What evidence has been found to support the theory that these ancient cities must have had a governing body?

- Written laws

- Architectures resembling town hall

Explanation:

One example of written laws that founded by Archeologists  was the Stele of Hammurabi's codex that found in 1901. It's a set of written laws that originated from Ancient Mesopotamia. The existence of law indicates that ancient society had a governing body that create legislations to maintain the order in society.

On top of that, a lot of ancients civilizations had some form of architectures that's designed to accommodate a lot of people with one podium in center. Those architectures is predicted to be used as a place where government officials communicate with the people about legislations.

yes he did writing dots to make the answer