The domestication of the horse around 3500 BCE on the Eurasian steppe was among the most consequential events in human history -- transforming warfare, trade, agriculture, and political organization across Eurasia, enabling the steppe empires that repeatedly reshaped the ancient and medieval world, and reverberating through the Columbian Exchange to transform the Americas.
Before the horse, the fastest thing a human being could do was run. After the horse, a person could travel hundreds of miles in days, strike and retreat before an enemy could respond, and coordinate military action across vast distances. This single change -- domesticating an animal that could be ridden -- reorganized the military, political, and economic geography of Eurasia for five thousand years. The horse made the Mongol Empire possible. It carried the Indo-European languages across Eurasia. It transformed the Great Plains of North America after 1492. The internal combustion engine replaced it -- but we still measure engine power in horsepower.