The study reports the detection of simian immunodeficiency virus (SIV)cpz antibodies and nucleic acids in fecal samples from wild-living chimpanzees in southern Cameroon, where prevalence rates reached 29 to 35%. By analyzing endemic SIVcpz strains, the origins of pandemic (group M) and nonpandemic (group N) HIV-1 were traced to distinct, geographically isolated chimpanzee communities. These findings establish *Pan troglodytes troglodytes* as a natural reservoir of HIV-1. The study used noninvasive methods to detect SIVcpz-specific antibodies and nucleic acids in fecal samples and genotyping approaches to identify individual apes. Phylogenetic analysis revealed that SIVcpz strains from different communities formed well-separated clades, indicating distinct SIVcpz lineages. The evolutionary relationships of these strains to HIV-1 groups M and N were also analyzed, confirming that the SIVcpz strains from the MB/LB and EK communities are closely related to HIV-1 groups M and N, respectively. This study provides a clear picture of the origin of HIV-1 and the seeds of the AIDS pandemic, suggesting that the SIVcpz strain that gave rise to HIV-1 group M persisted in *P. t. troglodytes* apes in southeastern Cameroon and likely spread to Kinshasa, where the group M pandemic originated.The study reports the detection of simian immunodeficiency virus (SIV)cpz antibodies and nucleic acids in fecal samples from wild-living chimpanzees in southern Cameroon, where prevalence rates reached 29 to 35%. By analyzing endemic SIVcpz strains, the origins of pandemic (group M) and nonpandemic (group N) HIV-1 were traced to distinct, geographically isolated chimpanzee communities. These findings establish *Pan troglodytes troglodytes* as a natural reservoir of HIV-1. The study used noninvasive methods to detect SIVcpz-specific antibodies and nucleic acids in fecal samples and genotyping approaches to identify individual apes. Phylogenetic analysis revealed that SIVcpz strains from different communities formed well-separated clades, indicating distinct SIVcpz lineages. The evolutionary relationships of these strains to HIV-1 groups M and N were also analyzed, confirming that the SIVcpz strains from the MB/LB and EK communities are closely related to HIV-1 groups M and N, respectively. This study provides a clear picture of the origin of HIV-1 and the seeds of the AIDS pandemic, suggesting that the SIVcpz strain that gave rise to HIV-1 group M persisted in *P. t. troglodytes* apes in southeastern Cameroon and likely spread to Kinshasa, where the group M pandemic originated.