This chapter examines the role of selection in driving certain aspects of pelvic morphology, particularly the differences between mediolateral breadths and anteroposterior breadths. The chapter is divided into three sections, representing the three key selection pressures researchers have spent the most time on – namely, obstetrics, locomotion and thermoregulation. Data for the role of each of these on pelvic morphology are considered, as is discussion of the myriad ways human populations have mixed and matched morphological traits to manage these selection pressures. Clearly, there is not a single strategy for handling the interactive nature of these pressures.

In this chapter, we discuss evidence about the evolutionary forces that have shaped the evolution of the human pelvis, both in its entirety as well as portions of the pelvis, focusing on studies that have investigated pelvic evolution using experimental and quantitative genetic methods. These methods are tied to information from Chapter 4 about pelvis development, with emphasis placed on the importance of understanding the difficulty of tying development and growth with evolutionary processes. Special attention is placed on the concept of the palimpsest. Further, we review these findings in light of three principal hypotheses broadly offered about the processes that selected for pelvic shape (as reviewed in Chapters 2 and 3): locomotion, obstetric sufficiency and thermoregulation. We show from multiple studies that the human pelvis evolved in response to natural selection as well as through neutral evolutionary processes (e.g. genetic drift). A key conclusion from these studies is that parts of the pelvis evolved in different manners in response to these (and other) selection factors; thus, the shape of the human pelvis reflects a modular response to various sources of selection.

This chapter provides an overview of the anatomy of the primate pelvis, with a particular focus on the features of the hominoid (ape) and human pelvic morphologies. Underlying sources of morphological variation such as phylogenetic signals, sexual dimorphism and obstetric function are examined, as well as general patterns of pelvic anatomical variation within Homo sapiens.

In light of the various sources of evidence presented in the preceding chapters, we are left to conclude that the human (in the broadest sense of recent humans and their ancestors) pelvis represents various experiments in evolution. A diversity in pelvic sizes and shapes has marked hominin history, as each population and each species responded to selection pressures in sometimes unique and sometimes convergent ways. These were situated in the distinctive population histories of each of these groups, creating a mosaic of patterns underlain both by responses to evolution and changing patterns of covariance within development. To understand the diversity we observe among fossils, as well as variation within recent humans, we must therefore cultivate a multidisciplinary expertise in biomechanics, kinematics, fossil evidence, developmental biology and evolutionary theory. This book represents an attempt at bringing together these various sources of evidence to better understand the factors, patterns and potential processes that shaped the evolution of the pelvis.

Even before the focus on bipedalism as the ‘hallmark’ of the human lineage (Robinson, 1972), interest in the pelvis was stimulated by discussions across different disciplines, including the growing field of obstetrics (see review in Walrath, 2003), as well as by multiple fossil discoveries (Pycraft, 1930; Dart, 1949). It was also clear at the outset that the pelvis was going to serve as a crucial part of the evolutionary history of humans, given that it had obvious functional implications in its role in locomotion, which included dramatic differences between other species grouped with humans taxonomically and then phylogenetically, including the African and Asian apes.

To investigate pelvis evolution and to understand the sources of its variation, we must comprehend its development. We review developmental processes that form the pelvis from three perspectives: cell layers and tissues, genomic information and overall growth. At tissue level, the pelvis forms largely from lateral plate mesoderm as well as from somites. The os coxa forms from cartilaginous precursor tissues that grow and ossify, especially in two major regions that become the ischiopubis and the ilium. The sacrum and coccyx form from multiple ossification centres. Numerous secondary ossification centres form, with the last fusing by the mid-twenties. The importance of multiple genes and molecular factors are discussed, including interactions among and differences in timing and locations of expression, emphasising Islet1, Emx2, Pbx and Hox. We review the importance of differences in timing of ossification among skeletal elements, their interactions with mechanical loading and sex hormones, and environmental factors affecting individual growth. These processes are linked to morphological integration and are the origins of the morphological variation on which evolutionary forces act.

This chapter examines the fossil record of hominoid and hominin pelvic remains from the Miocene through to the Late Pleistocene. The interpretation of functional demands shaping hominin pelvic morphology including locomotion, obstetrics and thermoregulation are discussed, as well as evidence for pelvic sexual dimorphism in hominin species. The long-standing view of a relatively linear pattern of hominin pelvic evolution from Australopiths, through early Homo, to Neanderthals, broken only by the appearance of the somewhat divergent morphology of Homo sapiens is examined in light of recent fossil pelvis discoveries that point to greater diversity in the hominin pelvic morphology. These fossils add to evidence from elsewhere in the postcranium that indicate there were multiple ways to be a bipedal hominin.