But the major change if you're looking for the one answer is probably going to be climate change at the end of the Ice Age. The end of the Ice Age, the melting of the glaciers, is going to cause global changes in the way that the land and the environment is structured. So it might be in the Middle East, say in Syria, where suddenly there is kind of a fertile valley where food will grow more easily giving people the opportunity to sit down. In other areas there might be a lack of an animal food supply which is going to cause people to have to sit down and figure out how do we eat. And climate change would be probably top on my list. But there's also other reasons that archaeologists and archaeogenetics people have put out there including the population just growing too large to be a huntering and gathering society. So when you have too many people you have to figure out how to feed them.