To understand why Hendrie’s team looked at hip bones in the first place, one must examine the obstetrical dilemma. Coined by anthropologist Sherwood Washburn in 1960, the hypothesis describes the competing evolutionary pressures placed on the human female pelvis. Walking upright on two legs requires a compact, narrow pelvic canal to optimize locomotor efficiency. Delivering encephalized, large-brained infants requires a broad, open birth canal.
This biological compromise shaped human reproduction for millennia. Human neonates are born neurologically altricial, developmentally fragile and reliant on constant maternal care, because gestation cannot extend past the point where the fetal head fits through the maternal pelvis. Childbirth adaptation pushed female pelvic anatomy to the absolute biomechanical limit of bipedal efficiency.
The Leeds team postulated that this bottleneck created psychological adaptations. If a narrow pelvis elevated the risk of fatal obstructed labor, casual sex without an investing partner represented a catastrophic evolutionary gamble. Conversely, a wide pelvic canal lessened the mortality baseline. In theory, women with broader pelvic architecture could sustain a diversified mating strategy with lower maternal risk.
