Answer:
First and foremost is the change from nomadic to sedentary life. A sedentary society is one that doesn't move around and is permanently settled in one place. When early humans began farming, they were able to produce enough food that they no longer had to migrate to their food source. This meant they could build permanent structures, and develop villages, towns, and eventually even cities.
Closely connected to the rise of settled societies was an increase in population. The ability to farm also meant a greater ability to control the amount of food produced, which meant that, for the first time in human history, there was a surplus of food. This, along with the lower rates of fatal injuries that were common amongst nomadic societies, led to population booms. For the first time, there was enough food to sustain larger populations, and those small settlements turned into some pretty sizable cities.
Hope it helps...