The Empire of Ghana and the Gold Trade
In the grasslands south of the Sahara, the Empire of Ghana grew immensely wealthy as the middleman in a trade of two things the world craved: West African gold and Saharan salt, exchanged nearly pound for pound. Camel caravans crossed the desert like ships crossing a sea, and Ghana taxed every load. It was the first of a series of great, rich West African empires — a reminder that sub-Saharan Africa was never isolated but a wealthy hub of a trans-Saharan world economy.
The path or medium of exchange moves, and a place or power rises or declines because it sits on or off the new route.