system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If places like Baghdad and Cairo can reach 40-50 C in the summer, how did they become the center of civilization in ancient and medieval times when there was no air conditioning?
gpt
Ancient peoples knew how to put up shade and keep things cool in very hot climates, and both cities lie along major rivers. Almost all ancient cities are along rivers or oceans, since it (a) moderates the climate relative to nearby areas, (b) provides food and water and (c) allows trade up and downriver.