For centuries the Swahili depended greatly on trade from the Indian Ocean. The Swahili have played a vital role as middle man between east, central and South Africa, and the outside world. Trade contacts have been noted as early as 100 A.D. by early Roman writers who visited the East African coast in the first century. Trade routes extended across Tanzania into modern day Zaire. Along these goods were brought to the coasts and were sold to Arab, Indian, and Portuguese traders and even reached as far as China and India. Materials of this trade were also found at Great Zimbabwe. During the Middle Ages, ivory and slaves became a substantial source of income. Many slaves who were sold in Zanzibar ended up in Brazil, which was then a Portuguese colony. Swahili fishermen of today still rely on the ocean to supply their primary source of income. Fish is sold to their inland neighbors in exchange for products of the interior.