Environment of evolutionary adaptedness, a term from evolutionary biology often used in evolutionary psychology. For humans the long period in which our ancestors lived as hunter-gatherers is most of the EEA. The EEA is not a single environment, but a weighted composite of the physical and social aspects of our ancestors environments.
The effect of the EEA on gene frequencies is highly dependent on the intensity of selection. For example, genes (several of them) for adult lactose tolerance have spread rapidly in cultures where milk is an important food source. (Links here [1] and many on Google for EEA lactose tolerance.)