This thesis represents, to date, the most comprehensive investigation into the influence of integration (covariation) and modularity (the organisation of integrated units) on the morphological evolution of the primate pelvis. The concepts of integration and modularity are core tenets of evolutionary biology, yet their evolutionary role remains poorly understood. In this thesis, I quantified primate pelvis morphological variation across 4 clades encompassing the main primate locomotory specialisations. Shape was captured in detail, using a surface-based geometric morphometric approach, to test five alternative models of pelvis organisation, calculate integration levels, and reconstruct pelvis evolution.

In this thesis, I demonstrate that the primate pelvis is dominantly modulated by developmental pathways, with ilium, ischium, pubis, acetabulum, and sacrum having the capacity to vary and evolve in a relatively independent manner (Chapter 2). This main modular pattern of primates is different to that of carnivores where in the latter group the ischium and pubis covary more closely together. The pubis-ischium parcellation is present in all examined primate phylogenetic groups (Lemuroidea, Ceboidea, Cercopithecoidea, and Hominoidea – humans excluded), suggesting that this parcellation was present in basal primates. Notably, a significant modular signal is also present for the functional hypothesis (locomotion-obstetrics). This suggests that the bony birth canal may vary and evolve relatively independently from the rest of the pelvis shape, alleviating the obstetric dilemma. Overall, this study demonstrates that the modularity pattern of the primate pelvic girdle is not simply limited to its developmental units. Instead, I find modular patterns acting in a complex multi-layered way, with developmental processes synergistically meeting functional needs.

Few studies have tried to explicitly clarify the role of integration plays in morphological variability and the evolutionary consequences this entails. In Chapter 3, I calculated the integration levels and tested whether integration may constrain or facilitate evolutionary flexibility and diversity. I found an inverse relationship between integration magnitudes and disparity levels, indicating that the impact of primate pelvis integration is best supported by the hypothesis of constraint across the primate order, its phylogenetic and locomotory groups. My findings highlight the need to consider the impact of integration when modelling shape changes and reconstructing evolutionary pelvic trajectories.

In Chapter 4, I examined the role of integration in the morphological divergence of the human pelvis. Human integration levels are marked by a reduction across its developmental and functional pelvic constituents compared to the other sampled primates (Gorilla beringei, Hylobates lar, Pan paniscus and Macaca mulatta). The reduction of inherent human constraint is paired with elevated levels of disparity, indicative of inherent high levels of evolvability present within the human pelvis. Particularly of interest is the low integration signal between the human pubis and ischium, yet the integration levels within these elements are remarkably high. In the case of the pubis, this translates into limited evolutionary possibilities and reduced disparity. Conversely, the high ischium integration acts as a facilitator to morphological disparity, aiding evolutionary responsiveness. The increased evolutionary flexibility of the human ischium played a pivotal role in both bipedal efficiency and increased levels of sexual dimorphism, whereby ischium disparity is also an important aspect in easing parturition. The reduced integration levels between the human developmental and functional pelvis modules provide its pelvic bauplan with increased flexibility to respond to multiple selective pressures, facilitating the complex morphological modifications and divergence of the human pelvis along an evolutionary trajectory that may have otherwise been difficult or even impossible to achieve.

This thesis represents a significant advance in the study of pelvic modularity and morphological evolution. Chapters 2 and 3 form a comprehensive baseline for primate pelvis structuration and integration magnitudes, providing an in-depth exploration of hypotheses of modularity and the impact of integration on macroevolutionary patterns. The thesis is also novel in that it investigates developmental and functional integration patterns, and does so across and within species. This provides a multi-layered view of the role of modularity and integration of the primate pelvic girdle.