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| Description: This tree shows the evolutionary relationships among the two major forms of HIV, called HIV-1 and HIV-2, and the immunodeficiency viruses that afflict nonhuman primates. Note that the viruses that branch off near the orange arrow at the base of the tree parasitize monkeys. Based on this observation, researchers conclude that virus strains jumped from monkeys to humans. (b) This tree shows a more detailed analysis performed by Hahn et al. (2000). (The asterisk marks the same branch point on both trees.) The arrows indicate the places on the tree where immunodeficiency viruses were transmitted from chimpanzees to humans. According to this tree, each major strain of HIV-1 originated in a different transmission event from a chimpanzee host, as represented by the gray arrows. Redrawn from Hahn et al. (2000).
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