Archaeology findings show that Sierra Leone has been inhabited for thousands of years. Traditional historiography has customarily presented it as peopled by successive waves of invaders; but the language pattern suggests that the coastal Bulom (Sherbro), Temne, and Limba have been in continuous settled occupation for a long time, with sporadic immigration from inland Mende-speaking people, including Vai, Loko, and Mende. Populations were historically organized in small political units of independent kingdoms or chiefdoms, the powers of whose rulers were checked by councils. Secret societies, notably the Poro society, also exercised political power and instructed initiates in the customs of the culture. Muslim traders brought Islam, and European contact began with Portuguese sailors Alvaro Fernandez in 1447 and Pedro Da Cintra in 1462. Located near present day Freetown, the Rokel estuary was established as an important source of fresh water for sea traders and explorers who traded goods such as swords and kitchen and other household utensils in exchange for beeswax and fine ivory works. By the mid-1550s, slaves replaced these items as the major commodity. British traders of the Royal African Company established forts along the coast for trading after the 1650s, but the British did not have a monopoly on the area. In 1808, the Freetown area became an official British Crown Colony. Trade flourished and in 1896 a protectorate was declared, which unified all the areas that form the basis of modern day Sierra Leone. Building on that foundation, the country developed a single identity with a shared history and culture. The language Krio developed out of a mix of various languages, but English became the official language. During British colonialism, Sierra Leone served as the seat of government for other British colonies along the West Coast of Africa. The first college for higher education south of the Sahara was established in 1827. The country is well known for its early achievements in the fields of medicine, law, and education, earning the nickname "the Athens of West Africa." Sierra Leone achieved independence from the British on April 27, 1961. Since independence, many changes have been experienced politically, economically, and socially in Sierra Leone. The 1990s brought successive civil disturbances, culminating into a ghastly war that led to the intervention of the international community, led by the United Nations Mission in Sierra Leone (UNAMSIL). Various international organizations helped provide an environment for the elections of 2002. The war has officially been over since 2002 and the process of rebuilding Sierra Leone and re-establishing its institutions is well underway. Since the war, Sierra Leone has held two multiparty elections which have been generally perceived as free and fair. Under a multiparty democracy, Sierra Leone's economy is gradually picking up. Mining and oil extraction have, in recent years, played a key role in providing additional income to the government and citizenry. With peace, the tourism sector is growing and shows signs of great potential. Many Sierra Leoneans who left during the war years are returning and bringing needed investment and expertise back to the country. Political stability and tolerance have fostered a peaceful co-existence and the country is making strides in many aspects of development.