The history of Saudi Arabia as a nation state began with the emergence of the Al Saud dynasty in central Arabia in 1727 and the subsequent establishment of the Emirate of Diriyah. Pre-Islamic Arabia, the territory that constitutes modern Saudi Arabia, was the site of several ancient cultures and civilizations; the prehistory of Saudi Arabia shows some of the earliest traces of human activity in the world. The world's second-largest religion, Islam, emerged in modern-day Saudi Arabia. In the early 7th century, the Islamic prophet Muhammad united the population of Arabia and created a single Islamic religious polity. Following his death in 632, his followers rapidly expanded the territory under Muslim rule beyond Arabia, conquering huge and unprecedented swathes of territory (from the Iberian Peninsula in the west to modern-day Pakistan in the east) in a matter of decades. Arab dynasties originating from modern-day Saudi Arabia founded the Rashidun (632–661), Umayyad (661–750), Abbasid (750–1517), and Fatimid (909–1171) caliphates, as well as numerous other dynasties in Asia, Africa, and Europe. The area of modern-day Saudi Arabia formerly consisted of mainly four distinct historical regions: Hejaz, Najd and parts of Eastern Arabia (Al-Ahsa), and Southern Arabia ('Asir). The modern Kingdom of Saudi Arabia was founded in 1932 by Abdulaziz bin Abdul Rahman, known in the West as Ibn Saud. Abdulaziz united the four regions into a single state through a series of conquests beginning in 1902 with the capture of Riyadh, the ancestral home of his family. Saudi Arabia has since been an absolute monarchy governed along Islamist lines. Saudi Arabia is sometimes called "the Land of the Two Holy Mosques", in reference to Al-Masjid al-Haram (in Mecca) and Al-Masjid an-Nabawi (in Medina), the two holiest places in Islam. Petroleum was discovered on 3 March 1938 and followed up by several other finds in the Eastern Province. Saudi Arabia has since become the world's second largest oil producer (behind the US) and the world's largest oil exporter, controlling the world's second largest oil reserves and the sixth largest gas reserves. From 1902 until his death in 1953, Saudi Arabia's founding father, Abdulaziz, ruled the Emirate of Riyadh (1902–1913), the Emirate of Nejd and Hasa (1913–1921), the Sultanate of Nejd (1921–1926), the Kingdom of Hejaz and Nejd (1926–1932), and as the King of Saudi Arabia (1932–1953). Thereafter, six of his sons in succession have reigned over the kingdom: Saud, the immediate successor of