As far as we know modern humans have been around for ~ 150,000-200,000 years. During this time until ~11,000 the Earth was in an Ice Age. Release from that Ice Age is correlated with the development of settled agricultural city-states, which dramatically increased our ability to ratchet. I believe, as Jared Diamond outlined in "Guns, Germs, and Steel" that the disproportionate ratcheting seen in different areas of the world was a product of several environmental factors. The most importan of those factors being a) domesticatable animals and plants, b) a continent w a large east-west axis (as opposed to north-south which impedes diffusion on domesticated plants and animals), and c) the development of a large network of civilizations which feed off of each others growth.