Understanding the forces that shaped the appearance and development of modern humans has been a leading goal of biological anthropology for decades. As technology has improved, our capability to investigate key questions about the factors affecting the shape of our anatomy have advanced significantly. Here, an interdisciplinary team of anthropologists and engineers will use engineering and experimental methods to examine how the shape of the skull has evolved in order to adapt to the forces associated with feeding on different types of food items. Specifically, the researchers will take a highly interdisciplinary approach to examining whether the skulls of these early humans were well designed to crack open and chew such hard, brittle objects. Dietary adaptations are thought to have been critical factors influencing the course of early human evolution, so this research project will provide valuable insights into the functional anatomy, diet, ecology and behavior of the earliest human ancestors.
With respect to intellectual merit, this project will: (a) examine the functional and evolutionary relationships between diet and skull form, (b) test a leading hypothesis explaining the evolution of the earliest humans, (c) collect and integrate multiple types of raw data critical to an understanding of feeding biomechanics, (d) develop methods for the rapid construction of engineering models that can be applied to research questions in a wide range of disciplines, (e) integrate ecological, comparative, experimental, and engineering techniques for the investigation of evolutionary questions, and (f) rapidly disseminate data, models and findings to the scientific community.
With respect to broader impacts, this study will: (a) promote interdisciplinarity, diversity and internationalism in science, (b) collect data about skull biomechanics that are relevant to dentistry and craniofacial medicine, (c) support the research of three junior investigators each in the first year of their academic appointments, (d) support female graduate students at several universities, (e) provide support to undergraduates at a university whose student body has a high proportion of minorities, (f) provide training for international students in developing nations (Brazil, Suriname), which will ultimately support the development of scientific infrastructure and institutions in those countries, (g) provide content to an exhibit focusing on human biology and evolution at the Georgia Children?s Museum, (h) using engineering models, limit the need for, or at least increase the analytical power of, future experimental studies requiring the use of live animals, (i) generate data relevant to conservation efforts by documenting the relationship between ecology and adaptation in certain primates, (j) strengthen collaborations between anthropologists and engineers in ten universities and two countries, (k) heighten awareness in the engineering community about how their methods are applicable to evolutionary questions.
Bones in the skull are separated by sutures - a kind of joint made up of soft tissues, and thus a soft area compared to rigid bones. Sutures are found to be important growth sites, yet their biomechanical significance is an open question, because their function during chewing activities is unclear, and the effects of their fusion status and placements have not being systematically studied. It is hypothesized that sutures act as energy absorbers protecting skulls. In order to test this hypothesis, biomechanical and morphometric analyses were conducted to examine, [1] the biomechanical impact of craniofacial sutures with under different conditions with variants including sutural fusion status, sutural elastic properties, and loading regimes, and [2] morphological impact of the placement of sutures. First, the biomechanical effects of sutures were tested using Finite Element Analysis, a powerful computer-aided program that allows the modeling of sutures in the context of an assessment of synchronous global strain patterns using various loading designs. [1] Static Analysis - Using loading simulations corresponding to incisor, premolar, and molar maximal biting conditions, a sensitivity analysis of the mechanical effect of sutures in Finite Element models of a macaque cranium with eight bone-suture functional units representing eight facial sutures. . Results demonstrate that the presence of sutures does not profoundly influence global strain patterns, regardless of their material properties and fusion status (Fig. 1). More specifically, strain patterns remained relatively unaffected away from the suture sites, and bite reaction force was likewise barely affected. Thus, the biomechanical significance of sutures in a global context would therefore appear to be limited. [2] In simulation of physiological dynamic loading conditions, the biomechanical effects of the zygomaticotemporal suture in a juvenile Rhesus macaque cranium were tested (Fig. 2). The dynamic analyses produced similar results compared to static simulations in terms of strain patterns and reaction forces, indicating that the zygomaticotemporal sutures have limited impact on global skull mechanics regardless of loading design (Fig. 4). Thus, contrary to the functional hypothesis tested, sutures did not absorb significant amounts of energy during either static or dynamic simulations. Second, based on findings from mechanical simulations of sutures, it is alternatively hypothesized that sutures are mechanically significant only insofar as they are weak points on the cranium that must be shielded from unduly high stresses so as not to disrupt vitally important growth processes. This derived hypothesis is tested in two projects. [1] The placement of a facial suture in the face (maxilla-zygomatic suture/MZS) was investigated in Old World Monkeys (OWM) and New World Monkeys (NWM). Results demonstrated that the MZS in NWM has a more lateral position compared to that in OWM. Consequently, the ratio of facial surface vs. temporal surface of the zygoma in NWM is relatively smaller than that in OWM, which is coupled with different configuration patterns in the orbital and pterion areas. Variation is also present within closely related taxa. For example, the MZS is more laterally placed in Cebus apella than in C. albifrons. These findings suggest different bone interaction patterns related to differences in dietary ecology. The significance of the placement of sutures thus warrants careful ontogenetic, phylogenetic, and biomechanical studies. [2] The impact of naturally developed extra sutures in the highly stressed in the midface, such as the zygoma (divided zygoma with intrazygomatic suture), which is a naturally-happened experiment defying the derived hypothesis. Investigations of the divided zygoma condition were conducted in Rhesus macaques and great apes. Results demonstrated that, [1] there is relatively low incidence of divided zygoma in Rhesus macaques (0.3%), no presence in gorillas and common chimpanzees, yet high in orangutans (3% - compared to that of humans with incidence of 3% or higher in Eastern Asian populations). Morphometric analysis in macaques demonstrate that overall the cheek with divided zygoma is smaller than that of the normal side, yet the zygomatic part of the zygomatic arch at the affected side is significantly stockier than that at the normal side (Fig. 3), which is in line with the results of a simulation using Finite Element Analysis - normal dietary activities may induce unduly high stress in the lower midface (Fig. 4). Thus the presence of the intrazygomatic sutures in divided zygoma condition and has an unfavorable impact on the midface in both growth and function. Insights into this rare and unfavorable divided zygoma condition in the midface may deepen our understanding of the craniofacial form and evolution. The mechanism behind the divided zygoma phenomenon is unclear, disturbed normal developmental process or hybridization-related genetic background is speculated. In conclusion, sutures do not absorb significant amounts of energy during either static or dynamic simulations, they are mechanically significant only insofar as they are weak points on the cranium that must be shielded from unduly high stresses so as not to disrupt vitally important growth processes.
