New strains of SIV were found - and dated - in the drill (pictured), a close relative of the baboon. Credit: Cyril Ruoso/Minden Pictures/FLPA

The HIV-like virus that infects monkeys is at least 100,000 if not millions of years old, scientists reported this week at a meeting of the New York Academy of Sciences. The vast age of the monkey virus, which does not cause illness in most of its hosts, suggests that it may take a long time for HIV to become equally benign in humans.

"Don't expect human evolution to unfold in a timeframe that will do anything good for us," Michael Worobey, an evolutionary biologist at the University of Arizona in Tucson, told meeting attendees. "We're not going to evolve adaptations that will mitigate this virus in any acceptable timescale, so we need other solutions."

Most researchers agree that the pandemic strain of HIV that currently infects more than 33 million people worldwide started in central Africa around 100 years ago, when hunters contracted the virus through tainted bushmeat. But the age of HIV's primate ancestor — simian immunodeficiency virus (SIV) — remains hotly contested. Using DNA sequence data taken from SIV strains, some have estimated that SIV is a few thousand years old1, whereas others suggest that the virus dates back only a couple of hundred years2.

These projections, however, assume that SIV DNA sequences mutate at the same rate as HIV's modern pace of evolution, which many say is much faster than historic rates of change. So some researchers have sought other lines of evidence. A related virus found embedded within the genome of lemurs from Madagascar pointed to a timescale of millions of years3. And although SIV-infected chimpanzees remain susceptible to disease (see 'Wild chimpanzees get AIDS-like illness'), other wild monkeys that have coexisted with SIV for longer, including sooty mangabeys and African green monkeys, seem to have evolved complete immunity to the virus, indicating an extended period of coevolution.

Geological clock

To pin down the age of SIV, Worobey teamed up with Preston Marx, a virologist at the Tulane University in New Orleans, Louisiana, who, since 2001, has been looking for viruses in monkey bushmeat taken from Bioko, an island located 32 kilometres off the west coast of Cameroon. Marx and his colleagues discovered new strains of the virus in three monkey species that had never before been shown to be exposed to SIV, including the red-eared guenon (Cercopithecus erythrotis), as well as a fourth monkey, drills (Mandrillus leucophaeus), close relatives of baboons.

Worobey compared DNA sequences taken from SIV strains infecting drills from both Bioko and the mainland. Crucially, he knew from geological records that the island separated from continental Africa around 12,000 years ago. Assuming that the strains had had at least 12,000 years to evolve apart, he determined that the mutation rate of SIV is much slower than originally thought. Using this approach, he suggested that earlier DNA-based calculations were wrong, and estimated that SIV must be at least 100,000 years old.

"It's compelling and orthogonal evidence that SIV is a lot older than we previously thought," says Sarah Schlesinger, a cellular immunologist at Rockefeller University in New York, who co-organized the New York meeting.

Using the biogeographical data from Bioko "is certainly a good way of getting some handle on this", says Paul Sharp, an evolutionary biologist at the University of Edinburgh, UK, who studies HIV. "But what it does is it gives you a minimum age rather than fixing an age." Sharp, who was not involved in the study, says that SIV is probably closer to five million years old.

Regardless of the exact age, Beatrice Hahn, an HIV researcher at the University of Alabama in Birmingham, says that the study demonstrates that it probably took a long time before SIV turned harmless in most monkeys. As such, people should not rely on evolution alone to fight the threat of HIV, she cautions. "Will humans eventually turn into monkeys and learn to cope? Perhaps. Do I want to wait for that? No."