The Domestication of the Horse
On the grasslands of the Eurasian steppe, herders learned to ride and harness the horse, and the effect rippled across a continent. Mounted and mobile, steppe peoples could move faster and strike harder than any settled society, carrying their languages and bloodlines deep into Europe and Asia. The domesticated horse redrew the human map: it spread the Indo-European tongues that half the world still speaks, and made the steppe a wellspring of conquerors for the next three thousand years.
Large movements of peoples — pushed or pulled — reshape the societies they leave and the ones they enter.