Evolution of the human birth canal.

It seems puzzling why humans have evolved such a small and rigid birth canal that entails a relatively complex process of labor compared with the birth canal of our closest relatives, the great apes. This study reviewed insights into the evolution of the human birth canal from recent theoretical and empirical studies and discussed connections to obstetrics, gynecology, and orthopedics. Originating from the evolution of bipedality and the large human brain million years ago, the evolution of the human birth canal has been characterized by complex trade-off dynamics among multiple biological, environmental, and sociocultural factors. The long-held notion that a wider pelvis has not evolved because it would be disadvantageous for bipedal locomotion has not yet been empirically verified. However, recent clinical and biomechanical studies suggest that a larger birth canal would compromise pelvic floor stability and increase the risk of incontinence and pelvic organ prolapse. Several mammals have neonates that are equally large or even larger than human neonates compared to the size of the maternal birth canal. In these species, the pubic symphysis opens widely to allow successful delivery. Biomechanical and developmental constraints imposed by bipedality have hindered this evolutionary solution in humans and led to the comparatively rigid pelvic girdle in pregnant women. Mathematical models have shown why the evolutionary compromise to these antagonistic selective factors inevitably involves a certain rate of fetopelvic disproportion. In addition, these models predict that cesarean deliveries have disrupted the evolutionary equilibrium and led to new and ongoing evolutionary changes. Different forms of assisted birth have existed since the stone age and have become an integral part of human reproduction. Paradoxically, by buffering selection, they may also have hindered the evolution of a larger birth canal. Many of the biological, environmental, and sociocultural factors that have influenced the evolution of the human birth canal vary globally and are subject to ongoing transitions. These differences may have contributed to the global variation in the form of the birth canal and the difficulty of labor, and they likely continue to change human reproductive anatomy.

The cliff edge model of the evolution of schizophrenia: Mathematical, epidemiological, and genetic evidence.

How has schizophrenia, a condition that significantly reduces an individual's evolutionary fitness, remained common across generations and cultures? Numerous theories about the evolution of schizophrenia have been proposed, most of which are not consistent with modern epidemiological and genetic evidence. Here, we briefly review this evidence and explore the cliff edge model of schizophrenia. It suggests that schizophrenia is the extreme manifestation of a polygenic trait or a combination of traits that, within a normal range of variation, confer cognitive, linguistic, and/or social advantages. Only beyond a certain threshold, these traits precipitate the onset of schizophrenia and reduce fitness. We provide the first mathematical model of this qualitative concept and show that it requires only very weak positive selection of the underlying trait(s) to explain today's schizophrenia prevalence. This prediction, along with expectations about the effect size of schizophrenia risk alleles, are surprisingly well matched by empirical evidence. The cliff edge model predicts a dynamic change of selection of risk alleles, which explains the contradictory findings of evolutionary genetic studies.

The variation in shape and thickness of the pelvic floor musculature in males and females: a geometric-morphometric analysis.

INTRODUCTION AND HYPOTHESIS: In women, the risk of pelvic floor prolapse is known to be associated with age and parity. Different studies suggested that it is also related to pelvic dimensions, e.g. biomechanical modelling showed that a larger pelvic canal results in higher values of displacement, stress and strain in the pelvic floor muscles, which can increase the risk of pelvic floor disorders. To better understand the multiple factors contributing to pelvic floor disorders, we assessed how age, body weight, body height, parity (in women), pelvic canal size and overall muscle development affected pelvic floor geometry. METHODS: A comprehensive geometric morphometric analysis of variation in pelvic floor muscle shape was conducted based on a dense set of 3D landmarks measured on CT scans in a cohort of 49 deceased men and 52 deceased women. The multivariate association between biological variables (parity, dimensions of the true pelvis, age, body weight, height) and pelvic floor muscle morphology was explored by reduced rank regression in both sexes. RESULTS: In women, advanced age, high body weight relative to body height and a large pelvic canal were associated with a deeper pelvic floor. Surprisingly, parity did not have any strong association with overall pelvic floor shape. In men, high body weight was associated with a deep pelvic floor. Age had little effect on male pelvic floor shape, except for the thickness of the ischiocavernosus muscle, which reduced with age. CONCLUSION: These results suggest that age, relative body weight and the size of the pelvic canal contribute to the risk of female pelvic floor disorders via their effect on pelvic floor shape, independently of birth-related factors such as injury and avulsion of pelvic floor muscles.

