The islands were uninhabited until their discovery by Portuguese explorers in the 15th century. Gradually colonized and settled throughout the 16th century, they collectively served as a vital commercial and trade center for the Atlantic slave trade. The rich volcanic soil and proximity to the equator made São Tomé and Príncipe ideal for sugar cultivation, followed later by cash crops such as coffee and cocoa; the lucrative plantation economy was heavily dependent upon African slaves. Cycles of social unrest and economic instability throughout the 19th and 20th centuries culminated in peaceful independence in 1975. São Tomé and Príncipe has since remained one of Africa's most stable and democratic countries. 