Immunodeficiency virus? No problem (Image: Preston Marx/Tulane University)

HIV’s close cousin, the simian immunodeficiency virus, has been around for tens of thousands of years at least – much longer than the few hundred years that some earlier studies had suggested. Because SIV does not cause AIDS in its primate hosts, some have speculated that HIV too might stop being lethal within a few centuries – but the discovery that SIV has had millennia to evolve into peaceful coexistence dashes these hopes.

The age of a virus can be determined by measuring its rate of mutation, then calculating how long it would take for this to generate its current genetic diversity.

Preston Marx, a virologist at Tulane University in New Orleans, Louisiana, and his colleagues studied SIV samples from drills, close relatives of baboons. They compared SIV from drills living on Bioko Island, off the coast of Cameroon, to those on the mainland. Because the island has been isolated from continental Africa since the last ice age, 10,000 years ago, the genetic differences between the two virus samples must represent 10,000 years of evolutionary divergence.


Using their comparison of the viruses to estimate the rate of genetic change, the team reckoned that it must have taken at least 32,000 and probably 76,000 years to generate the genetic diversity seen in SIV throughout Africa.

“It hammers home the point that these viruses have been around for a long, long time,” says Beatrice Hahn, an HIV researcher at the University of Alabama at Birmingham, who was not connected with the research.

Newcomer virus

If the calculated age is correct, SIV has had millennia to evolve its ability to live in its hosts without causing AIDS. That makes it highly unlikely that HIV, which seems to have first infected humans barely a century ago, will lose its virulence any time soon.

The finding may also be relevant to attempts to control the spread of HIV. If SIV is millennia old, it is likely that humans had been exposed to it off and on for many thousands of years before it made the leap to infect them, becoming HIV.

The increased mobility of the past 100 years has increased the ease with which the virus can be transmitted. Cities, steamboats, railways and roads brought people into a socially connected network that gave HIV the nudge it needed to take hold, says Michael Worobey, an evolutionary biologist at the University of Arizona in Tucson and co-author of the study.

Since digging up the roads to break up the network of transmission isn’t an option today, we need to focus on other ways of knocking the virus back below the threshold of viability, he says. This would include diligent use of condoms, antiviral drugs, and other measures to reduce the transmission of HIV.

Journal reference: Science, DOI: 10.1126/science.1193550