History

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It is obvious that bridges were made by nature itself. Even in the most historic human communities must have created bridges from material that can be easily gathered. For example, A fallen tree can be dragged into position to serve as a plank. The leaves from specific plants can be made as an elementary suspension bridge. Or rafts can be tied together to make a floating bridge. 

Then later people started to study concepts of building bridges"The greatest bridge builders of antiquity were the ancient Romans. The Romans built arch bridges and aqueducts that could stand in conditions that would damage or destroy earlier designs." (1)