Since the 1980s, there has been a lot of debate around the origin of HIV.  HIV is a type of lentvirus, which means it attacks the immune system.  In a similar way, the SIV virus attacks the immune systems of monkeys and apes.  Research found that HIV related to SIV and there are many similarities between the two viruses.  HIV-1 is closely related to a strain of SIV found in chimpanzees and HIV-2 is closely related to a strain of SIV found in mangabeys.  In 1999, a strain of SIV was found in a chimpanzee that was almost identical to HIV in humans.  The researchers who discovered this connection concluded that it proved chimpanzees were the source of HIV and that the virus had crossed species from chimps to humans.  The same scientists then conducted more research into how SIV could have developed in the chimps.  They discovered that the chimps had hunted and eaten 2 smaller species of monkeys and became infected with 2 different strains of SIV.

Studies of some of the earliest known samples of HIV provide clues about when it first appeared in humans and how it evolved.  The most studied strain of HIV is HIV-1 Group M, which is the strain that has spread throughout the world and is responsible for the vast majority of HIV infections today.