“This is how we tend to learn about world history: as discrete civilizations that developed on their own over time. Now imagine a different kind of museum, one organized solely by era. You could stroll through a gallery, for example, and see a Roman silver denarius coin, a bronze coin from China’s Han dynasty, and a punch-marked coin from India’s Mauryan Empire. You would see right away that three major civilizations were going through remarkably similar changes at roughly the same time, despite the vast distance between them: each had become an empire, and each was running an economy based upon coin currency.”