The acquired immunodeficiency syndrome (AIDS) is a global pandemic caused by the human immunodeficiency virus (HIV). Modern molecular biology has provided a clear, evidence-based answer, tracing the virus’s lineage back to non-human primates in Central and West Africa. This scientific consensus reveals that HIV is the result of multiple, distinct transmissions of a precursor virus from animals into the human population.
Identifying the Primate Host and Precursor Virus
The scientific investigation into HIV’s origin established that the virus is a mutated form of the Simian Immunodeficiency Virus (SIV), which is naturally carried by various African primates. SIV strains that crossed the species barrier gave rise to the two main types of human HIV: HIV-1 and HIV-2. These two types are genetically distinct and originated from different animal hosts in different geographic regions of Africa.
The precursor to HIV-1, the virus responsible for the global AIDS pandemic, is Simian Immunodeficiency Virus of chimpanzees (SIVcpz). This strain originated in the central chimpanzee subspecies, Pan troglodytes troglodytes, whose natural range is in West-Central Africa, including countries like Cameroon and Gabon. SIVcpz itself is thought to be the result of a recombination event between two different SIV strains acquired by chimpanzees from smaller monkeys they hunt and consume.
The origin of the second, less widespread form of the virus, HIV-2, is the Sooty Mangabey (Cercocebus atys). These Old World monkeys carry Simian Immunodeficiency Virus of sooty mangabeys (SIVsmm) in West Africa, where HIV-2 is primarily found today. SIV infection in these natural primate hosts is typically non-pathogenic. Despite high levels of the virus circulating in their blood, SIV-infected sooty mangabeys do not develop an AIDS-like illness.
The Mechanism of Cross-Species Transmission
The scientific explanation for how SIV transitioned from the primate host to humans is known as the “bushmeat” or “cut hunter” theory. This mechanism involves the hunting, butchering, and consumption of infected chimpanzees and monkeys, which are a common source of protein in parts of Central and West Africa. The virus crossed the species barrier when an infected animal’s blood or other bodily fluids came into direct contact with a hunter’s open cut, wound, or mucous membrane during the process of preparing the meat.
These initial spillover events were accidental and likely occurred numerous times over many decades, but only a few led to sustained human-to-human transmission. The first instance of SIVcpz jumping from a chimpanzee to a human that led to the global pandemic occurred in West-Central Africa, most likely in southeastern Cameroon. Phylogenetic analysis suggests that this single zoonotic event gave rise to the HIV-1 Group M strain.
The subsequent spread of the virus from a localized infection to a widespread epidemic was facilitated by social and geographic factors. The virus found an ideal environment for dissemination in Kinshasa, then Léopoldville in the Belgian Congo. This rapidly growing urban hub, with its expanding colonial infrastructure of railways and river travel, allowed the newly adapted virus to travel far and wide. This established the HIV-1 Group M lineage in the human population around the 1920s.
Viral Evolution and the Diversification of HIV Strains
Following the initial cross-species transmission, the Simian Immunodeficiency Virus evolved and diversified into the various strains of Human Immunodeficiency Virus. The most significant finding from genetic studies is that HIV-1 and HIV-2 originated from at least eight to eleven separate transmissions from primates to humans. These multiple, independent jumps explain the genetic differences between the major types of HIV.
HIV-1, which is the most virulent and transmissible, is further divided into groups, with Group M (for “Main”) being responsible for over 90% of all AIDS cases worldwide. Other less common groups, such as N, O, and P, resulted from separate, distinct SIVcpz or SIVgor transmissions that remained largely confined to West-Central Africa. HIV-2, which is less easily transmitted and less virulent, also comprises multiple groups, with only Groups A and B spreading significantly within human populations in West Africa.
Scientists confirmed this complex ancestry and timeline using phylogenetic analysis. This method uses genetic sequencing to compare viral genomes, tracing their evolutionary relationships and constructing a “family tree” for the virus. By applying molecular clock models, which measure the rate of genetic mutation, researchers estimated that the most recent common ancestor of the pandemic HIV-1 Group M strain existed around the 1920s.