surprisingly well preserved in these pitch finds and that hardly any degradation products were present. Today, comparable pitches can easily be produced with modern technical methods, i.e. using airtight laboratory flasks and temperature control facilities. However, any attempt at simulating the conditions of the Neandertal period and at producing these birch pitches without any of these modern facilities will soon be met with many difficulties. This implies that the Neandertals did not come across these pitches by accident but must have produced them with intent. Conscious action is, however, always a clear sign of considerable technical capabilities.
Article
Anatomical capacity for articulate speech is primarily determined by the limitations of the oral cavity. Nonetheless, previous research and reconstructions of the vocal tracts of early hominids have focused upon the pharyngeal cavity and the phonation of vowels. Investigators have misleadingly addressed the issue of articulation with data reflecting phonation.Articulation of speech sounds is a function of lingual position in the oral cavity. Production of consonants, not reported for chimpanzees and important for intelligibility for humans, is delimited by the extrinsic tongue and suprahyoid muscles. With similar configurations and identical innervations, these muscles in chimpanzees have different inclinations or angles of insertion than their homologs in humans. Such muscular features restrict articulation of consonants in the chimpanzee.This research concentrated on the greater length of the palate and mandible in the chimpanzee as a possible model for the earliest hominids. The position of these bony structures was correlated with the position of the hyoid bone and therefore, the tongue.Through the use of comparative radiological data, variation in anatomical relationships was documented. Differing measurements between the bony structures of the oral and pharyngeal cavities reflect variation in the anatomical ability or potential to produce articulate speech.Preliminary measurements of two fossil hominids were comparable to measures of the modern human sample. Subsequent measures of fossil hominids may provide insights into their relative “position” between chimpanzees and humans with regard to their potential for articulate speech.
Article
Sima de los Huesos is one of the most complex Pleistocene sites at Sierra de Atapuerca (Burgos, Spain). This pit has yielded a number of 28 hominids dated around 400 kyr. This is the most complete collection of Middle Pleistocene Homo heidelbergensis around the world. Sima de los Huesos was never a hominid occupation place, since no traces of habitation has been discovered, nor a carnivores net, because there are not herbivores remains. However, it contains a large variety of carnivores, such as foxes, large felidae, wolfs, mustelids, and bears. The presence of these specimens may be explained as several events of natural falling, hibernation and catastrophic death, particularly clear for the bears' case. This may be supported by the fact that all these specimens are present along the whole sedimentary sequence. On the contrary, human remains are mostly concentrated inside a quite discrete sedimentary level, which cannot be explained by any kind of catastrophic nor attritional event, according with the age's profile. The recent finding of an Acheulean handaxe at the Sima de los Huesos cave site casts light on the evolution of human behaviour during the Middle Pleistocene. It is a finely flaked quartzite handaxe, which is associated with the hominid assemblage. The particular nature of the deposit involving its taphonomy, palaeontology, and technology points to a symbolic meaning both of the tool and the human accumulation. This would support the hypothesis of human mortuary practices performed at the Sima around 400 kyr ago. This discovery allows us to extend human complex behaviour and symbolism of mortuary rituals 300 kyr earlier than broadly heretofore accepted. To cite this article: E. Carbonell, M. Mosquera, C. R. Palevol 5 (2006).
Article
Feeding and speech depend on integrated movements of the jaws, tongue surface and tongue base–hyoid complex. Phylogenetically and ontogenetically, the movements of feeding antedate those required for speech. The hypothesis that speech movements would fall within the range used in feeding was tested. Lateral projection videofluorographic records were made for 10 subjects eating 8 g samples of three foods and reading a standard diagnostic speech text (Grandfather Passage). Radiopaque markers were glued to the upper and lower canines and tongue. Marker positions (Cartesian coordinates) for each video frame were plotted relative to the upper occlusal plane (X axis) and to a perpendicular dropped from that plane at the upper canine (Y axis). A plot of all coordinates per record gives the spatial domain (in the sagittal plane) within which a given marker moved. Tongue marker domains showed an extraordinary range of movement in feeding with extensive palatal contact. In speech, there was little palatal contact, and markers moved within a smaller sagittal domain. Although speech domains fell within the range for feeding, their centroids were highly statistically different, P < 0·001 (Hald test for differences in bivariate populations). In contrast, the hyoid domain for speech was anterior to that used in feeding and had almost no overlap with it (P < 0·0001). Our hypothesis is confirmed for the tongue surface markers but not for the hyoid. We conclude that patterns of hyoid movement in speech are a specific adaptation for speech. This research was supported by USPHS NIDCD Award 02123.