Maxillary lateral incisor agenesis is associated with maxillary form: a geometric morphometric analysis.

BACKGROUND AND OBJECTIVE: Agenesis of the maxillary lateral incisor occurs in up to 4% of all individuals and requires multidisciplinary treatment. Its developmental origins, however, are not fully understood. Earlier studies documented genetic factors contributing to agenesis but also an association with craniofacial morphology. In this study, we assessed the association between maxillary morphology and lateral incisor agenesis by a geometric morphometric approach to disentangle the roles of developmental plasticity and genetic factors. MATERIALS AND METHODS: We quantified the maxillary alveolar ridge by 19 two-dimensional landmarks on cross-sectional images of 101 computed tomography scans. We compared the shape and size of the alveolar ridge across patients with unilateral or bilateral agenesis of maxillary lateral incisors and patients with extracted or in situ incisors. RESULTS: The maxillary alveolar ridge was clearly narrower in patients with agenesis or an extracted incisor compared to the control group, whereas the contralateral side of the unilateral agenesis had an intermediate width. Despite massive individual variation, the ventral curvature of the alveolar ridge was, on average, more pronounced in the bilateral agenesis group compared to unilateral agenesis and tooth extraction. CONCLUSIONS: This suggests that pleiotropic genetic and epigenetic factors influence both tooth development and cranial growth, but an inappropriately sized or shaped alveolar process may also inhibit normal formation or development of the tooth bud, thus leading to dental agenesis. CLINICAL RELEVANCE: Our results indicate that bilateral agenesis of the lateral incisor tends to be associated with a higher need of bone augmentation prior to implant placement than unilateral agenesis.

The microstructure and the origin of the Venus from Willendorf.

The origin and key details of the making of the ~ 30,000 year old Venus from Willendorf remained a secret since its discovery for more than a hundred years. Based on new micro-computed tomography scans with a resolution of 11.5 µm, our analyses can explain the origin as well as the choice of material and particular surface features. It allowed the identification of internal structure properties and a chronological assignment of the Venus oolite to the Mesozoic. Sampling numerous oolite occurrences ranging ~ 2500 km from France to the Ukraine, we found a strikingly close match for grain size distribution near Lake Garda in the Southern Alps (Italy). This might indicate considerable mobility of Gravettian people and long-time transport of artefacts from South to North by modern human groups before the Last Glacial Maximum.

Age dependent changes in pelvic shape during adulthood.

The human pelvis has been reported to change in shape throughout adult lifetime, and also the expression of parturition scars, or "pelvic features", increases with age. However, little is known about the causes and timing of these changes. Here we investigate changes in pelvic shape and the expression of pelvic features by applying a comprehensive geometric morphometric approach to a modern sample of 167 adult individuals with known age, parity, and cause of death. Our results confirm that the pelvis changes in shape during adult life, but to a larger magnitude in females compared to males. Moreover, females showed three different phases of pelvic shape change, coinciding with the main period of reproduction (from 17 to about 37-40 years), the period after that but before menopause, and the postmenopausal period (after 50 years of age). Males exhibited two phases with relatively similar shape changes. The expression of parturition scars increased in females until about 40 years of age and stayed relatively constant thereafter. Only a very weak increase of feature expression was found in males. We hypothesize that changes of adult pelvic shape and feature expression result from a combination of hormone-mediated and mechanically induced bone remodeling. Estrogen-induced and pregnancy-related bone remodeling dominates in premenopausal women, whereas bone remodeling in response to mechanical factors may underlie pelvic shape changes in men and postmenopausal women. The continual widening of the birth canal during the reproductive period eases childbirth in a population, but it is unlikely that this remodeling pattern specifically evolved as an obstetric adaptation in the human lineage. The highly conserved endocrine system and estrogen-induced pelvic bone remodeling were already in place when the neonatal brain increased in the human lineage. But the regularity control of this conserved pathway may have been "fine-tuned" by selective forces in the human lineage.

Thirty years of geometric morphometrics: Achievements, challenges, and the ongoing quest for biological meaningfulness.

