How did the Bhutia people originally come to Bhutan?

The Bhutia people, also known as the Ngalop, are believed to have migrated southward from Tibet into Bhutan beginning in the 8th or 9th century CE.12

As descendants of these Tibetan immigrants, the Bhutia settled primarily in the western and central regions of the country. Today, they constitute the largest ethnic group in Bhutan and play a dominant role in the nation's political life.12