If large amounts of our knowledge base was encoded through evolution, we would see people with weird, specific cognitive deficits - say, the inability to use nouns - as > a result of genetic mutations.

That's not necessarily the case. You are assuming a much more narrow encoding system than necessary. One doesn't need a direct encoding of specific genes going to nouns or the like. Remember, evolution is messy and doesn't encode data in the direct fashion that a human would. Moreover, some problems we see are in fact pretty close to this. For example, many autistic children have serious problems handling how pronouns work (such as some using "you" to refer to themselves and "I" to refer to anyone else). Similarly, there's a clear genetic distinction in language processing between humans and other primates in that many of the "sentences" constructed by apes which have been taught sign language completely lack verbs and almost never have any verb other than an imperative.