More Words, Deeper Hole

Okay, I wasn't very clear. Initial occupation of North America was probably confined to coastlines because large parts of the North American landmass were, well, glaciers. Generally speaking, a glacier doesn't support a whole lot of life. And their sheer size can block off access from the coasts to chilly but non-glacier inland areas that DO support plant and animal life (so you'll get big islands surrounded by ice rather than water, and regional fauna populations that are cut off from each other and from predators that aren't already on their "island"). So one fairly well-supported hypothesis right now is that humans have been in the Americas for a very long time, but in fairly small numbers confined to limited areas along the coasts where they could scrape a living. As the world warmed up again, sea levels rose (submerging many initial settlement sites) but there was a heck of a lot more habitable territory opened up inland, so they actually gained usable land despite the continents technically becoming smaller.

Vegetation and animal life moved into formerly glaciated areas with a vengeance, and humans followed them. They had to adapt from a beachcombing lifestyle to an inland hunting and gathering lifestyle, but the benefit was that there were major population surges because they had so many new places to spread into. One of the reasons it was long believed that Clovis kill sites represented the first North Americans was because there are, comparatively speaking, a heck of a lot more Clovis sites than there are earlier. Which is consistent with a population boom, and a move to a different hunting style. There's not a lot of big game along the coastlines, except marine mammals, which are more dangerous to hunt than are comparably-sized land mammals. Coastal peoples are more likely to get their daily protein from fish and shellfish, if it's plentifully available (which it often is in non-Arctic conditions). They generally hunt big marine mammals for status/prestige/religious reasons rather than as a major component of their diet.

So, the kind of managed ecology that Charles C. Mann talks about in "1491" is typical of what happened well *after* humans had expanded into this new inland niche. They had already expanded to live pretty much anywhere it was possible for human life to be supported, and if they wanted to increase their population, they had to either switch to agriculture (which happened in some areas) or they had to find a way to make hunting and gathering more high-yield. Hence the controlled burns etc.