Why is childbirth so hard in humans?

Author(s): Fischer B

Summary:

Compared to other primates, childbirth is remarkably difficult in humans, the reason being that the head of the human fetus is large in comparison to the birth-relevant dimensions of the human pelvis. Modern human pelvic morphology must serve more than one purpose: It is thought to have evolved as a compromise between being shaped for upright walking and giving birth to large-headed neonates. This “obstetric dilemma” arose as a consequence of bipedal humans evolving increasingly larger brains. Although anatomically modern humans have existed for at least 100,000 years, and although the selection pressure for evolving wider birth canals has probably been considerable throughout, we do not see any evolutionary response in birth-relevant pelvic dimensions. We hypothesize that this lack of response is due to evolutionary constraints that inhibit the evolution of wider birth canals in humans. In this study we try to identify allometric constraints using morphometric techniques on a human skeletal sample.