like the myth of the "peaceful" Maya or the mother goddess peaceful societies of the Minoans etc. it is projection of what the outsiders wish.
The myth has gotten publicity with the film "lost city of Z", but also because oil exploration surveys have found evidence of extensive farming.
PhysOrg has an article about the real history of that region, and how indeed a lot of what is believed about the Amazon is a myth.
Gee, Father Carvajal might have been telling the truth.
essentially, the "lost tribes" are the survivalist remnant of these sophisticated farmers, not an example of humanity's primitive ancestors.
Father Carvajal's diary of the Orellana expedition has achieved prominence recently. For over four centuries, scholars dismissed its reports of large cities, well developed roads, monumental construction, fortified towns, and dense populations. It was thought that the acidic soils of Amazonia could not support the level of agriculture necessary to sustain such a civilization. His writings were largely dismissed as fabrications and propaganda.
I pointed this out awhile back when I did a scathing review of Tom Wolfe's book about language, where a missionary/anthropologist studying one such tribe found linguistic holes that shattered Chomsky's hypothese on neurolinguistics.
I pointed out that since the tribe's kids easily learned Spanish, which included these concepts, that maybe there was indeed a place in the brain already wired to allow them to learn these things, and so their language's lack might have been degeneration of language in an inbred group of a genetic agnosia from interbreeding that discouraged those without the trait to use these concepts.