In 750, the Abbasids overthrew the Umayyads. Arabs Muslims and Persians of all strata made up the rebel army, which was united by the converted Persian Muslim, Abu Muslim. In their struggle for power, the society in their times gradually became cosmopolitan and the old Arab simplicity and aristocratic dignity, bearing and prestige were lost. Persians and Turks began to replace the Arabs in most fields. The fusion of the Arab nobility with the subject races, the practice of polygamy and concubinage, made for a social amalgam wherein loyalties became uncertain and a hierarchy of officials emerged, a bureaucracy at first Persian and later Turkish which decreased Abbasid prestige and power for good.