Strains of the simian immunodeficiency virus (SIV), which infects monkeys such as the Bioko drill, revealed the virus has been around thousands of years longer than previously thought, scientists say. (Preston Marx/Tulane University)

The roots of an HIV-like virus in monkeys go back thousands of years more than thought, scientists have found.

Simian immunodeficiency virus, or SIV, is between 32,000 and 75,000 years old and may even be much older, according to a genetic analysis of SIV strains found in monkeys on Bioko, an island off the coast of Africa.

It took thousands of years for SIV to evolve into its mainly non-lethal state, which could have implications for human immunodeficiency virus, researchers say in Friday's issue of the journal Science.

Previous estimates of the virus's age, based on DNA sequencing data, were only a few hundred years.

Other species of immunodeficiency viruses are less deadly in their hosts than HIV, said Michael Worobey, a professor in the University of Arizona's department of ecology and evolutionary biology, who led the study in conjunction with virologist Preston Marx of Tulane University.

'Quite powerful'

It would likely also take a long time for HIV to follow the same path as SIV to evolve into a less lethal state, the researchers said.

In the study, the researchers used virus strains from bush meat samples of monkeys on Bioko Island to reconstruct the evolutionary past of the virus.

"When I see pictures of these rare animals, it is quite powerful to know that they are infected with viruses that help us see into the deep past of one of humanity's most devastating plagues," Worobey said in an email.

Some of the primate species on the island have only a few hundred individuals left and might become extinct, Worobey said.

Four SIV strains from four different primates — drills, red-eared guenons, Preuss’s guenons and black colobuses — were very different, the researchers found. But each was also close to the strain infecting members of the same four genuses on the mainland, so they must have existed before the island separated.

The island was cut off from what is now Cameroon when sea levels rose 10,000 years ago at the end of the last ice age.

The findings suggest that something changed in the human population that made HIV viable. Worobey suspects that when there were enough steamboats, cities, roads and railways to stitch together large transmission networks in Africa, then the virus began its spread.