Circular frame of hard material capable of turning on an axle. Wheels may be solid, partly solid, or spoked. The oldest known wheel was a wooden disk of planks held together by crosspieces. A pottery wheel or turntable was developed c. 3500 BC in Mesopotamia. The spoked wheel appeared c. 2000 BC on chariots in Asia Minor. Later developments included iron hubs that turned on greased axles. Perhaps the most important invention in human history, the wheel was essential to developing civilizations, and has remained essential to power generation, transportation, industrial manufacturing, and countless other applications.

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