It started in the 10th Millennium BC after major climate changes and the end of the glaciation. This touched off the neolithic age or "new stone" age where surviving humans started developing agriculture and the domestication of animals. The first permanent villages were created at this time and there was massive flooding due to the melting glaciers for hundreds of years. Over the next few thousand years, these agricultural and domestication techniques were perfected and humans migrated to major flooding areas--Nile Valley, Mesopotamia, Indus Valley, and Huang Valley. The Neolithic Age is the answer.