Meeting Abstract
9.5 Thursday, Jan. 3 Fetal load adaptations in the axial skeleton of early bipeds: Australopithecus africanus WHITCOME, KW*; SHAPIRO, L; LIEBERMAN, DE; Harvard University; University of Texas at Austin; Harvard University whitcome@fas.harvard.edu
Although humans are not the only living bipedal species, some aspects of human morphology are uniquely adapted to the human style of bipedality. One key feature shared by humans and earlier hominins is an elongated lumbar region with a marked lordosis that places the trunk�s center of mass above the hips stabilizing the upper body over the supporting limbs. Bipedalism poses an additional challenge to the lumbar lordosis in pregnant females because changes in body shape and mass distribution associated with fetal growth shift the maternal center of mass anterior to the hips. Human females have evolved a derived curvature of the lumbar vertebrae in order to compensate for this bipedal obstetric load. We investigated the lumbar vertebrae of the two known early hominin Australopithecus africanus specimens, one a putative female (Sts 14) and the other a likely male (Stw 431) using a human loading model. Sts 14 exhibits the typical human female pattern with three dorsally wedged vertebrae, while Stw 431 has a more human male-like pattern with fewer lordotic vertebrae. Additionally, the prezygapophyses of Sts 14 (L1-L6) are angled 9o – 12o more coronally than the measurable facets of Stw 431 (L3, L5, L6), as is typical of the human female and male patterns, respectively. These features function to mitigate shearing force generated during gravid loading in bipeds. Analysis shows that the biomechanical demands of pregnancy exerted an early selection pressure on the evolution of lumbar lordosis in bipedal hominins. This research was supported by NSF and the L.S.B. Leakey Foundation.
