How are scholars able to find out how early people with no written records lived?
Scholars use several techniques in order to establish and conclude a portrayal of early people’s lives, before any written records. They simply rely on findings that were left behind. Tools, bones, graveyards, hunting equipment, as well as weapons, art on pottery or cave walls, are some of the prehistoric finds, to name a few. These findings, when examined can tell scholars a lot about peoples lifestyle pre-written records. For example, by examining bones, a scholar would determine if people were malnourished, or whether they knew how to take care of broken bones or used any healing methods.
What was prehistoric society like around the Amazon?
Populations from North American and Europe did not find the prehistoric Amazon population interesting. The cultural differences were too vast, as opposed to societies around the Amazon. The Amazonian people believed in coexisting with their gods and relied heavily on their dangerous environment. I suppose the Amazon was a threat to societies around it, since they had not developed an acquired set of skills to survive.
What ensured European domination in the New World?
The Europeans were able to dominate the New World, by having armies that would take over natives ground, and have their own people settle. They also were in advantage knowing how to trade, how to build weapons, which were made of steel, as opposed to native culture who used leather, wood, and bones to build their weapons.

How do you think an entire prehistoric group of people could simply disappear, such as the people around the Marajo of the Amazon?

A lot of factors could wipe out a whole group. There are many ways this could happen. It could be due to diseases that they were unable to cure, or seek help for. Neighboring villages would eradicate a whole village’s population just to prevent a mortal disease from spreading onto their grounds. So, war could also eliminate a native culture, whether it is invasion or fighting a disease from spreading. Today, this happens as well, yet in a “civilized” way, where a native culture is broken up and disappears from their natural environment. This happens when many investors buy out native cultures to leave their islands, so they can build their hotel resorts on them.

References:
Sayre, H. M. (2010). Discovering the Humanities. Upper Saddle River, NJ; Prentice Hall-Pearson