As farming developed, grain agriculture became more sophisticated and prompted a division of labor to store food between growing seasons. Labor divisions then led to the rise of a leisured upper class and the development of cities. The growing complexity of human societies necessitated systems of writing and accounting. Many cities developed on the banks of lakes and rivers; as early as 3000 BCE some of the first prominent, well-developed settlements had arisen in Mesopotamia, on the banks of Egypt’s River Nile, and in the Indus River valley. Similar civilizations probably developed along major rivers in China, but archaeological evidence for extensive urban construction there is less conclusive.