Human shoulder development is adapted to obstetrical constraints

Abstract

Significance During human birth, the risk of complications is relatively high because of the comparatively large dimensions of the fetal head and shoulders relative to the maternal birth canal. Here we show that humans exhibit a developmental mode of the shoulders that likely contributes to mitigating obstetrical problems. Human shoulder growth is decelerated before birth but accelerated after birth, which stands in contrast to the more uniform shoulder growth trajectories of chimpanzees and macaques. This indicates that fetal developmental modifications were required during human evolution not only in the head but also in the shoulders to compensate obstetrical constraints.

There is an obstetrical dilemma: Misconceptions about the evolution of human childbirth and pelvic form

“…Nonetheless, their association with pelvic canal size, even in younger and nulliparous women (e.g., Hendrix et al, 2002 ; Lawrence et al, 2008 ; Nygaard et al, 2008 ), imposes natural selection toward a smaller birth canal. In addition, not only the maternal pelvis but also the fetal head and shoulders are subject to obstetric selection favoring a smaller fetus (Kawada et al, 2022 ). Simultaneously, however, smaller fetal size at birth reduces infant survival rate and thus is selected against (Alberman, 1991 ).…”

Pelvic shape change in adult Japanese macaques and implications for childbirth at old age

“…A pelvis that favors obstetrical sufficiency could compensate these aging-related intrinsic changes in females. On the neonatal side, it has been shown that developmental adjustment is present in the shoulders in perinatal humans ( 20 ). Our findings suggest that obstetrical constraints have more pervasive effects on the ontogeny of the primate skeleton than previously thought.…”