The genital prolapse of Australopithecus Lucy?

Introduction and hypothesis The female bony pelvis has to fulfil opposing functions: it has to be sufficiently closed to support the pelvic viscera in the upright position, while remaining sufficiently open to allow vaginal delivery. We aim to give an evolutionary perspective and the possible evolution of the bony pelvis from Lucy to the modern female with the implications in terms of genital prolapse. Methods Thirteen pelvimetric measurements were performed on 178 bony pelves: 1 fossil pelvis from Australopithecus Lucy, 128 female Caucasian modern adult pelves and 49 female Catarrhine pelves (29 gorillas and 20 chimpanzees). Results Lucy’s pelvis shape was the most transversely oval, short and broad, termed platypelloid. Modern female pelves were transversely oval only at the inlet. A protruding ischial spine, fairly small ischial tuberosities and a sacral concavity made Lucy closer to Homo sapiens and less like the great apes. In the last group, pelvic planes were anteroposteriorly oval, except in the gorilla, where the outlet was round or slightly transversely oval. The subpubic angle was narrowest in Lucy, whereas it was greater than 90° in the great apes. Conclusions The female pelvis is involved in both visceral support and parturition and represents a compromise. The narrower pelvis of Australopithecus Lucy provided protection against genital prolapse, but resulted in complex obstetrical mechanics. From an evolutionary perspective, the pelvis of Homo sapiens became modified to make parturition easier, but increased the risk of genital prolapse: the ilia became wide open laterally and the sacrum broadened with a shorter distance between the sacroiliac and coxofemoral joints.