Is ethnic diversity good or bad for economic development? Most studies find corrosive effects. This paper shows that historical exposure to economic exchange can mitigate these effects in the long run. I collect data from a natural experiment of Peru's colonial history: the forced resettlement of native populations in the sixteenth century. Where the resettlement concentrated ethnically diverse populations with a history of internal crop exchange, contemporary populations perform better systematically. Additional evidence suggests that prior experience with mutually beneficial crop exchange shaped more open attitudes toward out-group members. Economic complementarities helped sustain long-run, market-oriented cooperation and local trade. (JEL F54, J15, N16, N36, N56, O15, Q12)