Water workers in northern Chile have discovered a collection of human mummies 3,000 years older than the first mummy of an Egyptian pharaoh, and scientists are now revising their theories of South America's earliest societies.

Excavations for a new water pipe in the city of Arica, said to be the driest city in the world, led archaelogists to one of their richest finds -- 96 mummies preserved in the hot desert sands for periods ranging from 3,670 to 7,810 years.

Recent carbon dating tests have confirmed their antiquity.

"This discovery will change our view of primitive societies," said Dr. Marvin Allison, 64, an American pathologist who taught for 24 years at the Medical College of Virginia in Richmond and who now works at Arica's University of Tarapaca.

The discovery of the mummies was called a "major find" by one of Allison's colleagues, who has also worked on the project. Dr. Enrique Gerszten, professor of pathology at the Medical College of Virginia, said that the discovery is particularly significant in the fields of anthropology and archaeology.

Allison said, "I think it shows that these societies were much more complicated than originally thought. They must have had a good social structure. Don't forget they maintained themselves twice as long as Christianity."

His laboratory dissections have shown that the techniques used to preserve the bodies are far more complicated than the embalming practiced on Egyptian pharaohs.

Allison said the mummies were prepared by skinning the body and removing large parts of flesh, the internal organs and the brain. Then they were dried over hot coals and stuffed with minerals, feathers and vegetable matter. Sticks were used to reinforce the limbs and, sometimes, the trunks.

The skin was pulled on again "just like a glove" and sewn up neatly. The dry, rigid corpses were decorated with clay masks for faces, wigs of their own hair and, perhaps, a cement-like covering painted ocher, red or black.

The elaborate preparation suggests that the bodies were being converted into statues for ritual purposes.

"This is probably the beginning of a complicated system of religious beliefs, if you will, or at least magic, that they were trying to utilize," Allison said.

The degree of social organization required for such burial rites has led Chilean scientists to rethink ideas about the Indians who settled on the Pacific coast of South America.

"We have always thought that man built his first villages in the years 500 to 1,000 B.C., and now we find a high degree of settlement long before," said Hans Niemyer, director of Chile's Natural History Museum, who also found mummies in the area six years ago.

Among the new-found mummies were 52 adults and 44 children. Thirty-two could be identified positively as male and 34 as female.

Allison, who specializes in the history of disease in man and whose work was financed in part by The National Geographic Society, said different physical ailments in the males and females have thrown light on the way the society was organized.

The men show abnormalities in their ears, suggesting that they spent most of their time diving for food in the ocean. The women have signs of arthritis, which, Allison said, was the result of squatting to prepare food beside the sea.

He said they appeared to live off an all-protein diet from sea birds' eggs, deer, seals and sea lions, and their homes were built of whale ribs covered with the skins of sea lions.

"It's the most ideal place in the world to live, because you put your hand in the ocean and pull out your food in half an hour," he said.