This dissertation project explores the links between the ultimate drivers of variation in primate growth rates and their proximate (i.e., hormonal) underpinnings via a hard-tissue structure, the sella turcica. In doing so, it proposes a novel, non-destructive method for estimating individual somatic growth rates, which are presently difficult to infer in the hominin and primate fossil records. It also investigates the inter- and intraspecific effects of ecology and environment on the growth rates of extant primates.The ultimate causes, or selective pressures, shaping growth rate have long been the subject of anthropological research, but the proximate mechanisms that underpin variation in growth rate are less well studied. At the proximate level, somatic growth is the direct result of hormones produced by endocrine glands such as the pituitary. This project builds upon the relationship between the size of the pituitary, which is positively correlated with growth rate across mammalian taxa, and the sella turcica, the bony structure within which the pituitary gland is housed. By pairing 3D cranial morphology data with growth data from a well-studied primate population, this research tests whether the size of the nonhuman primate sella turcica reflects somatic growth rate. It also assesses how aspects of ecology and demography (i.e., ultimate causes such as resource availability, food quality, extrinsic mortality) relate to somatic growth rates both within the study population and across a comparative sample of 51 extant primate species. It further explores whether these ecological variables also explain variation in relative sella turcica size; together, the complementary components of this dissertation contribute to a better understanding of primate growth.

This dissertation is an historical analysis of the science of human origins, paleoanthropology, examining the intersection of science and culture around fossil human ancestors (hominins) over the last century and a half. Focusing on fossils as scientific objects, this work examines three controversial fossils from the science’s history asking, how do fossils formulate, challenge, and reconfigure notions of what it means to be human? The introduction reviews the historiography of paleoanthropology and the gaps that exist in the literature. Chapter two examines the first case study, the type specimen of Homo neanderthalensis, known as the Feldhofer Neanderthal, providing a biography of the object from its discovery in Germany in 1856 until its species designation in 1864. Chapter three briefly links the Neanderthal’s story in time and space to the next fossil’s story. Chapter four picks up the story of paleoanthropology in 1924 in South Africa, with the discovery and initial analysis of a specimen nicknamed the Taungs Baby, which was labeled a new hominin species, Australopithecus africanus. Chapter five is another brief chapter connecting the Taungs Baby story in time and space to the final specimen examined in this work at the end of the century. Chapter six examines the final case study, a specimen discovered in 2003 in Indonesia, designated a new species named Homo floresiensis and nicknamed the Hobbit. Through comparing contrasting, and connecting the stories of these three specimens, three major conclusions emerge about the field. First, the fossils themselves play an important role in knowledge production about the hominin past. Second, scientific practice shaped both interpretations of fossils and larger questions of what it means to be human. Third, the scientific practice is itself shaped by local culture, which continually interacts with attempts to establish a global perspective about the human past. The perspective gleaned through the eyes of these three fossils therefore reveals the way shifting, rather than eternally true, claims are embedded in culture and intertwined with the perspectives of the humans conducting the science.

As the junction between the head and the trunk, the neck functions in providing head stability during behaviors like feeding to facilitating head mobility during behavior like grooming and predator vigilance. Despite its importance to these vital behaviors, its form and function remain poorly understood. Fossil hominin cervical vertebrae preserve a striking diversity in form despite the commitment to orthograde bipedality. Do these differences in cervical vertebral form correspond to functional variations among our recent ancestors? This dissertation attempts to understand 1) how does the neck function in head stability and mobility 2) how do these functions relate to cervical vertebral form. Kinematic and passive range of motion studies were conducted in several species of primate to obtain measures of function which were subsequently related to skeletal form.

Results show that cervical vertebral morphology does not significantly covary with differences in joint mobility. Rather, they implicate the critical role of ligaments and muscles in facilitating head mobility. Results of the kinematics study show that the neck plays a role in maintaining head stability during locomotion. However, the kinematic data do not significantly correlate with morphological variation among primate species. Given the negative results of the extant morphological analyses, it is difficult to apply them to the fossil record. As such, the functional significance of the disparate morphologies found in the hominin fossil record remain ambiguous.

I argued that the development of the Anterior Inferior Iliac Spine (AIIS), an entirely novel trait unique to the hominin pelvis, signaled a critical transformation from facultative (occasional) to obligate (exclusive) bipedality. The species that were considered included Homo sapiens, Australopithecus afarensis (Lucy), Ardipithecus ramidus (Ardi) and Pan troglodytes (chimpanzee); Ar. ramidus is believed to have been a facultative biped while both A. afarensis and H. sapiens were/are obligate bipeds, a stark contrast from the upright Bent-Hip Bent-Knee gait seen in chimpanzees, an organism that lacks an AIIS. It was found that the AIIS served a significant function in the advent of bipedality from Pan to Ardi because it allowed higher attachment for the rectus femoris muscle, a crucial knee extensor; however it is not heavily implicated in the transformation from facultative to obligate bipedality. Moreover, the appearance of the AIIS, first seen in Ardi, likely occurred following the lumbosacral changes that positioned the hominin body in an upright position so that the body's center of mass remained balanced over its supporting base. This provided the framework necessary to further select for organisms that had the AIIS and could walk upright, which perpetuated this change in the hominin lineage.

