It is commonly assumed that the strong sexual dimorphism of the human pelvis evolved for delivering the relatively large human foetuses. Here we compare pelvic sex differences across modern humans and chimpanzees using a comprehensive geometric morphometric approach. Even though the magnitude of sex differences in pelvis shape was two times larger in humans than in chimpanzees, we found that the pattern is almost identical in the two species. We conclude that this pattern of pelvic sex differences did not evolve de novo in modern humans and must have been present in the common ancestor of humans and chimpanzees, and thus also in the extinct Homo species. We further suggest that this shared pattern was already present in early mammals and propose a hypothesis of facilitated variation as an explanation: the conserved mammalian endocrine system strongly constrains the evolution of the pattern of pelvic differences but enables rapid evolutionary change of the magnitude of sexual dimorphism, which in turn facilitated the rapid increase in hominin brain size.
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We thank the Center for Academic Research and Training in Anthropology, the Primate Research Institute of the University of Kyoto, F. Zachos (Mammal Collection, Natural History Museum Vienna), M. Häusler, C. Fornai and V. Krenn (University of Zurich and University of Vienna) for access to the chimpanzee pelves. We thank H. Reynolds for sharing the human landmark data. B.F. and P.M. acknowledge support from the Austrian Science Fund FWF (Elise Richter grant no. V-826 to B.F., grant no. P29397 to P.M.). N.D.S.G. was funded by a postdoctoral fellowship from the Konrad Lorenz Institute. E.Z. was supported by an Erasmus+ student exchange fellowship from the European Union.
The authors declare no competing interests.
Peer review information Nature Ecology & Evolution thanks Philipp Gunz, Nicole Torres and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Peer reviewer reports are available.
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Extended Data Fig. 1 Principal component analysis (PCA) of pelvis shape, jointly for humans and chimpanzees.
The black arrows represent the allometry vectors, which were estimated here using a non-pelvic size measure for each species (femoral head diameter in chimpanzees and body height in humans, compare Extended Data Fig. 2) in contrast to Fig. 2, where they were estimated based on pelvic centroid size. For these non-pelvic size variables, z-scores were computed independently for each species. The allometry vectors were then estimated by regressing the shape coordinates on the z-scores for each species and projected into the PC spaces. As in Fig. 2, the allometry vectors are species-specific but not sex-specific. The vectors are shown twice, superimposed on the sex means for each species. The direction of allometry is distinct from the direction of sex differences in both species. The individual data shown are the same as in Fig. 2. Each small point corresponds to an individual (humans: males=blue, females=red; chimpanzees: males=light blue, females=light red). The larger points in the same colors correspond to the sex means for each species.
Extended Data Fig. 2 Pelvic centroid size vs. femoral head diameter in chimpanzees and body height in humans, respectively.
Correlations for these pairs of size measures were 0.62 in chimpanzees and 0.70 in humans. Femoral head diameter was available for 31 out of 34 chimpanzees and stature was available for all 99 human individuals.
Extended Data Fig. 3 Inlet area in mm2 in humans and chimpanzees for females (red) and males (blue).
Inlet area was calculated as the area of the polygon defined by the 2D inlet landmarks. Humans: female mean=11540 (sd=1248), male mean=10376 (sd=1047); Chimpanzees: female mean=9517 (sd=1107), male mean=8661 (sd=1263).
These data comprise means for females and males of 26 pelvic variables (linear distances, curved distances, and circumferences) from 6 populations (skeletal collections): White individuals from Hamann-Todd collection (HTW, 60 males, 59 females); Black individuals from Hamann-Todd (HTB, 60 m., 60 f.); Whites from Terry collection (TEW, 52 m., 52 f.), Blacks from Terry collection (TEB, 52 m., 52 f.); Coimbra collection (CO, 84 m., 71 f.); Spitalfields collection (SP, 31 m., 35 f.). We conducted a principal component analysis of these data. Shown are the sex means of males (blue) and females (red) for each population within the first three principal components (accounting for 94% of the total variance). The sex difference vectors (lines connecting the sex means) of the six populations are close to parallel in the first three principal components (panels A and B), illustrating the similar pattern of sex differences in the pelvis between human populations, despite some variation in magnitude (panel C). The magnitude was calculated as the Euclidean length of the sex differences vector.
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Fischer, B., Grunstra, N.D.S., Zaffarini, E. et al. Sex differences in the pelvis did not evolve de novo in modern humans. Nat Ecol Evol (2021). https://doi.org/10.1038/s41559-021-01425-z