Crop surpluses were possible due to the technological improvements that were implemented in agriculture. Former civilizations were not able to trade with each other, because they were only able to produce food for self-subsistence. On the contrary, when productivity increased, so did production, and the extra food that was not consumed could be traded with neighbour communities. Each community started to specialize on a certain product, to sell the production surpluses, and to use the money earned to buy the goods that they did not produce by themselves.
