In some cases, they were also used as farm laborers.

Parts of Portugal were suffering from a distinct shortage of farm laborers, and Africans filled the void.
At the beginning of the sixteenth century, in some sections of rural Portugal as much as one third of local population was African in origin.
Even so, European labor needs could not support much of a slave trade for long.

The enclosure system was under way, changing farming techniques, and it had created a labor surplus.

However, at the same time, emerging capitalism financed explorations in Africa, Asia, and the western hemisphere.

African sailors were involved in most of these explorations including Columbus's voyage in 1492.