The worldwide AIDS epidemic has been traced back to a single ancestor virus - the HIV Eve - that emerged perhaps around 1930.

Earlier research had suggested that the epidemic began in the first half of the 20th century, but the latest analysis, done at the Los Alamos National Lab in New Mexico, appears to be the most definitive so far. Bette Korber, who keeps a database of HIV genetic information at the lab, calculated HIV_s family tree by looking at the rate the virus mutates over time. She assumed these genetic changes happen at a constant rate, and using a supercomputer she clocked the mutations back through time to a common ancestor. Korber estimates that the current epidemic goes back to one or a small group of infected humans around 1930, though this ancestor virus could have emerged as early as 1910 or as late as 1950. From this single source, she suggests, came the virus that now infects roughly 40 million people all over the world. Her findings were released at a scientific conference this week in San Francisco. Experts believe that HIV_s ancestor is a virus that ordinarily infects chimpanzees. Somehow it spread to people - perhaps through a bite or hunting mishap - in west equatorial Africa. Just when this happened, though, is still a mystery, Korber said. The leap from chimp to man could have been around 1930. Or it may have occurred much earlier and the virus stayed within a small group of humans. Korber based her work on the genetic codes of 160 different copies of the AIDS virus. She analyzed them on a Los Alamos supercomputer, called Nirvana, that can perform 1 trillion computations per second. The earliest existing sample of HIV was found in a blood specimen obtained in Leopoldville - now Kinshasa - in 1959.