Agriculture is answer you're looking for! Nomadic bands had to hunt or gather their food which meant they had to move to find it if the weather changed or if their wasn't anything to eat in the area. That all changed when someone figured out how to grow their own food so they wouldn't have to leave a spot. About 9,500 BC is when we first got evidence of that although we think that wild grains were collected and eaten from at least 20,000 BC.
