How did the Portuguese slave trade start

They began to kidnap people from the west coast of Africa. The Spanish took the first African captives to the Americas from Europe as early as 1503, and by 1518 the first captives were shipped directly from Africa to America. The majority of African captives were exported from the coast of West Africa, some 3,000 miles between what is now Senegal and Angola, and mostly from the modern Benin, Nigeria and Cameroon.