Many of the derived features of the human skeleton can be divided into two adaptive suites: traits related to bipedalism and traits related to encephalization. The cervical spine connects these adaptive suites and is itself unique in its marked lordosis. I approach human cervical evolution from three directions: the functional significance of cervical curvature, the identification of cervical lordosis in osteological material, and the representation of the cervical spine in the hominin fossil record.

Mammals with a habitually orthograde trunk posture possess a more anterior foramen magnum than mammals with non-orthograde trunk postures. Russo & Kirk (2013) also found that bipedal orthograde mammals possess a more anteriorly placed foramen magnum than those that are just habitually orthograde. This finding has allowed us to use foramen magnum position as a predictor of trunk posture in early hominins. This prompts more research of how the other landmarks on the cranial base move in relation to this shift in foramen magnum positioning. I collected landmark data on images of 125 mammalian basicrania spanning 41 species that differed in trunk posture. Using Procrustes and Principal Components Analysis (PCA), I attempted to evaluate the effects of trunk posture on basicranial morphology, primarily focusing on the placement of the carotid and jugular foramina. The results supported Russo and Kirk's finding of a more anterior foramen magnum placement in orthograde mammals; in addition, the results displayed correlations between foramen magnum position and carotid foramen position among primates and diprotodonts.

Previous studies have demonstrated that cranial base anatomy is influenced primarily by three different characteristics: brain shape, positional behavior, and facial growth (Lieberman et al. 2000). Although the timing of cranial base growth is not fully understood, features of the cranial base are frequently used to interpret the hominin fossil record (Guy et al. 2005; White et al. 1994; Brunet et al. 2002). While specific aspects of cranial base morphology may be species-specific, there is sparse information on the developmental mechanisms driving these adult morphologies. The aim of this study is to 1) examine changes in the human cranial base form throughout ontogeny and 2) determine their relationship to the development of positional behavior and brain growth. This research asks: to what extent does human cranial base morphology vary before and after adult positional behavior is acquired? The null hypothesis is that there is no relationship between features of the cranial base and the development of positional behavior. Data are collected using 3D landmarks on n=35 human crania and analyzed with both Morphologika (O'Higgins and Jones 1999) and MorphoJ (Klingenberg 2011) to identify age related changes in shape. Results of this study demonstrate that most of the changes in cranial base form occur between dental eruption stages N and NJ1 between 0 and 2 years of age. These changes consist of a relative shortening of the anterior-posterior cranial base length, a more posterior positioning of the foramen magnum, and a more anterior position of the occipital condyles and separate the N and NJ1 dental development groups from other groups. This change coincides with the transition to upright posture in human children (Abitbol 1993), a significant period of brain growth (Neubauer 2009) and has implications for reconstructing positional behavior in fossil hominins. Despite new insights into the development of cranial base morphology, the utility of the cranial base in assigning hominin taxonomy remains inconclusive.

This dissertation begins to lay out a small slice of the history of morphological research, and how it has changed, from the late 19th through the close of the 20th century. Investigators using different methods, addressing different questions, holding different assumptions, and coming from different research fields have pursued morphological research programs, i.e. research programs that explore the process of changing form. Subsequently, the way in which investigators have pursued and understood morphology has witnessed significant changes from the 19th century to modern day research. In order to trace this shifting history of morphology, I have selected a particular organ, teeth, and traced a tendril of research on the dentition beginning in the late 19th century and ending at the year 2000. But even focusing on teeth would be impossible; the scope of research on this organ is far too vast. Instead, I narrow this dissertation to investigation of research on a particular problem: explaining mammalian tooth morphology. How researchers have investigated mammalian tooth morphology and what counts as an explanation changed dramatically during this period.

Modern primate diet is well-studied because of its considerable influence on multiple aspects of morphology, including the shape of the facial skeleton and teeth. It is well-established that differences in craniofacial form influence feeding abilities by altering the nature of bite force production. Tooth morphology, likewise, has been shown to vary with diet across primates, particularly in the details of occlusal form. It has also been suggested that tooth form (e.g., tooth root size and shape and crown size) reflects, in part, the demands of resisting the stresses generated during feeding. However, while they are central to our efforts to infer diet in past species, the relationships between bite force production, craniofacial morphology and tooth form are not well-established. The current study is separated into two parts. In Part I, the hypothesis that crown size and root surface area are adapted to resist masticatory stress is evaluated by testing whether these features show correlated variation along the tooth row in a taxonomically diverse sample of primates. To further explore the adaptive nature of this correlation, pair-wise comparisons between primates with mechanically resistant diets and closely-related species consuming less resistant foods are performed. If crown size and root surface area covary along the tooth row, past research suggests they may be related to bite force. To test this hypothesis, Part II examines the variation of these dental characteristics in comparison to theoretically-derived bite force patterns along the tooth row. Results suggest that patterns of maximum bite force magnitude along the tooth row are variable both within and between species, underscoring the importance of individual craniofacial variation on masticatory force production. Furthermore, it is suggested that some adaptations traditionally associated with high bite force production (i.e., facial orthognathy) may increase anterior biting force at the expense of posterior biting force. Taken together, results from the current study reveal that both tooth root and crown size vary in conjunction with the mechanical properties of diet and with bite force patterns along the tooth row in anthropoids.