My main thought about this is that it seems to have taken a tremendously long time for human beings to make it out of South Asia to places like North Asia, Australia, and the Americas. Given what I know about the rest of the chapter, it seems clear that Diamond's "Great Leap Forward" was a very important event. Once that leap happened, humans really started to disperse from the areas that were relatively easily reached from Africa. They were able to build boats that were good enough to get them to New Guinea and Australia. They were able to endure cold enough conditions to get to Siberia and then across the land bridge to the Americas. Something big clearly happened to cause this relatively sudden explosion into new areas after humans had been present in South Asia for so long.