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 strife. Chinese civilization was traditionally thought to have first emerged in the Yellow River or Huang He valley, which along with the Yangtze basin constitutes the geographic core of the Chinese cultural sphere. However, current views have considered the origin to be a simultaneous duality of both the Yellow River and the Yangtze. 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. This lens also tends to assume Chinese civilization can be traced as an unbroken thread many thousands of years into the past, making it one of the cradles of civilization.