Experts doubt that the people in the new Brazilian pictures, or others who have been sighted, are truly “uncontacted” tribes, as was reported in some news accounts. They think, instead, that the group in Acre was probably pushed out of Peruvian territory into Brazil by loggers sawing down their habitat. After years of ignoring the situation, Peruvian authorities in the region of Madre de Dios announced last week their intention to protect isolated tribes and clamp down on illegal logging.

Whoever they are, the protection and continued survival of people like this and their way of life involves striking a careful balance between the ambitions of developers of resources and the best interests of the native population. Anthropologists generally praise Brazil’s efforts in recent years.

Sydney Possuelo, a former director for unknown tribes at Funai, is given much credit for establishing the practice of not contacting isolated people. The government’s policy now is to demarcate lands where they still live and put them off limits to developers. Though the settlements are monitored from time to time, no attempt is supposed to be made to intrude or assimilate the people into the outside culture.

In an interview two years ago with BrazilMax.com, a travel Web site, Mr. Possuelo said he once believed that contact and assimilation benefited these people. “But as I went about my work, I saw that it was really bad for them,” he said. “They get diseases. They lose their autonomy. They become dependent on our world.”

This attitude is a far cry from previous experience. Almost ever since the rubber boom in the 19th century, indigenous people have been forced off their lands to make way for rubber extraction, logging, mining and ranching. Lately, cocaine trafficking in some parts of South America has contributed to the problem. As a result, the uncounted numbers of those surviving on their original land are now few.

“The hope is to avoid those mistakes,” said Janet Chernela, an anthropologist at the University of Maryland. “But some say they are unavoidable.”