Engineering has existed since prehistoric times out of necessity for survival. Some of the earliest engineering designs included irrigation systems, the wheel and axle, and the plow. Ancient civilizations like the Egyptians and Babylonians built impressive structures using engineering principles. Over time, innovations accelerated, including development of the steam engine, electricity, airplanes, computers and more. Several engineering feats over 100 years old are still in operation today, such as suspension railways and transporter bridges. Engineering has progressed through incremental innovations building on past ideas.