The Fourth Discovery

With the invention of the harness, mankind was no longer limited to his own physical strength but could now use beasts of burden. With the invention of the steam engine, mankind was no longer limited to the physical strength of animals but could now use machines to do labour in places that didn't have running and falling water. The result was the Industrial Revolution. Yes, the Industrial Revolution had unpleasant social effects, but having worked our way through those our modern society is very much a product of the steam engine.