[gcruse]

Interesting - so women's birth canals will never
evolve to accommodate bigger heads.

I don't see how unless there are mutations where
a wider canal coincides with a bigger brain/head.
But, I really don't understand everything I know :)

[meowmeow]

I don't see how unless there are mutations where a wider canal coincides with a bigger brain/head.

I don't know, sounds like you understand pretty well...isn't that what evolution is all about, random mutations that are somehow convenient to the organism? I think a third arm would be very convenient. Darn those mutations!