It is not known when HIV entered the human population although, using computer analyses, it has been estimated to be around 1930). It is thought that a type of Simian Immunodeficiency Virus (SIV) known as SIVcpz, that infects chimpanzees (Pan troglodytes), is the virus that jumped to the human population in central Africa. However, until recently, the  SIVcpz isolates found in laboratory animals were most closely related to HIV-1 type N which is much less common than type M (main) that probably gave rise to the AIDS epidemic. In 1999, a type of SIVcpz was discovered in a captive chimpanzee that is very similar to the major circulating type of HIV-1.

Since the SIVcpz most similar to HIV-1 type M was found in a captive chimpanzee, it was not possible to determine where in the wild HIV-1 first arose as a result of SIVcpz crossing over to humans. It is also possible that the natural SIV reservoir in Africa could have been another simian species. In 2006, the analysis of feces of wild chimpanzees showed that animals in south east Cameroon harbor SIVcpz that is extremely similar to HIV-1 type M and this is thought to be the precursor of the HIV epidemic. This finding is further evidence that SIVcpz did not cross over to humans as a result of the use of SIV-infected attenuated polio vaccine since the vaccine trials were carried out near Kingali in the Republic of Congo which is over 500 miles away. Type N HIV-1 is suspected to have arisen in humans in the same region. The origin of type O HIV-1 remains obscure.