Our intelligence could just as well be a complicated mesh of a huge number of 'rules' that have evolved over a long period of time in response to many difficult situations that had threatened the survival of our ancestors. They are also what we call intuition/pre-born knowledge/natural tendencies and reflex responses.
Having said that, we have not ruled out the possibility of intelligence (albeit of a form different than ours) arising as an emergent phenomena from simple rules. But if we were to replicate human intelligence on silicon, I'm more inclined to believe that we'll have to 'manually' encode a huge number of situation-specific rules, e.g. when you see wriggly thing run away.
I urge you to watch Andrew Ng's talk that I linked to in the post, and read On Intelligence (http://www.amazon.com/On-Intelligence-Jeff-Hawkins/dp/080507...) by Jeff Hawkins, a book that totally changed the way I look at intelligent behavior.
I surmise that that's the "nature" part, and what emerges is the "nurture" part. But these are all just potshots in the dark. Passing our eventual understanding on to faster machines with better retention seems suicidal.
http://www.amazon.com/dp/0738201421
I'm not sure if we can solve AI in an 8 bit cellular automata, but the general concept of intelligence emerging versus being designed certainly rings true with human evolution.