城市形成的原因？

我用英文回答！
In 3500BCE, Masopotamia. People live in small village, but the pupulation increase, so the farmer don't have land to farm. The people moved to Sumer. But it have a problem which is uncontroled water. The Sumerian (the people lived in Sumer) found a sulution, which is built irrigation system. A new problem came out again, people need to work together to toke care of the irrigation system. The sumerian lived togerther thich is became a city-state. Each city-state fought, so the Sumerian made a stronge wall to save the city-state.
That is the first city of the world!

The cause of the city made is the people need to work together of some reason, like weather or need to keep something. The people came together and made the village increase,so it became a large city!

房子多了人住得多了就变成城市了

让世界好分别一写啊！！！