There is genetic evidence as well as a huge amount of archaeological research showing that in the ancient world they had a much better ability to travel and migrate to far away places than the mainstream academic community has acknowledged. South America was certainly populated much earlier than North America. There are literally dozens of published studies from South America showing habitation there from as long ago as 300,000 years till some 50,000 years ago. North American archaeologists detest all of that information.Mounds were erected in South America some 10,400 years ago more than 5,000 years earlier than any in North America. Pottery was everywhere South America thousands of years before it appeared in North America.
In the book, Atlantis is only mentioned coincidentally because its dates are in line with significant and important events in the Americas and elsewhere. As to the idea of a "Lost civilization," I agree with what Graham has written. We were more focused on the underlying belief systems that appear to have been similar across cultures and distances. We focused on the movements of these ancient people across vast distances and vast time frames. The Denisovans, Neanderthals, modern humans, and other lineages obviously had to be mentioned in it all because we are looking at dates supposedly earlier than modern humans existed. Personally, I wouldn't say that "shamanism" was the unifying factor. The fundamental spiritual/religious beliefs were clearly shared among these people.
I'll add one more thing in response to your last question. It is known that in the Death Journey ceremony held in mound builder cultures (around the winter solstice) that hallucinogenics were used by some of the participants. That ceremony sent the souls of the dead to the Path of Souls-the Milky Way. Some of the most amazing artifacts ever excavated from American mounds (displaying very specific symbols) were containers used in these ceremonies. However, rather than the drugs being the focal point of it all, it's pretty clear that only some people used it, and that it was just one portion of a 10 hour ceremony that involved a lot of activity.