Answers (1)
A good explanation of the origin of HIV-1 is given at this site:
www.advert.org/origin-aids-hiv.htm
It comes from wild chimpanzees in the Cameroon in Africa, it is thought to be a natural mutation of the Simian Immunodeficiency Virus (SIV) that has existed for 32000 years to the SIVcpc strain which is almost identical to HIV-1
Transfer to humans may have happened by infected chimp being killed for food by a hunter who was infected through any cuts or consumption of partially cooked meat.
I'm voting this up because I believe the bushmeat answer is the most probable reason for the spread of SIV/HIV.