Extensive research has largely illuminated how Human Immunodeficiency Virus (HIV) transferred from chimpanzees to humans, providing crucial insights into its emergence and spread.
The Ancestor Virus: SIVcpz
The precursor to HIV-1 is Simian Immunodeficiency Virus (SIV), specifically SIVcpz, found in chimpanzees. These SIVs naturally infect many primate species across Africa, often causing no apparent illness in their natural hosts. SIVcpz, which infects the central chimpanzee subspecies Pan troglodytes troglodytes, is the closest genetic relative to HIV-1.
SIVcpz is a recombinant virus, meaning it formed from the genetic material of two different SIVs. Chimpanzees acquired these two distinct SIVs, SIVrcm from red-capped mangabeys and SIVgsn from greater spot-nosed monkeys, likely through their natural predatory behavior. This recombination event created the SIVcpz strain, which became capable of infecting humans.
The Zoonotic Leap: How SIV Became HIV-1
The primary mechanism for SIVcpz crossing the species barrier into humans is widely accepted as the “bushmeat” hypothesis. This involves hunters in Central Africa who hunted chimpanzees for food. During hunting, butchering, or handling infected chimpanzee meat, human exposure to the animal’s blood or other bodily fluids could occur through cuts, scratches, or open wounds.
This direct contact allowed the virus to enter the human bloodstream, initiating cross-species transmission. Evidence suggests multiple independent transmission events from chimpanzees to humans occurred. These distinct jumps gave rise to the different groups of HIV-1 known today, including Group M, N, O, and P. Group M is responsible for the global AIDS pandemic, accounting for approximately 90% of all HIV-1 cases worldwide.
The Early Spread and Pandemic Emergence
After SIVcpz jumped to humans, it began to spread within human populations, primarily in Central Africa during the early 20th century. Kinshasa, then known as Leopoldville in the Belgian Congo, emerged as an epicenter for the virus’s early amplification. This urban center allowed the virus to establish a foothold and spread widely.
Several societal and historical factors facilitated this early dissemination. Rapid urbanization and the development of extensive transportation networks, such as railways and river routes along the Congo and Sangha rivers, connected previously isolated communities. The influx of male laborers into cities and mining areas also contributed to the rise of the sex trade, which further accelerated viral transmission. Additionally, medical practices involving the re-use of unsterile needles in health campaigns contributed to the virus’s early spread among human populations.
Tracing the Origin: Scientific Investigations
Scientists pieced together HIV’s origin through scientific investigations. Phylogenetic analysis, which studies the evolutionary relationships of viruses based on their genetic sequences, has been a primary tool. By comparing the genetic makeup of various HIV and SIV strains, researchers constructed a viral “family tree,” tracing HIV’s lineage directly back to SIVcpz in chimpanzees.
This genetic analysis also helped approximate the timing of the initial cross-species jump. Phylogenetic analyses indicate that the common ancestor of HIV-1 Group M, the pandemic strain, likely emerged in the human population between 1915 and 1941, with estimates often pointing to the 1920s in Kinshasa. Further evidence came from archived tissue samples, including a blood sample from Kinshasa in 1959 and a lymph node sample from 1960, which contained some of the earliest known HIV strains. The genetic diversity in these early samples confirmed the virus had been circulating and diversifying in the human population before its identification.