Smallpox emerged in the Old World around 10,000 BC and is believed to have originated via cattle farming. It reached very high concentrations in Europe and became a common plague there; it was spread around the world to peoples who had never encountered it by European exploration and conquest. It and other Old World disease spread very rapidly among American native populations, rendering whole cultures extinct and reducing others to scattered survivors often incapable of rebuilding. The total population of the Americas lost to European diseases after the arrival of Columbus and Cortez is estimated at 90 to 95 percent.
Given that many Native nations were at least modestly dependent on agriculture (the Iroquois, Navajo, Aztecs, Incas, Mississipians -- indeed, most of the well-known groups), such population losses coming so quickly are nothing short of catastrophic. Most of your resource base collapses because one person is going to have to work MUCH harder to provide enough food for themselves -- fields go unplanted, vegetables don't get tended, wild game is much more dangerous to hunt by oneself, and one cannot expect any assistance with gathering. Even a small number of people used to an agriculture-enriched lifestyle are going to be hit much harder.
It's also worth noting that Cabeza da Vaca actually described the Coahuiltic as a healthy and prosperous people -- and ant eggs, lizards and so on were just normal parts of their diet. Ant eggs in particular are STILL a cultural delicacy among the Latino groups descended from the Coahuiltecs (escamole taco, anyone?). Diet adapts to local circumstances.
The only way these societies could have already been devastated is if epidemics had ravaged the whole continent immediately in the first decades after the first Europeans landed, ahead of any European contact > with the inland peoples.
That is precisely what happened. One infected slave from Spanish-held Cuba is believed to be the Patient Zero that transmitted an infection which would go on to wipe out about fifty percent of the Aztec population. Hernando de Soto, exploring the southeast, encountered many towns and villages abandoned just two years prior when most of their inhabitants died of the plagues. Isolated survivors often just abandoned their homes outright, since in many cases a handful of people or even a single survivor were all that was left out of a village of hundreds or thousands. Neighbors who showed up, unaware of what happened, might contract disease from the corpses in some cases, or simply welcome in the survivors who'd start the cycle anew. North America had extensive trade routes linking all major regions, from coast to coast. Foot and boat traffic carried diseases quite far from their initial outbreak sites.
If even the earliest accounts are of devastated societies, then how do we know anything about the better life they led before that?
Because they're not all dead, and they left their own records of what happened and there are records of contact with them in much better conditions*, and there are still plenty of Native people alive today, who often know rather more about said records of their lives before than the typical Euro-American? And because it's generally acknowledged within anthropological, archaeological and historical fields now that modern research bears out a picture of generally healthy, sustainable populations for most of the foragers of the Americas? And quite large, complex societies that were generally not recognized as such by early Anglo scholars into the matter?
(Malthus seriously* misrepresents Cabeza de Vaca's case -- the Floridians were in a bad way, but they were also right next door to Spanish early conquest -- his accounts of the Coahuiltecs of coastal and inland Texas describe them as a healthy and prosperous people...and their descendents STILL enjoy ant eggs as a dietary item; you don't have to be desperate to eat insects and many human groups actively enjoy it .
Where does this information come from? You cite an ethnography by Boas, who was born in 1858, as authoritative, but dismiss a compilation of far older accounts compiled by Malthus in the early 19th century.
Boas actually travelled to the civilizations he wrote about, lived among them, recorded their oral traditions and analyzed their languages, investigated their history and their environmental circumstances. For many people, especially in the Northwest, far North and other relatively late-contacted areas, these events occured within the living memory of their elders.
Malthus wasn't an expert on Native American civilizations or history, and basically went with the prevailing account available at the time. He relied on a consensus that wasn't yet well-understood to be false. So I reject Malthus' picture of pre-Columbian America for the same reason I reject Lysenko's account of evolution. The difference is that Malthus was an influential thinker within the development of Western thought, and his role means that a lot of people who agree with what insights he did make are unwittingly buying into cached arguments about related subjects (often ones that don't support his case) which hadn't yet been discovered as such when Malthus wrote in the first place.
Scholarship in the field since Malthus' time has seriously changed the outlook -- Charles C. Mann and Jared Diamond are good, accessible sources for a summary overview ("1491" and "Guns, Germs and Steel"). If I seem to be vague, it's mostly because this is domain-specific knowledge that's not widely understood outside the domain, but as domain insider it's fairly basic stuff.
And because it's generally acknowledged within anthropological, archaeological and historical fields now that modern research bears out a picture of generally healthy, sustainable populations for most of the foragers of the Americas?
How exactly does this modern research reconstruct the life of American foragers centuries ago, and based on what evidence? Could you cite some of this work? (I'd like to see the original work that presumably explains its methodology rigorously, not popular summaries.)
I also note that you haven't answered Wei Dai's questi...
From Mike Darwn's Chronopause, an essay titled "Would You Like Another Plate of This?", discussing people's attitudes to life:
Conclusion, graphs, and references in article. As usual, I recommend reading Chronopause.com as Darwin has many good articles; to quickly link a few: