Source : http://zh.wikipedia.org/wiki/%E8%92%99%E5%8F%A4%E6%97%8F
From the Shang Dynasty (商代) to Western Han (西汉), there was a nomadic group of people known as DongHu (东胡, ) who lived north of the present Hebei Province of PRC.
Circa 206BC, they were defeated by the XiongNu (匈奴) and the remnants were split into the ancestors of the WuHuan (乌桓) and XianBei (鲜卑).
During the Age of Fragmentation from the Jin Dynasty to the North-South Dynasties, the XianBei people further divided into groups known as MuRong (慕容部), TuoBa (拓拔部) and RouRan (柔然部) etc.
The RouRans waged wars against the TuoBas of Northern Wèi (北魏). Later, they were defeated by the Tujue (突厥) and split into northern and southern groups.
The southern group went to the upper reaches of the Liao River (辽河) and became known as the Khitans (契丹).
The northern group went to an area which was east of Yablonovy mountain ranges (雅布洛诺夫山脉) and south of the Stanovoi Range (外兴安岭). These became the ancestors of the ShiWei (室韦/失韦) people which was recorded to have relationship with the Tang Dynasty (唐朝).
The Mongols descended from a branch of the ShiWei.
