The first substantial documentary information on the Wolof dates from the travels of Ca da Mosto from 1455 to 1457. According to oral traditions, however, it was probably during the preceding century that the Wolof were unified into a loose political federation known as the Dyolof Empire, centered in northwestern Senegal. Around the middle of the sixteenth century, this empire fragmented into its component parts, giving rise to the four major Wolof kingdoms of Baol, Kayor, Dyolof proper, and Walo. The subsequent history of these kingdoms is rife with political intrigue, rebellions, exploitation, and warfare, both against one another and against the Moors. European contacts did not become of major significance, except for the slave trade, until the nineteenth century. Gradually, a few commercial centers were established along the coast, the principal ones being the key slave ports of Saint Louis and Gorée. Peanut growing was introduced into Senegal around 1840, and peanuts soon became the main export. In the 1850s, primarily to protect their economic interests, the French launched their first serious attempts to conquer the Wolof kingdoms. The Wolof put up a bitter resistance, but, by the end of the century, they were completely subjugated; French colonial rule lasted until the independence of Senegal in 1960. During this same period, the Wolof, who had a long and ambivalent (often hostile) involvement with Islam, became rapidly and thoroughly Islamicized. The French stimulated the development of urban centers, which became the major sources of Westernization during the twentieth century.