Understanding the origin of Human Immunodeficiency Virus (HIV) is crucial for comprehending its global impact. Scientific investigation has largely unraveled how this virus jumped from non-human primates to humans, detailing its viral ancestors, cross-species transmission, early dissemination, and the methods used to uncover these facts.
The Viral Ancestor
The story of HIV begins with simian immunodeficiency viruses, or SIVs, which have circulated naturally in various non-human primate species in Africa for thousands of years. Over 45 African non-human primate species host SIVs, including chimpanzees and sooty mangabeys. Unlike HIV in humans, SIV infections are generally benign in their natural hosts, who typically do not develop AIDS. Their immune systems have adapted to coexist with the virus.
Genetic evidence strongly links specific SIV strains to the two main types of human HIV. Simian immunodeficiency virus found in chimpanzees (SIVcpz) is the direct ancestor of HIV-1, the type responsible for the global pandemic. HIV-2, which is less widespread and primarily found in West Africa, originated from SIVsmm, a strain naturally infecting sooty mangabeys. SIVcpz itself originated from a recombination of two other SIV lineages from Old World monkeys, likely acquired by chimpanzees hunting and consuming these monkeys.
The Zoonotic Leap
The transfer of SIV from primates to humans is a process known as zoonosis, and it occurred through multiple, independent events. The leading hypothesis for this cross-species transmission involves exposure to infected primate blood or bodily fluids. This most likely happened when humans hunted, butchered, or consumed bushmeat. For instance, if a hunter sustained a cut while butchering an infected chimpanzee, the virus could have entered their bloodstream.
This was not a single occurrence but rather several separate instances where SIV strains successfully jumped into the human population. HIV-1, the predominant type, has at least four distinct groups (M, N, O, and P), each representing an independent transmission event from SIVcpz or SIVgor (gorilla SIV) to humans. Group M of HIV-1, which accounts for the vast majority of global infections, originated from a specific SIVcpz strain in southeastern Cameroon. Similarly, HIV-2 comprises at least eight known groups (A-H), with groups A and B being the most prevalent, stemming from multiple independent transmissions of SIVsmm to humans in West Africa.
Early Global Dissemination
Once SIV successfully crossed into humans, several societal changes in Central Africa during the early to mid-20th century facilitated the virus’s spread beyond isolated communities. The rapid growth of urban centers, particularly cities like Kinshasa (then Léopoldville) in the Democratic Republic of Congo, played a significant role. These burgeoning cities created denser populations and new social dynamics that allowed the virus to spread more effectively.
Improved transportation networks, such as extensive railway and river systems, further amplified the virus’s movement. These networks connected remote areas to major urban hubs, enabling infected individuals to travel widely and introduce the virus to new populations. Colonialism, with its associated population mobility, forced labor, and shifts in social structures, also contributed to an environment conducive to viral dissemination. While initial human infections might have remained localized, urbanization and increased connectivity provided the conditions for HIV to establish a broader foothold and eventually spread globally.
Unraveling the Mystery
Scientists pieced together the complex origin story of HIV using advanced molecular biology techniques. A cornerstone of this research is phylogenetic analysis, which studies the evolutionary relationships between viruses by comparing their genetic sequences. By analyzing the genetic differences and similarities among numerous HIV and SIV samples, researchers could construct “family trees” of the viruses, tracing their lineages back to common ancestors. This method allowed for the identification of SIVcpz and SIVsmm as the direct precursors to HIV-1 and HIV-2, respectively.
The analysis of stored blood and tissue samples from early patients has been crucial in establishing a timeline for the virus’s emergence and early spread in humans. For instance, the earliest known HIV-1 sample, designated LEO70, was identified in a blood sample collected in 1959 from a man in Kinshasa, Democratic Republic of Congo. Another significant sample, DRC60, from a woman in Kinshasa in 1960, provided further insights into early viral diversity. By applying molecular clock models, which estimate evolutionary rates based on genetic mutations over time, scientists determined that the most common ancestor of HIV-1 Group M, the pandemic strain, likely emerged around 1920, with estimates ranging from 1908 to 1930. These historical samples and genetic analyses continue to refine our understanding of HIV’s journey.