Sex differences in the pelvis did not evolve de novo in modern humans

• Published in: Nature ecology & evolution Vol. 5; no. 5; pp. 625 - 630
• Main Authors: Fischer, Barbara, Grunstra, Nicole D. S., Zaffarini, Eva, Mitteroecker, Philipp
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 01.05.2021; Nature Publishing Group
• Subjects: 631/181/19/2471; 631/181/2468; 631/181/2806; Animals; Biological and Physical Anthropology; Biological evolution; Biomedical and Life Sciences; Chimpanzees; Ecology; Endocrine system; Evolutionary Biology; Extinct species; Female; Fetuses; Gender aspects; Gender differences; Hominidae; Hominids; Humans; Life Sciences; Male; Mammals; Paleontology; Pan troglodytes; Pelvis; Sex; Sex Characteristics; Sex differences; Sexual dimorphism; Zoology
• ISSN: 2397-334X; 2397-334X
• DOI: 10.1038/s41559-021-01425-z

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. Comparing pelvic sex differences across modern humans and chimpanzees reveals a similar pattern despite differences in magnitude of pelvis shape dimorphism, suggesting that this pattern did not evolve de novo in modern humans but was present in the last common ancestor of humans and chimpanzees.