[stanz]

why hasn't evolution allowed mother's birth canals to develop large enough for the increased brain size?

Because then they couldn't walk. I'm no expert in biomechanics, but if the hips are widened, the gait would be affected.

Any biologists out there who can comment?

[Jonathan]

Well I'll be a monkey's uncle!!

Ask a biologist if he can explain the evolution of reciprocal altruism. Can't - it alone debunks the faith of scientists in their evolutionary god.

But they won't ever mention that.

[general_re]

I'm no biomechanic either, but what I do know is that the narrowness of the female pelvis is a result of the gait changing to permit bipedalism. In order to have a reasonably smooth gait when upright, the pelvis had to narrow somewhat - compare the way chimps walk upright when they're on two feet. It's more of a "waddle" than a walk, and running upright is right out of the question for them. It's a tradeoff - they get easier births, and we gain some altitude.

The incomplete formation of a baby's skull is a partial solution to the problem - the fontanels allow the infant's skull to compress somewhat as it passes through the birth canal. Those of you who have witnessed a birth will probably have noticed that babies tend to come out pointy-headed in many cases. Another part is the hormone "relaxin", which softens and loosens the (anatomical parts alert - stop reading now) cervix and the vaginal canal, and makes it a bit more flexible and open, along with the pelvis.

Like I said, it's only a partial solution to the problem of squeezing a baby through the birth canal, though - as my wife readily assures me ;)