The Portuguese explorer, Fernão do Pó, seeking a route to India, is credited with having discovered the island of Bioko in 1472. He called it Formosa ("Beautiful"), but it quickly took on the name of its European discoverer. The islands of Fernando Pó and Annobón were colonized by Portugal in 1474. The Portuguese retained control until 1778, when the island, adjacent islets, and commercial rights to the mainland between the Niger and Ogoue Rivers were ceded to Spain in exchange for territory in the American continent (Treaty of El Pardo, between Queen Maria I of Portugal and King Charles III of Spain). From 1827 to 1843, Britain established a base on the island to, supposedly, combat the slave trade. However human trafficking continue through existing networks of slave trade established long before Europeans arrived. The mainland portion, Rio Muni, became a protectorate in 1885 and a colony in 1900. Conflicting claims to the mainland by France and Spain were settled in 1900 by the Treaty of Paris, and periodically, the mainland territories were united administratively under Spanish rule. Between 1926 and 1959 they were united as the colony of Spanish Guinea. During the Spanish Civil War (1936-1939), the nationalist side from Fernando Poo (who rebeled on September 19, 1936) overtook the Republican forces of Río Muni on October 14, 1936.
The population of the colony was stratified as: