The article explores how different systems of trade can naturally develop, leading to the emergence of money as a medium of exchange. Through a game where players can choose to trade locally or pay to trade elsewhere, the researchers found that both decentralized and centralized markets can form. Centralized markets are more efficient and likely to emerge when people perform diverse tasks and interact in large groups with uncertain future encounters. This study suggests that understanding how markets organize themselves is crucial for understanding how money systems evolve.