Members of an unknown tribe in western Brazil threaten a government plane surveying the region for evidence of so-called "uncontacted" peoples. Survival International, a native-rights group, estimates that half of the world's hundred or so isolated tribes live in the rain forests of Brazil and Peru.

Some experts say there are few, if any, tribes that have had no outside contact. It's more likely that previous generations had negative encounters, prompting social taboos that continue to drive clans deeper into isolation.