When Madagascar separated from the African Continent the evolution of primates separated into two paths. One that led to the lemurs.lorises and tree shrews and the other which gave rise to the monkeys and later to higher evolved anthropoids which included the great apes and eventually hominids of which we are the only hominids in existence. We are genetically very close to chimpanzees and gorillas. It is thought that a type of virus that existed in chimps and gorillas mutated and was transmitted to humans who processed bush meat(the flesh of chimps or gorillas). This mutation was a type known today as HIV. This is only a plausible hypothesis and needs further exploration.