We are more closely related to apes, like gorillas and chimpanzees, than we are to monkeys. But even when HIV has passed into human populations from these apes, it has not necessarily turned into a widespread health issue.
HIV originating from apes typically belongs to a type of virus called HIV-1. One is called HIV-1 group O, and human cases are largely confined to west Africa.
In fact, only one form of HIV has spread far and wide after jumping to humans. This version, which probably originated from chimpanzees, is called HIV-1 group M (for "major"). More than 90% of HIV infections belong in group M. Which raises an obvious question: what's so special about HIV-1 group M?
It is not especially infectious, as you might expect. Instead, it seems that this form of HIV simply took advantage of events. "Ecological rather than evolutionary factors drove its rapid spread," says Nuno Faria at the University of Oxford in the UK.
Faria and his colleagues built a family tree of HIV, by looking at a diverse array of HIV genomes collected from about 800 infected people from central Africa.
The very first person to be infected with HIV-1 group M probably picked up the virus in the 1920s
Genomes pick up new mutations at a fairly steady rate, so by comparing two genome sequences and counting the differences they could work out when the two last shared a common ancestor. This technique is widely used, for example to establish that our common ancestor with chimpanzees lived at least 7 million years ago.
"RNA viruses such as HIV evolve approximately 1 million times faster than human DNA," says Faria. This means the HIV "molecular clock" ticks very fast indeed.
HIV originating from apes typically belongs to a type of virus called HIV-1. One is called HIV-1 group O, and human cases are largely confined to west Africa.
In fact, only one form of HIV has spread far and wide after jumping to humans. This version, which probably originated from chimpanzees, is called HIV-1 group M (for "major"). More than 90% of HIV infections belong in group M. Which raises an obvious question: what's so special about HIV-1 group M?
It is not especially infectious, as you might expect. Instead, it seems that this form of HIV simply took advantage of events. "Ecological rather than evolutionary factors drove its rapid spread," says Nuno Faria at the University of Oxford in the UK.
Faria and his colleagues built a family tree of HIV, by looking at a diverse array of HIV genomes collected from about 800 infected people from central Africa.
The very first person to be infected with HIV-1 group M probably picked up the virus in the 1920s
Genomes pick up new mutations at a fairly steady rate, so by comparing two genome sequences and counting the differences they could work out when the two last shared a common ancestor. This technique is widely used, for example to establish that our common ancestor with chimpanzees lived at least 7 million years ago.
"RNA viruses such as HIV evolve approximately 1 million times faster than human DNA," says Faria. This means the HIV "molecular clock" ticks very fast indeed.