Bhutan has long maintained a policy of strict isolationism, both culturally and economically and as a result the Bhutanese culture is one of the oldest (dating back to the mid 17th century) and well preserved cultures in the world. In fact it has only been in the recent decades of the 20th century that foreigners have been allowed to visit the country, and only in limited numbers. Bhutan has an estimated population of 600,000 which is sparsely distributed throughout the country. There are three main ethnic groups in Bhutan; the Ngalungs (often called Drukpas) live in the north-western region and their ancestors migrated from Tibet, the Sharchops who inhabit the eastern and central region and derive from northern Burma and northeast India and lastly the Lhotshampas are the third ethnic group who live in the southern foothill districts and are of Nepali origin. The Bhutanese revere the tantric guru Padmasambhava the founder of Himalayan Buddhism in the 8th century.