After reviewing the many elements of the puzzle piece by piece throughout this book, it is now time to assemble them into a coherent summary of the events that led to the transformation of SIVcpz into HIV-1, triggering the worst pandemic of modern times. Several pieces of this puzzle are irrefutable, while others remain the most plausible hypotheses explaining parts of the story, given the currently available circumstantial evidence. However, as the years go by, it becomes less and less likely that researchers will uncover novel information that could substantially alter this narrative.

We have seen in Chapter 2 that, for at least several hundred years, the Pan troglodytes troglodytes chimpanzee of central Africa has been infected with a simian immunodeficiency virus, SIVcpz, which is genetically identical to HIV-1. The distribution of SIVcpz among chimpanzees in the pre-colonial era was probably not much different from what it is today. Apart from the higher level of threat from humans, the social and sexual behaviour of chimps has not changed over time. SIVcpz is mainly transmitted within well-defined troops of chimpanzees, presumably through sexual intercourse, but only sporadically to other communities, with which contacts are infrequent. This resulted in a heterogeneous distribution of SIVcpz, absent from some communities while infecting a third of the members of other troops. Overall, around 6% of P.t. troglodytes chimps are infected with SIVcpz. Some naturally infected chimps develop a disease reminiscent of AIDS, but only after several years during which their intense sexual promiscuity allowed them to spread the virus. It is clear that the other three subspecies of Pan troglodytes are not the source of HIV-1. The other chimpanzee species, the Pan paniscus bonobo, has been less investigated but there is so far no evidence that it is infected with SIV.