The Human Immunodeficiency Virus (HIV), which causes Acquired Immunodeficiency Syndrome (AIDS), represents a significant global health challenge. Its origin is zoonotic, meaning it transferred from animals to humans. Understanding this cross-species jump is important for public health initiatives and preventing future spillover events.
Identifying the Source Species
The direct ancestors of HIV are simian immunodeficiency viruses (SIVs), which naturally infect various non-human primates in Africa. These SIVs do not cause disease in their natural primate hosts. Research identifies SIVs found in chimpanzees (SIVcpz) as the precursor to HIV-1, the more common and globally widespread type.
Similarly, SIVs in sooty mangabeys (SIVsmm) are the ancestors of HIV-2. These SIV strains are endemic to their respective primate populations, with SIVcpz found in central chimpanzees and SIVsmm prevalent in sooty mangabeys of West Africa.
The Transmission Event
SIV crossed into humans primarily through bushmeat hunting in Central and West Africa. During hunting or preparation of primates, individuals contacted infected blood or tissues. The virus likely entered the human bloodstream through cuts, scratches, or other wounds sustained by hunters or butchers.
This was not a single event but a series of rare, sporadic occurrences. Each independent cross-species transmission led to the emergence of different HIV strains or groups in humans. This highlights how close contact with infected animal fluids can facilitate zoonotic spillover.
Early Human Spread and Diversification
Once SIV entered a human host and evolved into HIV, it spread within human populations. Early spread is linked to societal changes in early 20th-century Central Africa, particularly around Kinshasa. Increasing urbanization, colonial practices, and transportation networks facilitated its dissemination.
Multiple independent cross-species transmissions and subsequent evolution led to HIV’s diversification into various groups. HIV-1 is categorized into groups M, N, O, and P; Group M caused the global pandemic. HIV-2 also diversified into distinct types, reflecting its multiple origins from sooty mangabeys.
Confirming the Zoonotic Link
The zoonotic origin of HIV is confirmed by various lines of evidence. Phylogenetic analysis, comparing SIV and HIV genetic sequences, demonstrates their evolutionary relationship and traces HIV to its primate ancestors. This genetic mapping shows a clear link between specific SIV strains in chimpanzees and sooty mangabeys and the different types of HIV in humans.
Discovery of SIV in wild primate populations further supports this link, confirming these viruses naturally circulate in African monkeys and apes. Identification of the virus in early human samples, like archived blood from 1959 Kinshasa, provides direct evidence of HIV’s presence before its recognition as AIDS. Epidemiological studies also trace early spread patterns, reinforcing the scientific understanding of HIV’s origins.