Evolution of human pelvic size, head size and walking

[Will__]

A dinner party discussion turned interesting:

I put forward a theory I'd heard that says that during human evolution, pelvic size had to decrease to allow for normal bipedal walking. The theory states that both walking and an increasing brain size meant that human babies used to spend more time in the womb but through evolution spent less and less time there so that a) their brains (and thus heads) could be larger and b) humans could walk better with a narrowed pelvic aperture.

My friend refuted this; I don't know where I heard the theory but would love to know if anyone else had heard this or knows of its source. And does anyone know how much longer they used to spend in there?

Thanks.

[heathfield]

Point your friend to this article.

[Will__]

heathfield, thanks for your answer.
I'm looking, however, for a more scholarly-sourced article or medically sourced page.

The article you pointed me to, while well written and argued, isn't written by an anthroplogist.....

Also, I'm looking for the amount of time anthropologists think human babies used to spend in the womb....

I'm going to cross-post to the science section - if this is a no-no, can an admin please tell me?

Thanks.