In Amazonia, Defending the Hidden Tribes

That kind of doubt spreads easily in towns such as Colider, where logging companies and farmers have cleared most of the surrounding area and small planes regularly fly overhead. From the air here, the land looks much like the American Midwest -- a patchwork of farms. The picture is much different less than 250 miles away in Kapot -- unreachable by car and boat -- on the edge of an Amazon forest that is almost as big as the continental United States.

But even some officials have expressed doubt. In Peru, the representatives of Perupetro have questioned the timing of the appearance, which came weeks before the country plans to auction 19 oil and gas exploration licenses. Some of the concessions are located near the border with Brazil, where some nongovernmental organizations argue that uncontacted tribes reside.

Last month, the Peruvian government rejected oil exploration plans by Barrett Resources, a U.S. company, and by Spain's Repsol YPF, in part over concern about uncontacted tribes.

While not denying the existence of some isolated groups, Quiroz -- Perupetro's legal counsel -- was skeptical about the recent appearance of the Kayapo tribe.

"In this age of globalization," Quiroz said, "you have to wonder why now and why there."

Vanishing Without a Word

Several years ago, Brazil's government changed its policy regarding isolated tribes: Instead of taking the initiative to try to contact them, it now aims only to protect them. Contact is made only if the Indians themselves initiate it or the tribe is in imminent danger.

Funai officials plan to fly over the forest in the coming weeks to try to locate the area where the tribe is based, Megaron said. The plan after that is to build a small field station in the forest -- not to contact them but to protect the area and make sure loggers and farmers do not come near them.

That plan, of course, would be unnecessary if the Indians chose to make contact again -- a possibility that many of the local Kayapo hope happens.

"Everybody wants to see them, because we love to compare them with ourselves," said Bepko, 26, a Kayapo who lives in the village of Kubenkokre in a nearby reserve. "We just want to hear their stories and learn about what their lives have been like."

According to the stealth tape recording made by the brothers, there is evidence that at least some in the tribe would like to return.

Megaron said he was able to decipher the language sufficiently enough to determine that a young member of the tribe was trying to convince his elders that the contact was a good thing.

"The son told his father not to be afraid, that they would protect each other," Megaron recounted. "He then talked to his mother and tried to tell her that everything was okay and that the other group of Kayapo was their relatives."

It was later, Funai said, that a tribal leader emerged from the forest and persuaded everyone to leave the village.

"They might have been scared of the sound of airplanes," said Luis Sampaio, a biologist who for 12 years has worked with the Kayapo in the reserve, which features a small landing strip. "Or they could have been scared by the clothes they saw people wearing -- we are not sure."

Megaron said they left without explanation or warning.

"Uncontacted Indians," he said, "don't say goodbye when they leave."

Special correspondent Lucien Chauvin in Lima, Peru, contributed to this report.