Part One

This is a wonderful question, and there is a lot to unpack here. I’ll start with a broad discussion on how contact unfolded across the Americas as a way to inform the later discussion of twentieth century “first contact” events in the Amazon.

One of the greatest challenges to understanding “first contact” events is that very few actually involve people completely lacking any context for understanding one another. With the exception of Columbus’s arrival on San Salvador, and all but the very first contact events along the coast of the Atlantic and the Gulf of Mexico, the tendrils of colonialism advance faster and farther than the wave of first contact. Word of mouth spread rumors of new arrivals, trade networks carried imported goods further inland, and the displacement of coastal populations started a cascading domino effect of refugee nations pushing against one another deep inland. All these events served as context, influencing the trajectory of nations whose descendants would not see a white individual for another century.

Historians of the U.S. southeast describe the impact of colonialism as a shatterzone. Like a hammer striking a vase, the impact of small European enclaves were restricted to immediate Atlantic coastline, but the cracks and damage radiate out from that point of impact to penetrate deep into the continent. In the southeast the major disruption came in the dual form of the deerskin and Indian slave trade. The Indian slave trade caused havoc throughout the southeast, spurring the rise of large confederacies, and setting the stage for the region’s first smallpox epidemic in the late 1690s. When sustained first contact occurred along the middle Mississippi (which was really third contact after the brief encounters of de Soto in 1542, and La Salle in 1673) in 1699 d’Iberville reported “a prodigious number of human skeletons that they formed a mountain” from “a numerous nation who being pursued and having withdrawn to this region, had almost all died here of sickness.” This was not a pristine first contact event of legend, but rather one deeply influenced by the impact of systemic violence, epidemics, territory displacement, and instigated warfare hundreds of miles away.

The southeast was not alone in this pattern of traumatic shocks preceding the first contact events of legend. Popular culture reinforces the romantic notion of first contact, but as alluded to above, in the unfortunate history of colonialism the bulk of true first contact events involved outright violence and criminal acts. Spain outlawed unofficial entradas into new lands, but as Reséndez eloquently described in The Other Slavery: the Uncovered Story of Indian Enslavement in America slavers were often the vanguard of contact throughout the Americas. Even during the first official entrada in North America in 1513, twenty years after Columbus’s first voyage, Ponce de Leon suspected slavers preceded his arrival when most Tequesta fled to the surrounding woodlands, and the unfortunate captured individuals already understood a few words of Spanish. In a pattern that would be replicated with nations as diverse as the Mono from Central Valley of California to the Mapuche of South America, exploitation of Native American labor was often the driving force behind first contact.

Now, why did I dive into that deep and violent history before talking about modern first contact events? Because what follows is not a new story in the history of the Americas. We inherit a legacy of colonialism built on the extraction of resources through predominantly violent means, and the subjugation of indigenous populations in the quest for personal enrichment. That history informs Peru’s official policy of no/minimal contact with nomadic populations, just as it influences the Mascho-Piro’s decision to violently confront outsiders. Unfortunately, a violent history provides a perspective on how these policies may ultimately prove futile in the wake of continuing encroachment from illegal loggers, gold miners, and drug runners.

This question would go great in /r/AskAnthropology!

In short, most of the world is connected to the rest of the world in some fashion, or otherwise has had some kind of cultural or material transaction in the recent past. The same is true of the "lost tribes" in the Amazon or Papua New Guinea.

Even though some might not have ever seen a European directly, they are still not isolated from global culture. For example, this famous photo taken in June 2010 of an uncontacted Amazonian people reveal some telling elements. In the foreground is a child holding a machete, and a stainless steel pot next to a rock by his feet. Less subtle indicators are a basket of banana leaves at the top right and on the ground by the basket to the left is an actual green banana (more bananas being grown can be seen in the documentary video. Bananas are not native to the New World, but those metal objects probably tip you off the most.

Non-industrial societies do not live a static existence totally isolated from their surroundings. Even though the uncontacted people in this photo have not made formal contact with the modern world and had actually been fleeing the encroachment of loggers, they have still been interacting with our modern culture via trade networks that extend to peoples we have contacted. So in that thought, the phrase 'uncontacted' or 'lost tribe' is actually quite the misnomer.

In the modern world, there is tremendous pressure by both governments and NGOs to prevent contact with many of these people, or at least arrange contacts on the indigenous group's terms. The main fear in contact is the easy spread of modern diseases to the natives, followed by fears of forced assimilation, cultural destruction and the possibility of violence toward either group. Basically, the idea is to leave these people alone and let them decide what to do. As for the uncontacted group I mentioned, they ultimately wound up meeting up with the Ashaninka people and a local environmental group; medical staff were then promptly flown in to deal with any infections that may occur.