Kind of obvious ones for you but im going to go with people and recourses.
cities and goverment
agriculture, and writing
The two things that kept China from civilization and interacting with the rest of the world is Yang and Yin.
Drought and a decline in trade
Penis
A civilization needs to have a structured government and a military. These two things of course are pointless without having population and architecture that is unique to their culture.
It means two things are alike but not in every way!
An ellipse have two focal points.
A common obstacle that every civilization must face is hurricanes,storms,etc.
wings and feathers
A number and a unit.
every civilization starts with water. in this case the hung he river.