The foundations of geometric morphometrics were worked out about 30 years ago and have continually been refined and extended. What has remained as a central thrust and source of debate in the morphometrics community is the shared goal of meaningful biological inference through a tight connection between biological theory, measurement, multivariate biostatistics, and geometry. Here we review the building blocks of modern geometric morphometrics: the representation of organismal geometry by landmarks and semilandmarks, the computation of shape or form variables via superimposition, the visualization of statistical results as actual shapes or forms, the decomposition of shape variation into symmetric and asymmetric components and into different spatial scales, the interpretation of various geometries in shape or form space, and models of the association between shape or form and other variables, such as environmental, genetic, or behavioral data. We focus on recent developments and current methodological challenges, especially those arising from the increasing number of landmarks and semilandmarks, and emphasize the importance of thorough exploratory multivariate analyses rather than single scalar summary statistics. We outline promising directions for further research and for the evaluation of new developments, such as "landmark-free" approaches. To illustrate these methods, we analyze three-dimensional human face shape based on data from the Avon Longitudinal Study of Parents and Children (ALSPAC).

The evolution of pelvic canal shape and rotational birth in humans.

BACKGROUND: The human foetus typically needs to rotate when passing through the tight birth canal because of the complex shape of the pelvis. In most women, the upper part, or inlet, of the birth canal has a round or mediolaterally oval shape, which is considered ideal for parturition, but it is unknown why the lower part of the birth canal has a pronounced anteroposteriorly oval shape. RESULTS: Here, we show that the shape of the lower birth canal affects the ability of the pelvic floor to resist the pressure exerted by the abdominal organs and the foetus. Based on a series of finite element analyses, we found that the highest deformation, stress, and strain occur in pelvic floors with a circular or mediolaterally oval shape, whereas an anteroposterior elongation increases pelvic floor stability. CONCLUSIONS: This suggests that the anteroposterior oval outlet shape is an evolutionary adaptation for pelvic floor support. For the pelvic inlet, by contrast, it has long been assumed that the mediolateral dimension is constrained by the efficiency of upright locomotion. But we argue that the mediolateral elongation has evolved because of the limits on the anteroposterior diameter imposed by upright posture. We show that an anteroposteriorly deeper inlet would require greater pelvic tilt and lumbar lordosis, which compromises spine health and the stability of upright posture. These different requirements of the pelvic inlet and outlet likely have led to the complex shape of the pelvic canal and to the evolution of rotational birth characteristic of humans.

Biomechanical trade-offs in the pelvic floor constrain the evolution of the human birth canal.

Compared with most other primates, humans are characterized by a tight fit between the maternal birth canal and the fetal head, leading to a relatively high risk of neonatal and maternal mortality and morbidities. Obstetric selection is thought to favor a spacious birth canal, whereas the source for opposing selection is frequently assumed to relate to bipedal locomotion. Another, yet underinvestigated, hypothesis is that a more expansive birth canal suspends the soft tissue of the pelvic floor across a larger area, which is disadvantageous for continence and support of the weight of the inner organs and fetus. To test this "pelvic floor hypothesis," we generated a finite element model of the human female pelvic floor and varied its radial size and thickness while keeping all else constant. This allowed us to study the effect of pelvic geometry on pelvic floor deflection (i.e., the amount of bending from the original position) and tissue stresses and stretches. Deflection grew disproportionately fast with increasing radial size, and stresses and stretches also increased. By contrast, an increase in thickness increased pelvic floor stiffness (i.e., the resistance to deformation), which reduced deflection but was unable to fully compensate for the effect of increasing radial size. Moreover, larger thicknesses increase the intra-abdominal pressure necessary for childbirth. Our results support the pelvic floor hypothesis and evince functional trade-offs affecting not only the size of the birth canal but also the thickness and stiffness of the pelvic floor.

Respiratory adaptation to climate in modern humans and Upper Palaeolithic individuals from Sungir and Mladec.

As our human ancestors migrated into Eurasia, they faced a considerably harsher climate, but the extent to which human cranial morphology has adapted to this climate is still debated. In particular, it remains unclear when such facial adaptations arose in human populations. Here, we explore climate-associated features of face shape in a worldwide modern human sample using 3D geometric morphometrics and a novel application of reduced rank regression. Based on these data, we assess climate adaptations in two crucial Upper Palaeolithic human fossils, Sungir and Mladec, associated with a boreal-to-temperate climate. We found several aspects of facial shape, especially the relative dimensions of the external nose, internal nose and maxillary sinuses, that are strongly associated with temperature and humidity, even after accounting for autocorrelation due to geographical proximity of populations. For these features, both fossils revealed adaptations to a dry environment, with Sungir being strongly associated with cold temperatures and Mladec with warm-to-hot temperatures. These results suggest relatively quick adaptative rates of facial morphology in Upper Palaeolithic Europe.

