The tea used for yak butter tea most probably comes from the mountains just west of Yunnan and Sichuan. What I have seen is a dark, formidable leaf with an intense flavor bordering on bitterness and smoke. Every Tibetan I have ever met, drinks it continuously and urges onto everyone whom he might meet.
As the name makes clear, tea and horses were the major commodities traded along this route from the Tang to the late Qing Dynasties, from around 600 AD to the 20th century. Trade and travel definitely took place along this route for several thousands of years before the Tang, as archaeological sites in Tibet and in the city of Chengdu demonstrate, but for us, this period is the most significant.