The history of China spans several millennia across a wide geographical area. Each region now considered part of the Chinese world has experienced periods of unity, fracture, prosperity, and hardship. Classical Chinese civilization first emerged in the Yellow River valley, which along with the Yangtze and Pearl River basins now constitute the geographic core of China and have for the majority of its imperial history. China maintains a rich diversity of ethnic and linguistic people groups. The traditional lens for viewing Chinese history is the dynastic cycle: imperial dynasties rise and fall, and are ascribed certain achievements. Throughout pervades the narrative that Chinese civilization can be traced as an unbroken thread many thousands of years into the past, making it one of the cradles of civilization. At various times, states representative of a dominant Chinese culture have directly controlled areas stretching as far west as the Tian Shan, the Tarim Basin, and the Himalayas, as far north as the Sayan Mountains, and as far south as the delta of the Red River.