Sex differences in the pelvis did not evolve de novo in modern humans.

It is commonly assumed that the strong sexual dimorphism of the human pelvis evolved for delivering the relatively large human foetuses. Here we compare pelvic sex differences across modern humans and chimpanzees using a comprehensive geometric morphometric approach. Even though the magnitude of sex differences in pelvis shape was two times larger in humans than in chimpanzees, we found that the pattern is almost identical in the two species. We conclude that this pattern of pelvic sex differences did not evolve de novo in modern humans and must have been present in the common ancestor of humans and chimpanzees, and thus also in the extinct Homo species. We further suggest that this shared pattern was already present in early mammals and propose a hypothesis of facilitated variation as an explanation: the conserved mammalian endocrine system strongly constrains the evolution of the pattern of pelvic differences but enables rapid evolutionary change of the magnitude of sexual dimorphism, which in turn facilitated the rapid increase in hominin brain size.

Sacrum morphology supports taxonomic heterogeneity of "Australopithecus africanus" at Sterkfontein Member 4.

The presence of multiple Australopithecus species at Sterkfontein Member 4, South Africa (2.07-2.61 Ma), is highly contentious, and quantitative assessments of craniodental and postcranial variability remain inconclusive. Using geometric morphometrics, we compared the sacrum of the small-bodied, presumed female subadult Australopithecus africanus skeleton Sts 14 to the large, alleged male adult StW 431 against a geographically diverse sample of modern humans, and two species of Pan, Gorilla, and Pongo. The probabilities of sampling morphologies as distinct as Sts 14 and StW 431 from a single species ranged from 1.3 to 2.5% for the human sample, and from 0.0 to 4.5% for the great apes, depending on the species and the analysis. Sexual dimorphism and developmental or geologic age could not adequately explain the differences between StW 431 and Sts 14, suggesting that they are unlikely to be conspecific. This supports earlier claims of taxonomic heterogeneity at Sterkfontein Member 4.

A model of developmental canalization, applied to human cranial form.

Developmental mechanisms that canalize or compensate perturbations of organismal development (targeted or compensatory growth) are widely considered a prerequisite of individual health and the evolution of complex life, but little is known about the nature of these mechanisms. It is even unclear if and how a "target trajectory" of individual development is encoded in the organism's genetic-developmental system or, instead, emerges as an epiphenomenon. Here we develop a statistical model of developmental canalization based on an extended autoregressive model. We show that under certain assumptions the strength of canalization and the amount of canalized variance in a population can be estimated, or at least approximated, from longitudinal phenotypic measurements, even if the target trajectories are unobserved. We extend this model to multivariate measures and discuss reifications of the ensuing parameter matrix. We apply these approaches to longitudinal geometric morphometric data on human postnatal craniofacial size and shape as well as to the size of the frontal sinuses. Craniofacial size showed strong developmental canalization during the first 5 years of life, leading to a 50% reduction of cross-sectional size variance, followed by a continual increase in variance during puberty. Frontal sinus size, by contrast, did not show any signs of canalization. Total variance of craniofacial shape decreased slightly until about 5 years of age and increased thereafter. However, different features of craniofacial shape showed very different developmental dynamics. Whereas the relative dimensions of the nasopharynx showed strong canalization and a reduction of variance throughout postnatal development, facial orientation continually increased in variance. Some of the signals of canalization may owe to independent variation in developmental timing of cranial components, but our results indicate evolved, partly mechanically induced mechanisms of canalization that ensure properly sized upper airways and facial dimensions.

Three-dimensional surface scanning methods in osteology: A topographical and geometric morphometric comparison.

OBJECTIVES: Three-dimensional (3D) data collected by structured light scanners, photogrammetry, and computed tomography (CT) scans are increasingly combined in joint analyses, even though the scanning techniques and reconstruction software differ considerably. The aim of the present study was to compare the quality and accuracy of surface models and landmark data obtained from modern clinical CT scanning, 3D structured light scanner, photogrammetry, and MicroScribe digitizer. MATERIAL AND METHODS: We tested 13 different photogrammetric software tools and compared surface models obtained by different methods for four articulated human pelves in a topographical analysis. We also measured a set of 219 landmarks and semilandmarks twice on every surface as well as directly on the dry bones with a MicroScribe digitizer. RESULTS: Only one photogrammetric software package yielded surface models of the complete pelves that could be used for further analysis. Despite the complex pelvic anatomy, all three methods (CT scanning, 3D structured light scanning, photogrammetry) yielded similar surface representations with average deviations among the surface models between 100 and 200 µm. A geometric morphometric analysis of the measured landmarks showed that the different scanning methods yielded similar shape variables, but data acquisition via MicroScribe digitizer was most prone to error. DISCUSSION: We demonstrated that three-dimensional models obtained by different methods can be combined in a single analysis. Photogrammetry proved to be a cheap, quick, and accurate method to generate 3D surface models at useful resolutions, but photogrammetry software packages differ enormously in quality.

