WASHINGTON - AIDS evolved from a benign simian infection into a human-killer in the early 1930s, long before it was recognized as a disease, but it stayed in remote Africa until jet travel, big cities and the sexual revolution spread it worldwide, a new study suggests.
Researchers measuring the rate of genetic change in HIV, the virus that causes AIDS, found the current strains originated from a common ancestor that first evolved from a simian virus in southwest Africa between 1915 and 1941, with 1931 the most likely year.
''It could have been in humans even before that,'' said Tanmoy Bhattachary, a researcher at the Los Alamos National Laboratory in Los Alamos, N.M. The study appears Friday in the journal Science.
Bhattachary said the most common form of HIV worldwide evolved from simian immunodeficiency virus, or SIV, that was in the chimpanzee. SIV genetically converted to HIV either while it was in the chimp or after a human contracted SIV.
The disease did not become a worldwide menace, he said, until people left the isolated areas of Africa and carried the virus around the globe.
''It could have evolved in humans and stayed in a very small population, such as a village,'' said Bhattachary. ''That is typically what most new diseases do. They are in an isolated population and then something happens and it starts spreading all over.''
The findings are consistent with earlier studies that suggested that HIV originated early in this century and then was spread when Africa became less isolated.
Bhattachary said the date when SIV first evolved to HIV makes it ''very unlikely'' that a polio vaccination campaign in the late 1950s can be blamed for the rise of AIDS. Some researchers have suggested that a polio vaccine made using chimpanzee kidney cells could have transferred HIV into humans between 1957 and 1960.
Although the new research could not eliminate that possibility entirely, Bhattachary said, the fact that HIV originated before the polio vaccine means ''you can probably discount that scenario''
A type of AIDS virus called HIV2, which occurs mostly in Africa, may have originated from sooty mangabeys, a type of African monkey, the study says.
But HIV1, the virus that has caused a pandemic, came from the chimpanzee, a primate more closely related to humans.
A form of HIV1 called Group M is the major virus spreading throughout the world. It has infected about 50 million people and killed 16 million. Bhattachary said Group M may have crossed over from chimp to humans only once, or evolved from SIV in only one patient.
''Our study shows that the M group had a single origin,'' he said. ''It could have come from one animal or from one human.''
Other forms of HIV, he said, had ''multi-introductions.''
The most common form HIV in the United States, called subgroup B, first evolved between 1960 and 1971, with 1967 being the most likely year, said Bhattachary.
Clinical symptoms of what later became known as AIDS were reported in the United States in the late 1970s. AIDS was formally diagnosed and named in 1981. The HIV1 virus was isolated and confirmed in 1983.
John P. Moore, a microbiologist and AIDS researcher at Cornell University in New York, said the study by Bhattachary and his colleagues is ''outstanding and significant. They have looked into this very carefully.''
Moore said the findings emphasizes how science needs to be concerned and alert to the risks of cross-species viral transmissions, many of which are lethal or cause serious illness in humans.
Another researcher, Jim Moore of the University of California, San Diego, said the Los Alamos study is consistent with his findings that conditions in colonial Africa were ripe, starting in the late 19th century, for a new virus to take hold and spread.
Colonial powers forced people out of villages, causing many to live in the jungles, surviving by hunting and gathering, said Moore, who is unrelated to the Cornell scientists. A major food was meat from chimps and monkeys.
''This created conditions ideal for the transfer (of a virus) from chimps and a spread into small populations,'' said Moore.
Later, he said, large work gangs were organized to build roads and mines, with some construction organizations promoting prostitution to keep the isolated workers content. HIV is a sexually transmitted disease.
In more recent decades, an age with easy transoceanic travel and the sexual revolution, millions of people have been in and out of Africa.
Moore said campaigns to vaccinate the African population against small pox and other diseases may even have helped HIV spread, saying, ''They weren't using sterilized needles all the time.''
