A research team digitally reconstructed the pelvises of two exceptionally well preserved Neanderthal skeletons, a 1.5-year-old infant from Dederiyeh Cave in Syria and a young adult female from Sima de las Palomas in Spain, and reported in Proceedings of the National Academy of Sciences that the childbirth-relevant parts of those pelvises closely resemble those of modern women while the locomotion-relevant parts do not [1][8][9]. That combination matters because the standard account of human pelvic shape, the Obstetrical Dilemma Hypothesis, treats the pelvis as a compromise between a wide birth canal and the narrow distance between femoral head centres said to be needed for efficient walking on two legs [2][3].
The mechanics of the reconstruction are worth noting, because fossil pelvises are rare, usually fragmented, and earlier attempts to reconstruct Neanderthal birth canals produced conflicting conclusions [16]. The team combined anthropological methods with medical CT scans and digital cleaning software, stripping away the surrounding sediment in software rather than physically preparing the bones [4]. The new reconstructions were then compared with two previously reconstructed Neanderthal specimens, the Tabun female and the Kebara male, and with medical scans of modern humans [5]. That is four Neanderthal pelvises in the comparison set, two of them new, which is a real constraint on how far the result can be pushed [6].
Within that set, the pattern is consistent. Neanderthal pelvises were wider overall and already looked unlike modern human pelvises at a young age [7]. The 1.5-year-old infant carried the classic Neanderthal traits, flaring hip blades and a pelvis short from front to back relative to its width, which the authors read as evidence that the shape was laid down early in development rather than produced later by walking or environment [11]. As in modern humans, female Neanderthals had a wider birth canal than males [12], which is consistent with the pelvis being the most sexually dimorphic part of the human skeleton and the bone anthropologists routinely use to sex ancient remains [17].
The locomotor differences run the other way. Neanderthals show reduced pelvic tilt, less lower-back curvature and shorter muscle lever arms, a configuration the researchers describe as favouring stability and power over the long-distance running efficiency of modern humans [13]. Crucially, according to the study the character of those differences implies a wider birth canal did not necessarily make walking less efficient [9]. Since Neanderthals were our closest extinct relatives and also had large brains, they are the obvious test case for whether the same conflict shaped their hips [18].
Given that, the team proposes the real trade-off was not childbirth against locomotion but childbirth against pelvic floor stability [10]. This is not a new suspicion: critics of the Obstetrical Dilemma had already pointed out that wider hips do not measurably slow walking or running, and had nominated maternal metabolism or the need for a strong pelvic floor supporting the internal organs as the actual limiting factor [15]. There is also a phylogenetic reading. Evidence from Australopithecus and early Homo suggests the wide, side-to-side Neanderthal pelvis is the ancient condition shared with earlier ancestors, and the narrow, front-to-back human pelvis is the recent departure [14].
What to watch: whether the same non-destructive CT and digital-cleaning workflow gets applied to other fragmentary hominin pelvises, which would move the comparison set past four Neanderthals [6][4], and whether the pelvic floor hypothesis yields predictions that can be tested on soft tissue proxies rather than inferred from bone shape [10].
