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A comparison with chimpanzees: where do the sex differences in the human pelvis come from?

(25.03.2021) The pelvis is that part of the skeleton in which the greatest differences between the biological sexes occur. The birth canal in the female pelvis is wider, and the shape of the female pelvis makes the birth of our large babies with their particularly large heads possible in the first place.

Forensics, for example, makes use of this when determining the sex of human skeletons. Until now, it was not known when these differences in human evolution arose. Barbara Fischer from the University of Vienna and her co-authors have now published new insights into the evolutionary emergence of gender differences in the pelvis in a study in Nature Ecology & Evolution.

Gender differences in the pelvis in humans (first line) and in chimpanzees (second line). The two middle columns show the actual gender differences, while the two outer columns show the difference overdrawn. The female pelvis is shown on the left, the male on the right

Pelvic fossils of human ancestors are very rare, as the basin is usually not well preserved. It was therefore not yet clear whether this pattern of pelvic differences arose simultaneously with the upright gait or later, with the appearance of the large human brain. "We have now found that the pattern of pelvic differences is actually much older than previously assumed," says evolutionary biologist Barbara Fischer.

The biologists from the University of Vienna, the KLI for Evolution and Cognitive Research, and the University of Calgary compared pelvic differences in humans with those in chimpanzees, the closest living species of modern humans. Chimpanzees have a much easier birth than humans because their newborns are smaller.

"To do this, we examined 3D data from the pelvis of the two species and found that they have the same pattern of sex differences in the pelvis, despite the large species differences," says Fischer. The overall extent of the differences in the chimpanzee was only half as great as in humans. That the pattern is so similar in humans and chimpanzees indicates that it was already present in the common ancestor of the two species. This means that all extinct human species, such as the Neanderthals, probably also had the same pattern in the basin.

In fact, there are many species of mammals, e.g. bats or some primates, which give birth to even larger newborns relative to their birth canal than humans, and which have adaptations in the female pelvis for this purpose. At the same time, there are other mammal species, e.g. cats or possums, that have tiny newborns and that still have certain pelvic differences between the sexes that resemble the human pattern.

This indicates that this is an ancient, evolutionarily conserved mammalian specimen.

"We think that modern humans did not develop this pattern of pelvic differences anew, but rather inherited it from early mammals, which had the same problem as humans, namely having to give birth to very large newborns."

When the brain finally enlarged in the course of human evolution, the extent of the sex differences in the pelvis could therefore probably be increased rapidly. The developmental and genetic mechanisms that create these differences already existed and did not have to be newly created.

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