It has been widely believed that the first humans were infected with the AIDS virus in the 1950s. But a U.S. researcher says it probably happened much earlier, perhaps around 1930.

Bette Korber used a powerful computer to clock mutations in the virus back through time, assuming a constant rate of change.

She made the calculations at the Los Alamos National Lab in New Mexico and released her findings at a scientific conference in San Francisco.

The work challenges a theory that AIDS actually began in the 1950s when HIV was accidentally mixed with the polio vaccine.

Many 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 Africa.