Most of the English colonists had arrived as indentured servants, hiring themselves out as laborers for a fixed period to pay for their passage. In the early years the line between indentured servants and African slaves or laborers was fluid. Some Africans were allowed to earn their freedom before slavery became a lifelong status. Most of the free colored families formed in North Carolina before the Revolution were descended from unions or marriages between free white women and enslaved or free African or African-American men. Because the mothers were free, their children were born free. Many had migrated or were descendants of migrants from colonial Virginia. As the flow of indentured laborers to the colony decreased with improving economic conditions in Great Britain, planters imported more slaves, and the state's legal delineations between free and slave status tightened, effectively hardening the latter into a racial caste. The economy's growth and prosperity was based on slave labor, devoted first to the production of tobacco.