The history of Guyana begins about 35,000 years ago with the arrival of humans coming from Eurasia. These migrants became the Carib and Arawak tribes, who met Alonso de Ojeda's first expedition from Spain in 1499 at the Essequibo River. In the ensuing colonial era, Guyana's government was defined by the successive policies of Spanish, French, Dutch, and British settlers. During the colonial period, Guyana's economy was focused on plantation agriculture, which initially depended on slave labor. Guyana saw major slave rebellions in 1763 and again in 1823. Great Britain passed the Slavery Abolition Act in British Parliament that abolished slavery in most British colonies, freeing more than 800,000 enslaved Africans in the Caribbean and South Africa as well as a small number in Canada. It received Royal Assent on August 28, 1833, and took effect on August 1, 1834. Thus, in the immediate period following this historical law, slavery was ended in British Guiana. To address the labor shortage, plantations began to contract indentured workers mainly from India. Eventually, these Indians joined forces with the Afro-Guyanese descendants of slaves to demand equal rights in government and society, demands underscored by the 1905 Ruimveldt Riots. Eventually, after the second world war, the British Empire pursued policy decolonization of its overseas territories and independence was granted to British Guiana on May 26, 1966. Following independence, Forbes Burnham rose to power, quickly becoming an authoritarian leader pledging to bring socialism to Guyana. His power began to weaken with the international attention brought to Guyana in the wake of the Jonestown massacres in 1978. After his unexpected death in 1985, power was peacefully transferred to Desmond Hoyte, who implemented some democratic reforms before being voted out in 1992.