Often by engaging in sacred marriage, specifically scoodily-pooping with the high priestess of the city's temple. So the priests were overtaken by kings, who soon declared themselves priests. Thanks, Thought Bubble. So how do we know that these kings were scoodily-pooping with lady priests? Because they made a scoodily-pooping tape and put it on the internet. No, because there's a written record. Mesopotamia gave us writing, specifically a form of writing called cuneiform, which was initially created not to, like, woo lovers or whatever, but to record transactions, like how many bushels of wheat were exchanged for how many goats.