Human birth follows a pattern which is unique among mammals. Distinctions include the orientation of the fetus as it passes through the birth canal, the way the fetus emerges from the birth canal, difficulty during labor, and behavior by the mother and/or other individuals around the time of birth. Birth has important implications for the morphology of the pelvis, for sex differences in the pelvis, for such aspects of human biology as size (and maturity) at birth, and for behavior (including cooperative behavior). This paper reviews the fossil and comparative evidence for when and how the modern pattern of birth evolved.
The modern human pattern of birth evolved in a mosaic manner with some unique features appearing early in human evolution and others quite late. A human-like entry of the fetal head into the birth canal was already present among australopithecines as a result of their wide pelvic apertures. Other aspects of modern human birth such as the rotation of the head and body within the birth canal and the emergence of the fetal head in an occiput anterior position probably evolved later, when encephalization had placed increasing selection on both the form of the pelvis and the timing of birth. Cooperative behavior during and after birth accompanied the origin of the fully modern human mechanism of birth.
The unique phenomenon of modern human birth did not evolve in response to a single “obstetrical dilemma” but as part of a complex interplay between changes in a number of aspects of human biology. © 1992 Wiley-Liss, Inc.