The Silk Road
The 'Silk Road' is a 19th-century coinage — the geographer Ferdinand von Richthofen minted it in 1877 — not an ancient one. There was no single road, and silk was only one of many things travelling on it. What it names is the web of overland trade routes that connected China, central Asia, Persia, India, and the Mediterranean from roughly the second century BCE, when the Han envoy Zhang Qian opened contact with the west, to the fifteenth CE, when ocean routes made the caravans redundant. Chinese silk reached Roman senators — the empire fretted about the bullion draining east to pay for it. Roman glass reached Han emperors. Buddhism walked from India to China on the same network.
The Silk Road is the prototype of a particular kind of long-distance economic geography: a chain of intermediaries, each handling one segment, with no single merchant making the entire journey. Sogdian traders, operating from Samarkand and Bukhara, dominated the central Asian segment for centuries — their language became the trade's lingua franca and their letters survive in the watchtowers of Dunhuang; Persian and Arab merchants ran the western portions; Chinese officials regulated the eastern terminus at Chang'an. The road moved goods, but more consequentially it moved ideas: paper-making (carried west, by tradition, after the Battle of Talas in 751), printing, gunpowder, and the magnetic compass — the technologies Francis Bacon later credited with making the modern world — all reached Europe via the Silk Road, generally one to ten centuries after their Chinese origin. So did diseases. The Black Death almost certainly travelled west on the same routes that carried silk, reaching the Genoese colony of Caffa in 1347 and killing perhaps a third of Europe within five years. The road also moved religions: Buddhism from India to China and Japan, Manichaeism, Nestorian Christianity, and eventually Islam, which followed the trade until the oasis cities of central Asia were overwhelmingly Muslim. The caravans never moved much tonnage — camels, not container ships — so it was high-value, low-bulk goods and, above all, the things that travel free with people that mattered most. Traffic surged whenever a single power — Tang China, then the Mongol Pax Mongolica of the thirteenth and fourteenth centuries — secured the whole length at once, and collapsed when it splintered.