When I recently visited Europe and some Mediterranean countries, some ideas which I had considered before became pounding real hard on me and I statarted questioning myself: Why was the progress in civilizations so different between continents? So I started doing some research of my own on the theories which have been written in reference to such subject. I found a great site in the internet, called The Edge, an organization which gathers scholars from the different disciplines who according to it has as a goal: To arrive at the edge of the world's knowledge, seek out the most complex and sophisticated minds, put them in a room together, and have them ask each other the questions they are asking themselves.
Please go the link to initiate some of the discussions on the matter of how the cultures of the world grew at different rates.
We also have Jared Diamond's insight about this in the discussion : "Why did Human History Evolve Differently in Different Continents in the Last 13,000 years?"