Dramatic photographs of previously unfound Amazon Indians have highlighted the precariousness of the few remaining "lost" tribes and the dangers they face from contact with outsiders.
The bow-and-arrow wielding Indians in the pictures released on Thursday are likely the remnants of a larger tribe who were forced deeper into the forest by encroaching settlement, experts said.
Rather than being "lost," they have likely had plenty of contact with other indigenous groups over the years, said Thomas Lovejoy, an Amazon expert who is president of The Heinz Center in Washington.
"I think there is an ethical question whether you can in the end keep them from any contact and I think the answer to that is no," Lovejoy said.
"The right answer is to have the kind of contact and change that the tribes themselves manage the pace of it."
The Brazil-Peru border area is one of the world's last refuges for such groups, with more than 50 uncontacted tribes thought to live there out of the estimated 100 worldwide.
They are increasingly at risk from development, especially on the Peruvian side which has been slower than Brazil to recognize protected areas for indigenous people.
Arrows seen as a good sign
Jose Carlos Meirelles, an official with Brazil's Indian protection agency who was on the helicopter that overflew the tribe, said they should be left alone as much as possible.
"While we are getting arrows in the face, it's fine," he told Brazil's Globo newspaper. "The day that they are well-behaved, they are finished."
Contact with outsiders has historically been disastrous for Brazil's Indians, who now number about 350,000 compared to up to 5 million when the first Europeans arrived.
"In 508 years of history, out of the thousands of tribes that exist none have adapted well to society in Brazil," said Sydney Possuelo, a former official with Brazil's Indian protection agency who founded its isolated tribes department.
Concern over Peru policy
In recent years, though, tribes like the Yanomami have succeeded in winning greater protection by becoming more politically organized and forming links with foreign conservationists.
"It's not about making that decision for them. It's about making time and space to make that decision themselves," said David Hill of the Survival International group.
More than half of the Murunahua tribe in Peru died of colds and other illness after they were contacted as a result of development for the first time in 1996, Hill said.
Sightings of such tribes are not uncommon, occurring once every few years in the Brazil-Peru border area where there are estimated to be more than 50 out of the total global number of 100 uncontacted tribes.
In 1998, a 200-strong tribe was discovered by Possuelo living in huts under the forest canopy, also in Acre state near the Brazil-Peru border.
In September last year, ecologists looking for illegal loggers in Peru spotted a little-known nomadic tribe deep in the Amazon.
The sighting underscored worries among rights groups that oil and gas exploration being pushed by the Peruvian government, as well as logging, is putting tribes at risk.
Peru has no equivalent to Brazil's longstanding Indian affairs department, which has a policy of no contact with unknown tribes.
"There is a lot of logging going on over on the Peruvian side," Hill said. "It's had all kinds of effects on the groups living there, particularly on the uncontacted groups -- it's led to violent conflicts and deaths."
In May, Peru's petroleum agency Perupetro said it would exclude areas where isolated tribes live from an auction of oil and gas concessions. Perupetro had been under pressure to limit exploration activities near tribal areas, and had cast doubt on the existence of isolated groups, angering activists.