According to pious tradition, the concept of the rosary was given to Saint Dominic in an apparition of the Virgin Mary in the year 1214 in the church of Prouille. ... In the 15th century [the apparition] was promoted by Alanus de Rupe (aka Alain de la Roche or Saint Alan of the Rock), a Dominican priest and theologian, who established the "fifteen rosary promises" and started many rosary confraternities.
According to Herbert Thurston, it is certain that in the course of the twelfth century and before the birth of St. Dominic, the practice of reciting 50 or 150 Ave Marias had become generally familiar. Despite the popularity of Blessed Alanus's story about the origins of the rosary, there has never been found any historical evidence positively linking St. Dominic to the rosary. The story of St. Dominic's devotion to the rosary and supposed apparition of Our Lady of the Rosary does not appear in any documents of the Church or Dominican Order prior to the writings of Blessed Alanus, some 250 years after Dominic. Leonard Foley noted that although Mary's giving the rosary to St. Dominic is recognized as a legend, the development of this prayer form owes much to the Order of Preachers.
From the 16th to the early 20th century, the structure of the rosary remained essentially unchanged. There were 15 mysteries, one for each of the 15 decades. In the 20th century the addition of the Fatima Prayer to the end of each decade became more common. There were no other changes until 2002 when John Paul II instituted five optional new Luminous Mysteries.