Final answer:
Multiple cross-species transmission events have been evidenced by the diverse lineages of HIV groups like M, N, O, and P, each traceable to different SIV sources.
Step-by-step explanation:
Evidence that transmission of HIV from an SIV relative in chimpanzees to humans has taken place multiple times includes the finding that diverse groups of HIV have different lineages, which suggests multiple cross-species transmission events.
The presence of different HIV groups like M, N, O, and P, each with their unique lineage traceable to simian immunodeficiency virus (SIV), provides clear evidence of this phenomenon. These groups represent separate instances where the virus has crossed barriers from SIV hosts (non-human primates) to humans, each leading to the epidemic spread of distinct variations of the virus.