How did HIV originate?

In the early 1980s it was demonstrated that AIDS results from an infection with HIV-1, the human immunodeficiency virus. Based on comparisons of HIV sequences from various sources, the origin of the AIDS virus can be pinpointed to Africa in the 1930s. These comparisons reveal that the earliest sample of HIV is from a blood sample drawn from an African in 1959. 

HIV-1 was identified in 1993 by Dr. Robert Gallo and other medical scientists at the National Cancer Institute in Bethesda, Maryland. At about the same time, Dr. Luc Montagnier of the Pasteur Institute in Paris isolated HIV-2 from AIDS patients. These two HIV viruses are distinguishable by their genome make-up today, but are believed to have had a common ancestor in Africa. Scientists think that HIV-2 was transmitted to humans from a simian source, probably the sooty mangabeys that are hunted for food in Senegal, Guinea, the Ivory Coast and Gambia.

HIV-1, which is much more lethal, is most closely related to SIVcpz, Simian Immunodeficiency Virus, and was transmitted from the Central African Chimpanzee, Pan troglodytes troglodytes. HIV-1 mutates very rapidly, and Simian Immunodeficiency Virus crossed over to humans (on at least three separate occasions) as Human Immunodeficiency Virus. As the result of mutation, 11 different subtypes of HIV-1 have been identified, and are designated as A through K.