Article
The evolution of sexual dimorphism in quantitative characters under natural and sexual selection acting differently on the sexes is analyzed using population genetics models. The effects of genes when in males may be correlated with their effects when in females, producing correlated selective responses between the sexes, so that male and female phenotypes cannot evolve independently But under weak natural selection with constant relative fitnesses, in the absence of sexual selection, the joint evolution of the mean phenotypes of the sexes increases the mean fitness in a population, and if there is genetic variation for sexual dimorphism each sex eventually achieves a locally optimum phenotype. With sexual selection, fitnesses are generally frequency-dependent and evolution of the mean phenotypes does not maximize the mean fitness. Provided individual fitnesses exist, at equilibrium natural and sexual selection balance in each sex. A moderate intensity of sexual selection acting on a character under weak natural selection toward an intermediate optimum phenotype can produce a large deviation of the mean phenotype from the optimum and a substantial decrease of the mean fitness in a population, increasing the probability of extinction. When homologous characters in males and females vary similarly and are highly correlated genetically, the rate of evolution of sexual dimorphism may be one or more orders of magnitude slower than that for the average phenotype in a population. Methods for partitioning sexual dimorphism into contributions from natural and sexual selection are discussed, and genetic experiments are suggested for testing the involvement of non-equilibrium correlated selective responses between the sexes in observed cases of sexual dimorphism.
Article
Inferred patterns of mandibular bone stress derived from in vivo strain data are related to size and shape variation in the mandibular corpus of Macaca fascicularis . Size and shape are described in mechanical terms from the cross-sectional contours of compact bone at seven locations along the mandibular corpus. Area moments of inertia and section moduli are related to bending moment arms, cortical area is related to variation in total subperiosteal area, and an estimate of torsional strength is calculated. The distribution of masticatory stress in the macaque corpus is hypothesized to be highly inequitable; the symphysis and the most posterior region of the corpus (i.e., at the M2 and M3) are probably subjected to the highest normal stresses in lateral and parasagittal bending, respectively. The distribution of torsional shear stress in the mandible is difficult to model theoretically, although it is likely that these stresses also vary in magnitude in different regions of the corpus. These data indicate that the inference of masticatory stress patterns from purely morphological criteria in the absence of experimental corroboration is problematic. It is unlikely that regional variation in corpus mechanical properties directly reflects local differences in the relative magnitudes of different masticatory stress regimes.
Chapter
No matter how a primate acquires its food and what sorts of things it eats, mastication is an important step in food processing. According to Crompton and Hiiemae (1969), Hiiemae (1978, this volume), Hiiemae and Kay (1973), Kay and Hiiemae (1974), Beyron (1964) and Ahlgren (1966), chewing is done in all primates (so far investigated) in largely the same fashion. In a cyclic movement, the mandible is lowered (opening stroke), adducted (closing stroke), and finally moved in the power stroke upwards and inwards with the lower teeth exerting compressive and shearing forces on the food particles pressed against the upper teeth. This is done on one side only, called the biting side.
Chapter
All attempts to solve the “chin problem”, and understand the reasons why modern humans have a symphysis which differs in shape from that of all other primates, must necessarily incorporate a theoretical understanding of the mechanical principles involved in the lower jaw. Based on a knowledge of the kinetics of mastication provided by Hiiemae (1966, 1967, 1976, 1978), Hiiemae and Crompton (1971), Hiiemae and Kay (1973), Kay and Hiiemae (1974) and on the experimental results obtained and evaluated by Hylander (1975, 1978, 1979a, 1979b, 1979c), I have worked out a mathematical formulation on the principle of stresses involved with the mandible (Wolff, 1982).
Article
Traditionally, thick enamel has often been used to infer durophagy (i.e., hard nut and seed consumption) in extinct hominins. These inferences are based on the hypothesis that thick enamel is primarily an adaptation to prevent tooth fracture or chipping resulting from high-stress loads produced during the mastication of large hard foods. An alternative view argues that thick enamel may aid in maintaining tooth function in the face of gradual dental wear from grit, phytoliths and acid, which may be found in foods of widely varying hardness. We use estimates of primate dietary abrasiveness and recorded lifespan to test the hypothesis that enamel thickness is selectively responsive to lifetime dental wear resistance. We use data from the literature to relate enamel thickness to measures of dietary abrasiveness, diet profiles, and longevity for 17 primate species and performed linear regression using several combinations of these variables. We found a positive association between lifetime dietary wear and enamel thickness, suggesting that thick molar enamel in primates may have evolved as a means to resist wear apart from selection to resist tooth fracture. Assuming our estimates of lifetime dietary wear are accurate, we caution against ascribing thick enamel solely to the presence of hard-object feeding in paleoanthropological contexts without also considering primate lifespan and other aspects of feeding ecology.