Guyana is a majority Indo-Guyanese, with the next largest majority being Afro-Guyanese. The roots of this can be traced back to colonialism and slavery.

Basically, back when Guyana was British Guiana, it was full of plantations growing sugar. These plantations needed labor, so, like other sugar plantations in the Caribbean, they imported African slaves to do the work.

When the British abolished slavery in 1834 with its gradual emancipation law, about 89,000 slaves eventually earned their freedom by 1838. Now, these former slaves were not happy about how they were treated under slavery, and wanted to better improve their lives versus go back and work in the fields. However, as British Guiana still depended on sugar as an export cash crop, sugar planters still needed labor to harvest and process the sugar.

This problem was eventually resolved through indentured servitude. In order to fulfill the needs of the sugar plantations, indentured servants from other parts of the empire, especially British India, were imported by the thousands to work in the fields under five-year contracts. Indentured servitude was basically slavery all over again, but this time it was encouraged by antislavery proponents and involved compensation.

Eventually, by the end of World War II, the number of Indians in British Guiana outnumbered the number of Africans, and that's basically how it's been since. This would have significant consequences in British Guiana's (and later Guyana's) political history.

Source: Rabe, Stephen G, US Intervention in British Guiana, A Cold War Story.