Rethinking pelvic typologies and the human birth mechanism

Paleoanthropological reconstructions of childbirth in the genus Homo typically rely upon a model incorporating the evolution of a monotypic human birth mechanism. Two features characterize this proposed mechanism or pathway taken by the fetus through the birth canal: fetal rotation and neonate emergence in a position facing away from the mother. The evolution of these two features is said to facilitate birth through the bipedal pelvis but is also taken as evidence of the difficulty of human birth relative to that in other primates and our smaller-brained ancestors. In contrast, the present work takes the position that birth mechanisms vary now and probably did so in the past. The notion of a monotypic birth mechanism has been imported into paleoanthropological discourse from typological thinking in Euro-American biomedical practice and text. The history of anatomical descriptions of pelvic types and associated birth mechanisms shows a trend toward the concept of a singular normal birth mechanism in biomedical practice. This paper suggests that biomedically defined pelvic typologies constitute a static definition of human variation in pelvic morphology and that pelvic typology, in turn, has contributed to a static definition of the normal human birth process that has been incorporated into paleoanthropological models.