Sagittal sacral orientation within the pelvic girdle of humans is a key component of posture and obstetrics. On the one hand, sacral orientation has direct influence on the lumbar curvature; while on the other hand, it has an impact on the dorsoventral dimension of the birth canal. In this study, we aim to explore the evolution of sacral orientation in the sagittal plane and its relationship with the lumbar curvature in hominins. To do this, we measured sacral orientation using the pelvic incidence (PI) angle of the pelves of 53 modern humans, 19 nonhuman hominoids, and 4 fossil hominins. Our results show that the PIs of modern and fossil H. sapiens are the highest while the PI of nonhuman hominoids is the lowest (a nearly parallel sacrum in relation to the hip bone). Australopithecines PI is higher than that of nonhuman hominoids, but lower than that of modern humans. The PI of Homo heidelbergensis and H. neanderthalensis (Neandertal lineage hominins) is the lowest among hominins. We also found a strong correlation between lumbar lordosis and PI in nonhuman hominoids and hominins, indicating that PI angle is a good predictor of the lumbar lordosis when only the pelvis is preserved. We conclude that sacral orientation changed during the course of human evolution. When Neandertal lineage hominins are ignored, the results indicate a fairly simple path of evolution from nonhuman hominoid-like to human-like sacral orientation with two stages of the development. Neandertal lineage hominins show a reversal of this trend.
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