Fetal load and the evolution of lumbar lordosis in bipedal hominins

The mother load Backache is a leading cause of workplace absenteeism — testament to the difficulties of walking upright, a distinctive feature of humans and our hominin ancestors. But women face an extra evolutionary problem, given that for most of human history and prehistory, adult females have spent much of their lives either pregnant or nursing. Pregnancy makes the instability of upright walking even worse by constantly shifting the body's centre of gravity. Whitcome et al . detail those anatomical adaptations peculiar to female spines that balance the fetal load, and find that the bipedal australopithecines — but not the non-bipedal chimpanzee — had similar adaptations. Pregnancy makes the instability of upright walking even worse by its constant shifting the centre of gravity. The anatomical adaptations peculiar to female spines that balance the fetal load are detailed, and show that our australopithecine ancestors had much the same adaptations. As predicted by Darwin 1 , bipedal posture and locomotion are key distinguishing features of the earliest known hominins 2 , 3 . Hominin axial skeletons show many derived adaptations for bipedalism, including an elongated lumbar region, both in the number of vertebrae and their lengths, as well as a marked posterior concavity of wedged lumbar vertebrae, known as a lordosis 4 , 5 , 6 . The lordosis stabilizes the upper body over the lower limbs in bipeds by positioning the trunk’s centre of mass (COM) above the hips. However, bipedalism poses a unique challenge to pregnant females because the changing body shape and the extra mass associated with pregnancy shift the trunk’s COM anterior to the hips. Here we show that human females have evolved a derived curvature and reinforcement of the lumbar vertebrae to compensate for this bipedal obstetric load. Similarly dimorphic morphologies in fossil vertebrae of Australopithecus suggest that this adaptation to fetal load preceded the evolution of Homo .