Recent evolutionary decrease in the human pelvis size

Humans are still evolving.1 The “obstetrics dilemma” in hominin evolution2 resulted from a narrow pelvis producing better adaptation for erect bipedalism, while a larger birth canal was required to pass neonates with larger heads. In the last two centuries, the introduction of methods helping women with narrow pelves to deliver their children changed the selective pressures. Uniformly measured widths of bony pelvis in 8,866 adult humans born between 1880 and 1980 in Australia, Poland, and Mexico show a decrease of about 45 mm per century while their body heights increased by about 100 mm. Their shoulder widths remained essentially the same. The use of caesarean sections, forceps and vacuum-assisted birth increased in the last 200 years very substantially to now account for up to about 40% of deliveries in the countries studied.3 At the same time the total fertility decreased there to 1.32-1.79, several times below the traditional 6-7 live birth per woman4 due to birth control that completely removed the natural selection criterion of reproductive advantage of a large pelvis/small neonate head. The width of the bony pelvis in humans is strongly heritable5,6. In the future, caesarean section may become the only option for having a live birth.