Get a taste of what awaits you in print from this compelling excerpt.

We found fresh human tracks this morning. They all pointed in the same direction that we're walking through the virgin jungle of Brazil's westernmost Amazon Basin. Woolly monkeys hoot and chatter somewhere in the distance, their banter punctuated by the occasional zing of a machete and the shrill cries of screaming piha birds high in the canopy overhead. Our column of 34 men proceeds in silence, strung out single file far back into the forest. Only one or two companions are visible at any time in the blur of electric greens and rain-soaked browns. The rest are swallowed from view by a spray of overhanging branches and vines as thick as anacondas dangling a hundred feet (30 meters) from the treetops to the forest floor. Just ahead of me, Sydney Possuelo strides double-time across a stretch of level ground, a welcome break from the steep hillsides we've been scrambling over for days. "We're probably the only ones who have ever walked here," he tells me. "Us and the Indians."

A cantankerous iconoclast with bulging hazel eyes, scraggly salt-and-pepper beard, and wild locks flowing from beneath a floppy camouflage jungle hat, Possuelo, 63, is widely considered one of the Amazon's last great wilderness scouts and the leading authority on Brazil's remaining pockets of isolated Indians. After two weeks of river travel and 20 days of steady bushwhacking, Possuelo has led us into one of the most remote and uncharted places left on the planet, near the headwaters of two adjacent rivers, the Itaquaí and the Jutaí. This is the land of the mysterious Flecheiros, or Arrow People, a rarely glimpsed Indian tribe known principally as deft archers disposed to unleashing poison-tipped projectiles to defend their territory against all intruders, then melting away into the forest.

Suddenly Possuelo stops in his tracks. A freshly hacked sapling, still dangling by a shred of bark, lies across the path in front of us. In itself, the makeshift gate could not halt a toddler, much less a column of nearly three dozen armed men. But still, it bears a message—and a warning—that Possuelo instantly recognizes and respects. "This is universal language in the jungle," he whispers. "It means 'Stay Out. Go No Farther.' We must be getting close to their village."

Which is something Possuelo wants to avoid. He wheels around and with a silent, dramatic wave directs our column to veer off the path into the dense undergrowth on our flanks. A half hour later, after slogging through boot-sucking mud and dodging branches that swarm with fire ants, we arrive by the steep banks of a clear, narrow creek, where Possuelo orders a halt to the march while we wait for stragglers to catch up.

The Flecheiros figure among 17 so-called uncontacted tribes living in the far recesses of the Brazilian Amazon. In this part of the rain forest, the Vale do Javari Indigenous Area, there may be as many as 1,350 uncontacted indigenous people—perhaps the largest such concentration anywhere in the world. Most of them are descendants of the survivors of massacres perpetrated by white intruders over the centuries. The Indians then scattered into the rugged folds of the region's headwaters and continue to shun contact with the outside world.

But violent clashes account for only a fraction of the deaths suffered by native communities at the hands of outsiders. Most died from epidemic diseases, including the common cold, for which they had no biological defenses. Ivan Arapa, one of our scouts, is from the Matis tribe, who were first contacted by the outside world about 25 years ago. Ivan still remembers the wholesale death that accompanied these very first visits of Brazilian government officials to his village.

"Everyone was coughing, everyone was dying," he recalls. "Many, many Matis died. We didn't know why." More than half of the 350 Matis living along the Ituí River inside the Javari reserve perished in the months following contact, officials say.

Get the whole story in the pages of National Geographic magazine.

VIDEO Photographer Nicolas Reynard talks about working with trackers who "can see what we can't see." AUDIO (recommended for low-speed connections) RealPlayer  WinMedia

Experience the danger and discovery of the Possuelo expedition into Flecheiros territory.

Watch the expedition team carve and construct two large water-ready canoes. Later, Possuelo bonds with Korubo Indian Maya and her family.

What are the consequences of continued isolation for such people as Brazil's Flecheiros? What are the consequences of contact?

CONTACT! Should uncontacted, isolated cultures be left alone? Yes      No

View images and read dispatches from  Sydney Possuelo's  first contact with the Korubo people in 1996.

For hundreds of years Native Americans in the Northern and Southern Hemispheres have used urucú—more commonly known as annatto in the States—to dye their skin for ornamentation. The annatto tree (Bixa orellana) can reach 20 feet (six meters) in height. Bristly pods on the tree contain dozens of seeds surrounded by pulp. While the seeds can be rubbed directly on the skin to stain it red, more often the seeds are boiled down and mashed into a paste, then mixed with an oil, often from the nut of the piquia tree. Prepared in this method, the waxy ball lasts several weeks and can be used when convenient or necessary.

You may not be familiar with annatto used in this way, but you are surely familiar with its use as a food dye: Annatto is used to make cheddar cheese orange.

—Jennifer L. Fox