There are two forms of human AIDS virus, human immunodeficiency virus types 1 and 2 (HIV-1 and HIV-2). Related simian immunodeficiency viruses (SIVs) are found in more than 30 species of primates in sub-Saharan Africa; each species has its own form of SIV. HIV-1 is classified into three groups, M, N and O. HIV-2 is also classified into multiple groups, but only groups A and B have been found in more than one person.

The evolutionary tree shows that:

- HIV-1 and HIV-2 originate from SIVs in different primate species
   - HIV-1 strains are closely related to SIVcpz/s, from the chimpanzee subspecies Pan troglodytes troglodytes
   - HIV-2 strains are closely related to SIVsmm, from sooty mangabey monkeys (Cercocebus atys).

- HIV-1 groups M, N and O must have originated from three separate chimpanzee-to-human transmissions, because these lineages are interspersed among the SIVcpz/s branches.

-HIV-2 groups A and B must have originated from two separate sooty mangabey-to-human transmissions for similar reasons.

Key to viruses represented in the tree:

HIV-1/M HIV-1 group M
HIV-1/N HIV-1 group N
HIV-1/O HIV-1 group O
HIV-2/A HIV-2 group A
HIV-2/B HIV-2 group B
SIVcpz/t SIV from chimpanzees (subspecies Pan troglodytes troglodytes)
SIVcpz/s SIV from chimpanzees (subspecies Pan troglodytes schweinfurthii)
SIVsmm SIV from sooty mangabeys (Cercocebus atys)
SIVrcm SIV from red capped mangabeys (Cercocebus torquatus)
SIVdrl SIV from drills (Mandrillus leucophaeus)
SIVmnd-1 SIV from southern mandrills (Mandrillus sphinx)
SIVmnd-2 SIV from northern mandrills (Mandrillus sphinx)
SIVver SIV from vervet monkeys (Chlorocebus pygerythrus)
SIVgri SIV from grivet monkeys (Chlorocebus aethiops)
SIVtan SIV from tantalus monkeys (Chlorocebus tantalus)
SIVsun SIV from sun-tailed monkeys (Cercopithecus solatus)
SIVlho SIV from L'Hoest's monkeys (Cercopithecus lhoesti)
SIVsyk SIV from Sykes's monkeys (Cercopithecus albogularis)
SIVgsn SIV from greater spot-nosed monkeys (Cercopithecus nictitans)
SIVmus SIV from mustached monkeys (Cercopithecus cephus)
SIVmon SIV from mona monkeys (Cercopithecus mona)
SIVdeb SIV from De Brazzas monkeys (Cercopithecus neglectus)
SIVden SIV from Dent's mona monkeys (Cercopithecus denti)
SIVtal SIV from Gabon talapoins (Miopithecus ogouensis)
SIVcol SIV from mantled guerezas (Colobus guereza)