Trade did not happen in human history until crop surpluses were generated and, in turn, these were possible due to the technological improvements that were implemented in agriculture. Ancient civilizations had not been able to trade with each other, because they were only able to produce food for self-subsistence.
On the contrary, when new technology was implemented and productivity increased, so did the total outpu. The extra food that was not consumed started to be traded with neighbour communities. Each community started to specialize on cultivating 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.