The origins of HIV and the AIDS pandemic are traced back to cross-species transmissions of simian immunodeficiency viruses (SIVs) from African primates to humans. HIV-1 and HIV-2, the two human immunodeficiency viruses causing AIDS, originated from SIVs that infected chimpanzees and sooty mangabeys, respectively. The transmission of SIVcpz from chimpanzees in Cameroon to humans led to the emergence of HIV-1 group M, the main cause of the AIDS pandemic. Host restriction factors, such as tetherin, have played a critical role in shaping the emergence of new SIV zoonoses by imposing adaptive hurdles to cross-species transmission. The study highlights the genetic changes that occurred as SIVs crossed from monkeys to apes and then to humans, providing a framework to understand the requirements of successful host switches and future zoonotic risks. The AIDS pandemic began in the early 20th century, with the first identified HIV-1 strains emerging in west central Africa. HIV-1 evolved rapidly due to its error-prone reverse transcriptase and short generation time, allowing for detailed phylogenetic analysis to trace its origin. The AIDS pandemic is believed to have originated in Leopoldville (now Kinshasa), where the first HIV-1 group M strains were identified. The spread of HIV-1 involved population bottlenecks, leading to the predominance of different subtypes in various geographic regions. HIV-1 group M has diversified into nine subtypes and over 40 recombinant forms, with varying biological properties influencing their epidemiology. The study also discusses the origins of HIV-2, which originated from SIVsmm in sooty mangabeys. The natural history of SIVcpz in chimpanzees shows that it can cause disease, with significant impacts on health, reproduction, and lifespan. The study concludes that understanding host-specific adaptations and restriction factors is crucial for predicting future zoonotic risks. The role of tetherin in preventing cross-species transmission is emphasized, as its absence of effective antitetherin measures may limit the establishment of epidemic infections. Overall, the study provides a comprehensive understanding of the origins, evolution, and transmission of HIV and SIV, highlighting the importance of host factors in shaping the emergence of zoonotic diseases.The origins of HIV and the AIDS pandemic are traced back to cross-species transmissions of simian immunodeficiency viruses (SIVs) from African primates to humans. HIV-1 and HIV-2, the two human immunodeficiency viruses causing AIDS, originated from SIVs that infected chimpanzees and sooty mangabeys, respectively. The transmission of SIVcpz from chimpanzees in Cameroon to humans led to the emergence of HIV-1 group M, the main cause of the AIDS pandemic. Host restriction factors, such as tetherin, have played a critical role in shaping the emergence of new SIV zoonoses by imposing adaptive hurdles to cross-species transmission. The study highlights the genetic changes that occurred as SIVs crossed from monkeys to apes and then to humans, providing a framework to understand the requirements of successful host switches and future zoonotic risks. The AIDS pandemic began in the early 20th century, with the first identified HIV-1 strains emerging in west central Africa. HIV-1 evolved rapidly due to its error-prone reverse transcriptase and short generation time, allowing for detailed phylogenetic analysis to trace its origin. The AIDS pandemic is believed to have originated in Leopoldville (now Kinshasa), where the first HIV-1 group M strains were identified. The spread of HIV-1 involved population bottlenecks, leading to the predominance of different subtypes in various geographic regions. HIV-1 group M has diversified into nine subtypes and over 40 recombinant forms, with varying biological properties influencing their epidemiology. The study also discusses the origins of HIV-2, which originated from SIVsmm in sooty mangabeys. The natural history of SIVcpz in chimpanzees shows that it can cause disease, with significant impacts on health, reproduction, and lifespan. The study concludes that understanding host-specific adaptations and restriction factors is crucial for predicting future zoonotic risks. The role of tetherin in preventing cross-species transmission is emphasized, as its absence of effective antitetherin measures may limit the establishment of epidemic infections. Overall, the study provides a comprehensive understanding of the origins, evolution, and transmission of HIV and SIV, highlighting the importance of host factors in shaping the emergence of zoonotic diseases.