Many researchers have used models and equations to explain how human cooperation evolved. The current research shows that these cooperative behaviors can only spread throughout a population when there are public institutions that broadcast reputations and interactions. However, public institutions and public sharing of reputations have not been around forever. Ancient and medieval trading societies demonstrated hundreds of years of successful, cooperative economic activity acting under private assessment without publicized reputations. Here, I evaluate the evolution of cooperation in everyday trade in Ancient Pompeii and Viking Age Scandinavia that both existed and thrived under private assessment. Using a model and excavated archaeological and literary materials, I show that a new discriminator behavioral strategy with added human randomness and cumulative reputation values can produce evolved cooperation under private assessment. I also demonstrate that cumulative reputation values that include first and early impressions, compared to the current discriminator strategy that only accounts for an individual’s most recent impression, significantly increases evolved cooperation in a population.