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In page History of the United States:

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The first African slaves arrived in 1619.[3] Initially regarded as indentured servants who could buy their freedom, the institution of slavery began to harden and the involuntary servitude became lifelong[3] as the demand for labor on tobacco and rice plantations grew in the 1660s.[citation needed] Slavery became identified with brown skin color, and the children of slave women were born slaves (partus sequitur ventrem).[3] By the 1770s African slaves comprised a fifth of the American population.