Detecting Phylogenetic Signal and Adaptation in Papionin Cranial Shape by Decomposing Variation at Different Spatial Scales.

Phylogenetic reconstruction based on morphometric data is hampered by homoplasies. For example, many similarities in cranial form between primate taxa more strongly reflect ecological similarities rather than phylogenetic relatedness. However, the way in which the different cranial bones constitute cranial form is, if at all, of less functional relevance and thus largely hidden from selection. We propose that these "constructional details" are better indicators of phylogenetic history than any large-scale shape feature or raw form variable. Within a geometric morphometric context, we show how to analyze the relative extent of bones independently of differences in overall shape. We also show how to decompose total shape variation into small-scale and large-scale shape variation. We apply both methods to the midsagittal cranial morphology of papionin monkeys, which are well known for the discrepancy between morphological similarities and phylogenetic relationships. We study phylogenetic signal and functional adaptation using a molecular phylogeny and contextual data on feeding ecology and locomotor behavior. As expected, total cranial shape, bone outline shape, and large-scale shape features were only weakly associated with phylogenetic distance. But the relative bone contributions and small-scale shape features were both highly correlated with phylogenetic distances. By contrast, the association with ecological and behavioral variables was strongest for the outline shape and large-scale shape features. Studies of morphological adaptation and phylogenetic history thus profit from a decomposition of shape variation into different spatial scales. [Adaptation; canalization; cranial shape; geometric morphometrics; papionini; partial warps; phylogeny.].

The association of parturition scars and pelvic shape: A geometric morphometric study.

OBJECTIVES: Pelvic features, mostly known as parturition scars, have been extensively studied in the last decades and are frequently investigated in archaeological and forensic contexts. It is still unclear, however, whether they really relate to pregnancy and birth, or whether these features are caused by other biomechanical factors. Because the length and difficulty of labor correlates with the form of the birth canal, we studied the association between the expression of pelvic features and pelvic shape using geometric morphometrics. MATERIALS AND METHODS: We scored the expression of the preauricular sulcus, margo auricularis groove, sacral preauricular extension, dorsal and ventral pubic pitting for 54 individuals from a 19th century collection and 19 individuals from the Bronze Age cemetery of Hainburg-Teichtal, Austria. Based on photogrammetric surface models, pelvic shape was captured by 331 landmarks and semilandmarks. The multivariate association between pelvic features and pelvic shape was explored by partial least squares analysis. RESULTS: Within the female subsample, we detected a significant association of a constrained birth canal with a strong expression of the preauricular sulcus, the margo auricularis groove, and a retroverted position of the acetabulum. No significant association was found among males. DISCUSSION: This suggests that difficult or prolonged labor may indeed cause more strongly expressed pelvic features, presumably because of increased strain of the pelvic ligaments during birth. Furthermore, the retroversion of the acetabulum, which is known to cause sacroiliac joint dysfunction, changes the strain on pelvic ligaments and can thus also result in the development of pronounced pelvic features.

Evolution of the Mammalian Ear: An Evolvability Hypothesis.

Encapsulated within the temporal bone and comprising the smallest elements of the vertebrate skeleton, the ear is key to multiple senses: balance, posture control, gaze stabilization, and hearing. The transformation of the primary jaw joint into the mammalian ear ossicles is one of the most iconic transitions in vertebrate evolution, but the drivers of this complex evolutionary trajectory are not fully understood. We propose a novel hypothesis: The incorporation of the bones of the primary jaw joint into the middle ear has considerably increased the genetic, regulatory, and developmental complexity of the mammalian ear. This increase in the number of genetic and developmental factors may, in turn, have increased the evolutionary degrees of freedom for independent adaptations of the different functional ear units. The simpler ear anatomy in birds and reptiles may be less susceptible to developmental instabilities and disorders than in mammals but also more constrained in its evolution. Despite the tight spatial entanglement of functional ear components, the increased "evolvability" of the mammalian ear may have contributed to the evolutionary success and adaptive diversification of mammals in the vast diversity of ecological and behavioral niches observable today. A brief literature review revealed supporting evidence for this hypothesis.

