In the 13 mainland colonies of British North America, slavery was not the peculiar institution of the South. This development would occur after the American Revolution and during the first decades of the 19th century. Although slaves had been sold in the American colonies since at least 1619, slave labor did not come to represent a significant proportion of the labor force in any part of North America until the last quarter of the 17th century. After that time, the numbers of slaves grew exponentially. By 1776, African Americans comprised about 20% of the entire population in the 13 mainland colonies.