The origins of HIV, which causes AIDS, go back millions of years rather than the tens of thousands that had previously been believed, U.S. researchers say.

Writing in the journal PLoS Pathogens, scientists from the University of Washington and the Fred Hutchinson Cancer Research Center in Seattle report a virus with characteristics similar to the human immunodeficiency virus arose in African monkeys and apes 5 million to 12 million years ago.

While HIV emerged in humans in the 20th Century when an HIV-like virus jumped from chimps to humans, scientists had long understood similar viruses existed in monkeys and apes much earlier.

Earlier genetic studies had suggested these related viruses arose tens of thousands of years ago, but there had been suspicions that time frame was underestimated.

The new study of the genetic signatures of HIV-like viruses in a number of primates, including chimps, gorillas, orangutans and macaques, found changes in genes that have evolved in their immune systems suggest the viruses arose 5 million to 16 million years ago.

The findings could lead to a better understanding of HIV and AIDS, researchers in the field said.

"This kind of research helps us understand how the virus works," Sam Wilson of the University of Glasgow Center for Virus Research told the BBC.

"The hope is that one day this will translate into therapy."