The mystery was the catalyst for what became known as “the obstetrical dilemma,” a long-debated though widely accepted hypothesis suggesting that the upright gait of Homo sapiens was accompanied by a narrowing of the pelvis—an evolutionary trade-off that resulted in increased risks to pregnant mothers as they struggled to push large-brained babies through ever-slimmer birth canals. Among other things, the dilemma has been used to suggest that the wider, birth-giving hips of women have hindered them locomotively and athletically—and perhaps even evolutionarily—compared to men.

That has always struck some scientists as too pat an explanation, though it is only in the last decade or so that the theory, which still has many subscribers, has received substantive pushback. Today, challenges abound for the idiosyncrasies of human gestation and birth—including new notions that look beyond evolution to more proximate and modern factors like poor diet and obesity.