The nation of Tibet occupies the high-altitude Tibetan plateau and has since the beginning of the study period. Communications with the south are blocked by the Himalayas, and are tenuous in other directions, so Tibet has had few foreign visitors until recently.
The thirteenth century: Tibet became part of the Mongol Empire. The Mongols were impressed by and eventually adopted Tibetan Buddhism. This created a cultural connection lasting until the eighteenth century. If your ancestors were in Tibet in 1490, they might have had Mongolian ancestors as well in 1000.
1959: Although in the early 1700s the Manchus made Tibet a protectorate of their Qing Dynasty. China had little influence in Tibet until the rise of the People's Republic of China. In 1959, China took over Tibet and abolished its traditional government. Since then, there has been considerable ethnic Chinese immigration to this region. If your ancestors were in Tibet recently, then they or their ancestors might have lived in China proper in 1900.