The Evolution of Human Pregnancy

Abstract

Humans have a greater risk of complications and death during childbirth than any other primate. The obstetrical dilemma is a hypothesis that attempts to explain this fact. It suggests that human gestation time is shortened compared with other primate species due to a constraint between the size of the maternal pelvis and the fetus’ head. This hypothesis implies that humans are exceptional among primates because humans are born early -- a consequence of our large brains and bipedal posture. Previous studies supporting this hypothesis have not considered human reproductive traits within the comparative framework of primate biology and evolution. To address this hypothesis, we are collecting reproductive trait data (gestation length, adult and neonate body mass, etc.) in 100 primate species. We are applying phylogenetic generalized least squares models to analyze how these traits evolve. We expect that the results will clarify how the obstetrical dilemma has or has not shaped human evolution.