Upon the death of Muhammad, his father-in-law Abu Bakr became the first caliph and united the Arab people under one leader. As caliph, Abu Bakr led the conquest of much of the Byzantine Empire and all of the Persian Empire. There emerged two sects of Islam, the Shiites who believe leadership should pass directly through Muhammad's descendants, and the Sunnis who believe any pious Muslim male could lead. Sufism emerged as a mystical branch of Islam where followers dedicate their lives to studying Islam through practices like whirling dances intended as a form of prayer.