When farming first began, the villages were fairly small and farmers barely had enough to eat for the year. In addition, farmers didn't have a very big population. Finally, farmers didn't have a lot of experience with agriculture.
Bigger, Better Buildings
When farmers discovered new farming technologies they were able to harvest food more efficiently and quicker. Also, farmers were able to create a surplus so they could have extra wheat for the coming year.
After farmers had extra food more people began to move into their village which increased their population. By increasing their population they were able to attract more farmers. Finally, because of more farmers moving in to the village this allowed them to grow more food.
After the population increased the people in the village had to make bigger and better buildings so the new people could have a home and not be sleeping outside every day.
After people built bigger and better buildings the townspeople realized that they would not survive if everybody was working on one thing. So the people made other residents do other things such as fishing and animal taming which allowed other people to specialize in different things.
After humans completed Early Farming Villages, New Farming Technology, Surplus, Population Growth, Bigger, Better Buildings and Job Specialization they were able to use all of the things they have learned in the past to begin creating cities.