I assume they were brought in by Mongolian conquest, but I don't actually know. I guess I'm thinking of Sichuan, Gansu, and Xinjiang specifically.
Some time early in Han Dynasty, (around 200 BC), China began to establish military outposts in Western China, stretching toward the Gobi Desert, defend its borders from the Xiongnu/Hun incursions.
After the fall of Han and Jin Dynasty, the proto-Tibetan ethnic groups Di and Qiang rose in power and established several Chinese Kingdoms.
One of which is the Western Wei Kingdom. Around 380AD, Western Wei conquered most of Northern China (including the current day Xinjiang), and attempted to conquer all of Southern China, but failed.
Qinghai and Xinjiang have been intermittently under Chinese control since the Western Han Dynasty (roughly 200 BCE to 1 CE). At times they were lost to local ethnic groups or conquered by outsiders as well. However Sichuan has remained part of China since its conquest in 300 BCE.
One area of Asia in general is the Zomia region. This region was in Western Asia, and contained people who fled civilization. They were nomads, hill people, with no government. Most of the 100 million people living in this area still are not fully governed, despite being in modern nation states borders.
Links: http://www.boston.com/bostonglobe/ideas/articles/2009/12/06/the_mystery_of_zomia/
http://geography.about.com/od/politicalgeography/fl/Zomia-A-Region-Resisting-Modern-Statehood.htm
https://www.youtube.com/watch?v=wyzi9GNZFMU&list=UUX6b17PVsYBQ0ip5gyeme-Q
Picture: http://upload.wikimedia.org/wikipedia/commons/1/12/Zomia.jpg
