According to paleontologic and molecular studies, the chimpanzee (Pan troglodytes) is the closer relative to the humans (Homo sapiens) and that both lineages had a common ancestor at 5 to 7 million years ago. Moreover, the human-chimp lineage split from that of the rhesus monkey (Macaca mulatta) around 25 million years ago.

In work originating from the Bavarian Research Cooperation Prions (FORPRION), which ended in 2007, a team led by the scientist Prof. Dr. Christine Leib-Mösch has been able to show that prion proteins may activate endogenous retroviruses in infected brain cells.

Human resistance to a retrovirus that infected chimpanzees and other nonhuman primates 4 million years ago ironically may be at least partially responsible for the susceptibility of humans to HIV infection today.