The social and economic reasons for the appearance of the blues are not fully known. Blues has evolved from an unaccompanied vocal music of poor black laborers into a wide variety of styles and subgenres, with regional variations across the United States. African American work songs were an important precursor to the modern blues; these included the songs sung by laborers like stevedores and roustabouts, and the field hollers and "shouts" of slaves. The first appearance of the blues is not well defined and is often dated between 1870 and 1900, a period that coincides with the emancipation of the slaves and the transition from slavery to sharecropping and small-scale agricultural production in the southern United States.