A genetic study of the "family tree" of the AIDS virus suggests it may have appeared only about 40 years ago, scientists reported Thursday.

At most, the results suggest it probably evolved from its West African cousin within the past 100 years, said researcher Temple Smith.AIDS viruses are found in humans and monkeys, but the study could not determine whether an ancestral virus jumped from monkeys to humans, or vice versa, said Smith, director of the Molecular Biology Computer Research Resource at the Dana-Farber Cancer Institute of Harvard Medical School.

The work follows a study published last week that suggested an ancestral AIDS virus infected the primitive ancestors of humans and monkeys more than 20 million years ago. Smith said he doubts that conclusion.

The new work, which used a different method of analysis, appears in the British journal Nature. Smith wrote it with colleagues at the U.S. Centers for Disease Control in Atlanta and the Los Alamos National Laboratory in New Mexico.

It concerns the AIDS virus, called HIV-1, a West African relative called HIV-2, and a monkey AIDS virus called SIV.

To measure how fast HIV-1 was evolving, they compared a 1976 sample to others found during the 1980s. Results suggest that HIV-1 and HIV-2 split into separate evolutionary paths at least 40 years ago, Smith said in a telephone interview. And "unless there is something very unusual going on," he said, the split must have occurred no more than 80 to 100 years ago.