What has a baby which does not understand speech "heard before", that it can form variations on? Evolution is fine, but you do need a theory of abiogenesis, or in this case aontogenesis - knowledge-from-nothing-ness, in the vernacular.
While I agree with TheOtherDave's point, I'm not sure it's necessary. A baby doesn't understand new sounds the first time it hears them, but may understand them the hundredth time it's heard them- at which point it does have quite a bit of experience, both of hearing those noises in some situations and not hearing those noises in other situations. Then, once they've learned the general skill of acquiring words, they can correctly learn words quickly, sometimes even after hearing a single use- but that's drawing on their previous experience in learning thousands of words.
Another month has passed and here is a new rationality quotes thread. The usual rules are: