A finite element study of soft‐tissue deformation during birth in australopithecines: Assessing pelvic floor stress and fetal head rotation in early hominins

The emergence of bipedal locomotion during human evolution has considerably reshaped the hominin pelvis, which also impacted the pelvic floor musculature. Among early hominins, australopithecines have an anteroposteriorly narrow, transversely wide bony pelvis. This study investigates whether this particular shape of the birth canal weakens the pelvic floor during childbirth in these hominins and if the pelvic floor contributes to rotational birth. As such, we explored the stress values at the pelvic floor that occur during the descent of the fetal head, using 3D models of extant humans based on the reconstruction of pelvic-fetal dyads from hospital records with known birth outcomes. We then compared this to 3D models based on pelvic reconstructions of A.L. 288-1 (Australopithecus afarensis), Sts 14 (A. africanus), and MH2 (A. sediba) using a 110 g neonatal brain weight. The early hominin pelvic floor was reconstructed by warping the pelvis and pelvic floor extracted from an MRI of a modern adult woman to the australopithecine pelvic morphology. Our results suggest that the range of stress values on the pelvic floor is comparable between humans (5.3-10.5 MPa) and australopithecines (4.9-10.7 MPa), suggesting that their females may have been exposed to a similarly high risk of perineal laceration during vaginal delivery as modern humans. None of the four australopithecine simulations, and only one out of two modern human simulations, showed an internal rotation of the fetal head into an occiput anterior orientation, suggesting this movement is particularly complex and involves influences beyond just the levator ani musculature.