An ancient predecessor to the virus that causes Aids evolved in wild primates many millions of years earlier than previously believed, according to research published yesterday by the Stanford University School of Medicine.

The findings open a critical new avenue in the quest to understand the origins of the human immunodeficiency virus, or HIV, which unleashed a pandemic that since 1981 has claimed more than 25 million lives worldwide, and threatens millions more.

The discovery also bolsters scientific hunches that HIV-like viruses may be more broadly distributed among primates and other wildlife than previously thought, posing a potential risk for humans in close contact with these animals. Primates in Africa passed on the two strains of HIV circulating among humans, scientists are convinced.

"It points to the direction for future research, that we need to establish how widespread these viruses are," said Robert Gifford, an infectious disease researcher with Stanford, and lead author of the study.

Gifford and his colleagues studied gray mouse lemurs, squirrel-sized, saucer-eyed primates found in Madagascar.

They found that the lemur genomes they studied carried lentiviruses, which are among a family of viruses that include HIV. Lentiviruses have been intensely studied since the emergence of Aids.

While the discovery of lentiviruses in lemurs surprised the researchers, what especially stunned them was the realisation that lemurs must have carried the virus for at least 14m years.

That was the last time a land bridge might have existed between the island and mainland Africa, providing an opportunity for another species infected with the virus to pass it to the geographically isolated animals, which are native only to Madagascar.

At the outside, lemurs may have carried the virus for as long as 85m years, when the primate family that includes lemurs split from the evolutionary branch that gave rise to monkeys, apes and humans. The only other way lemurs could have acquired lentiviruses would have required mammals such as infected bats to wing their way across the 400km span of ocean between Madagascar and Africa – which the scientists consider unlikely.

The findings upend long-held beliefs about how long primates have carried HIV-like viruses, and how recently these viruses evolved. "There was a generally accepted maximum of a million years," Gifford said. "But many people thought it might be less than that."

Honing in on how long species carrying lentiviruses have harboured the pathogens helps scientists understand how they spread and evolve in wild populations, and the prevalence of biological defences against them. Certain species of primates, such as mandrills, sooty mangabeys and green monkeys, harbour the viruses without getting ill.

That knowledge could be used to protect humans from transmission of other HIV-like viruses still confined to wildlife. It could also help drug development by pinpointing features of the virus that remain fixed through its evolution. These fixed features may prove critical for normal functioning of the virus, and provide a target for new Aids drugs.

The HIV-2 strain is widely accepted to have been passed from sooty mangabeys in west Africa to humans, probably bushmeat hunters or those keeping the primates as pets, or both. Scientists believe HIV-1 was passed from chimpanzees to humans.

"There's an imperative to try to establish from these findings what the distribution of lentiviruses is, what the routes of transmission are, and the time frame," Gifford said.