Amigo, amigo," said Marcelo Santos, and with those words he may have reached across the divide of centuries. Santos, a "tracker" with Brazil's Indian-protection agency, had spotted a man and a women standing on a rise in the dense rain forest in southern Rondonia state. They carried bows and arrows and wore rough deerskin hats and many colorful necklaces. Santos tried to draw the couple to him, but they would not approach until he and his party had dropped their rifles, cameras and backpacks. He then declared himself a friend, and the couple replied in an unrecognizable language.

Who are they? The answer is not yet clear, but what has experts engaged in excited speculation is the possibility that they are what Brazilian authorities call "isolated Indians"-natives who live in remote corners of the Brazilian Amazon and who have had no contact with modern society. In recent years the Indian agency, Funai, has received reports of more than two dozen of these isolated bands. But the last time the agency confirmed such a report was 1986, when trackers made contact with a group of Iriri Indians in Para state just south of the Amazon River. What intrigues Indian experts is that the Rondonia couple may be part of a group of indigenous people unknown to anthropologists.

Though the couple was discovered Sept. 3, Funai has still not identified the tribe they belong to or the language they speak. "We are sending a native linguistics expert and an anthropologist into the area to try to establish a dialogue with these people and discover who they are," says Funai president Marcio Santilli.

Santos led his party, which included a reporter from a Sao Paulo newspaper, through inhospitable jungle for three days before encountering the couple, of whom he had found evidence on an expedition a month earlier. Another woman and a child may also live in the area. Despite their isolation, the pair's necklaces included manufactured glass and plastic, and they wore shorts made of rice sacks, items they might have picked up in the abandoned logging camps that dot the area.

The couple guided the visitors to a small clearing with two oval-shaped straw huts. In the middle of the clearing were papaya trees. A small monkey was tied on a leash. Inside the huts were wooden flutes, bows and arrows, and primitive tools. The couple offered their guests bananas and papayas, and were given a pocketknife, a machete and, because they admired it, Santos' watch.

The intrusion by the Indian workers was unusual; for the past decade Funai's policy has been to avoid contact with "isolated" Indians, whom satellite photos have spotted living in camps of as many as 50 people throughout the 5 million-sq-km Amazon region. Indeed, Funai's job is to protect them from homesteaders whose advance threatens their habitat. "We only make contact when the situation is dramatic," says Sidney Possuelo, who is in charge of Funai's isolated-Indian department.

That is the case in southern Rondonia, where the rain forest, which is on private land, is being steadily stripped away by farmers, ranchers and loggers. Eventually, the couple's home, and any others secreted in the deep woodlands, will be burned or bulldozed. Funai's task is to relocate them to an Indian reserve, once it is determined who their closest cultural relatives are. The Indians "keep moving on as the frontier advances," says Possuelo. "Pretty soon they won't have anywhere to go."

--By Michael S. Serrill. Reported by Ian McCluskey/Rio de Janeiro