Zoonosis is the crossover of the virus from animals to humans. It is possible that the mutated form of the SIV contracted by the chimpanzees, the red-capped Mangabeys and greater spot-nosed monkeys could have transferred to humans.
These are the possible theories about how SIV became HIV:
The 'Hunter' Theory: Humans have been hunters and gatherers since their origin. It is possible that the SIVcpz could have been transferred to humans when they hunted and ate the monkeys and chimpanzees. Or due to their infected blood getting into cuts and wounds of the hunters. Generally the body would have resisted the infection, but as these are lentiviruses they act slowly on the immune system or simply the virus could have mutated to HIV 1 and adapted itself in the body of the new host. There are many different strains of HIV each with a different genetic makeup, each time it passed from a chimp to a human it adapted developing a different strain - hence it was difficult to detect it too.
The Lancet published an article in 2004, which discussed how the retrovirus of SFV or the Simian Foamy Virus, transfers from primates to humans - when the hunters butchered and consumed monkey and ape meat. Such discoveries resulted in the ban of bushmeat to prevent passage of simian viruses into humans.
The Contaminated Needle Theory: As Africa is a poor yet highly populated continent, it is possible that one syringe was used to inoculate many people - one of these people could have been 'the Hunter' infected with the SIV/HIV. Even a drop of his blood could have carried the virus and transmitted it to others who were inoculated after him. The SIV could have mutated inside him or so many others who were infected by that needle used to inoculate him.
The Oral Polio Vaccine theory: This theory suggests that HIV can be traced to the testing of the oral polio vaccine - Chat - in the Congo, Urundi and Rwanda in the late 1950s. To reproduce it, the vaccine needs to be cultivated in live tissue, for which the kidney cells of local chimp population was used, possibly a carrier of SIV, leading to their mutation into HIV1. Though it has been argued that as the vaccine has to be taken orally and the mouth and throat lining are well-equipped to ward-off infections it is very unlikely that this could be the cause of the spread of HIV. Later it was even proven that only macaque monkeys are resistant to these viruses and their kidney cells were used for the production of the vaccine. As only one phial of the then produced vaccine was found and it happened to be clean does not remove the possibility of others being contaminated, also this phial was tested only for the group M strain of HIV, what about the others? It is also suggested that the virus could have existed in these humans before the vaccine trials.