Evolution of brain lateralization: A shared hominid pattern of endocranial asymmetry is much more variable in humans than in great apes.

Brain lateralization is commonly interpreted as crucial for human brain function and cognition. However, as comparative studies among primates are rare, it is not known which aspects of lateralization are really uniquely human. Here, we quantify both pattern and magnitude of brain shape asymmetry based on endocranial imprints of the braincase in humans, chimpanzees, gorillas, and orangutans. Like previous studies, we found that humans were more asymmetric than chimpanzees, however so were gorillas and orangutans, highlighting the need to broaden the comparative framework for interpretation. We found that the average spatial asymmetry pattern, previously considered to be uniquely human, was shared among humans and apes. In humans, however, it was less directed, and different local asymmetries were less correlated. We, thus, found human asymmetry to be much more variable compared with that of apes. These findings likely reflect increased functional and developmental modularization of the human brain.

Morphometric Variation at Different Spatial Scales: Coordination and Compensation in the Emergence of Organismal Form.

It is a classic aim of quantitative and evolutionary biology to infer genetic architecture and potential evolutionary responses to selection from the variance-covariance structure of measured traits. But a meaningful genetic or developmental interpretation of raw covariances is difficult, and classic concepts of morphological integration do not directly apply to modern morphometric data. Here, we present a new morphometric strategy based on the comparison of morphological variation across different spatial scales. If anatomical elements vary completely independently, then their variance accumulates at larger scales or for structures composed of multiple elements: morphological variance would be a power function of spatial scale. Deviations from this pattern of "variational self-similarity" (serving as a null model of completely uncoordinated growth) indicate genetic or developmental coregulation of anatomical components. We present biometric strategies and R scripts for identifying patterns of coordination and compensation in the size and shape of composite anatomical structures. In an application to human cranial variation, we found that coordinated variation and positive correlations are prevalent for the size of cranial components, whereas their shape was dominated by compensatory variation, leading to strong canalization of cranial shape at larger scales. We propose that mechanically induced bone formation and remodeling are key mechanisms underlying compensatory variation in cranial shape. Such epigenetic coordination and compensation of growth are indispensable for stable, canalized development and may also foster the evolvability of complex anatomical structures by preserving spatial and functional integrity during genetic responses to selection.[Cranial shape; developmental canalization; evolvability; morphological integration; morphometrics; phenotypic variation; self-similarity.].

Evolution of the human pelvis and obstructed labor: new explanations of an old obstetrical dilemma.

Without cesarean delivery, obstructed labor can result in maternal and fetal injuries or even death given a disproportion in size between the fetus and the maternal birth canal. The precise frequency of obstructed labor is difficult to estimate because of the widespread use of cesarean delivery for indications other than proven cephalopelvic disproportion, but it has been estimated that at least 1 million mothers per year are affected by this disorder worldwide. Why is the fit between the fetus and the maternal pelvis so tight? Why did evolution not lead to a greater safety margin, as in other primates? Here we review current research and suggest new hypotheses on the evolution of human childbirth and pelvic morphology. In 1960, Washburn suggested that this obstetrical dilemma arose because the human pelvis is an evolutionary compromise between two functions, bipedal gait and childbirth. However, recent biomechanical and kinematic studies indicate that pelvic width does not considerably affect the efficiency of bipedal gait and thus is unlikely to have constrained the evolution of a wider birth canal. Instead, bipedalism may have primarily constrained the flexibility of the pubic symphysis during pregnancy, which opens much wider in most mammals with large fetuses than in humans. We argue that the birth canal is mainly constrained by the trade-off between 2 pregnancy-related functions: while a narrow pelvis is disadvantageous for childbirth, it offers better support for the weight exerted by the viscera and the large human fetus during the long gestation period. We discuss the implications of this hypothesis for understanding pelvic floor dysfunction. Furthermore, we propose that selection for a narrow pelvis has also acted in males because of the role of pelvic floor musculature in erectile function. Finally, we review the cliff-edge model of obstetric selection to explain why evolution cannot completely eliminate cephalopelvic disproportion. This model also predicts that the regular application of life-saving cesarean delivery has evolutionarily increased rates of cephalopelvic disproportion already. We address how evolutionary models contribute to understanding and decision making in obstetrics and gynecology as well as in devising health care policies.