As Robert Winston pointed out long ago, being born is rather like trying to force your foot into a Wellington boot. The angle between the long axis of the uterus and that of the vagina is about 90 degrees. Is this angle present in the other apes and has there been any sexual selection going on with humans leading to an increase of this angle over the last million years or so?

Hi Roger,

Sorry for the length of time we've taken to get to this - it is usually a good indication that the answer is "we don't know." It may be that nobody has ever looked into this, or that this is a peculiarity of the erect nature of human posture. Don't really know I'm afraid.

I would suspect that this is soley a human feature, due to the rotation of the spine from it's natural plane to sitting directly above the legs. Having only dissected humans and no other primates, I can only give you my suspicions. It may be that species like the orang have a slightly different angle between the long axis of the uterus and the vagina (relative to other species), as of the primates, they spend more time in an upright position and it is likely that the common ancestor of humans, chimps and gorillas had a quadrapedal arboreal posture, similar to that of the orang. Complete supposition on my part though, I'm afraid.

At the risk of the automated banning system taking against this post, I should point out that it is unlikely that sexual, rather than natural selection, is involved. Sexual selection usually involves mating preferences and I am unsure how a human female would signal that she had a difference in the angles involved to make her more attractive to potential mates.

Apologies for the pedantry.

"Hope is a duty from which palaeontologists are exempt."
David Quammen

I think Dave has it on the head with his explanation. I've just been comparing the pelvises of a female gorilla, orang, chimp and human and the skeletal morphology supports Dave's supposition.