Razib Khan shares his model of the history of human misery, which addresses the truism of modern cultural anthropology that the move from hunting and gathering to farming was a big mistake:
There’s plenty of evidence that farming supports many more people per unit of land, so in pure demographic terms hunter-gathering was bound to be doomed. They didn’t have the weight of numbers. But why did the initial farmers transition from being hunter-gatherers to farmers in the first place? Because I think that farming was initially the rational individual choice, and led to more potential wealth and reproductive fitness. Remember, there’s a big difference between existing in a state of land surplus and one of labor surplus. American farmers were among the healthiest and most fertile human populations which had ever lived before the modern era. Pioneers had huge families, and continued to push out to the frontier. This was not the lot of Russian serfs or Irish potato farmers. But eventually frontiers close, and Malthusian logic kicks in. The population eventually has nowhere to go, and the surplus of land disappears. At this point you reach a “stationary state,” where a peasant society oscillates around its equilibrium population.
I suspect that new farming populations which slam up against the Malthusian limit suffered even more misery than their descendants. This is because I believe that their demographic explosion had outrun their biological and cultural capacity to respond to the consequences of the changes wrought upon their environment. First and foremost, disease. During the expansionary phase densities would have risen, and infectious diseases would have begun to take hold. But only during the stationary state would they become truly endemic as populations become less physiologically fit due to nutritional deficiencies. The initial generations of farmers who reached the stationary state would have been ravaged by epidemics, to which they’d only slowly develop immunological responses (slowly on a human historical scale, though fast on a evolutionary one). This is even evident in relatively recent historical period; Italians developed biological and cultural adaptations to the emergence of malaria after the fall of Rome (in terms of culture, there was a shift toward settlement in higher locations).But there would be more to adapt to than disease. Diet would be a major issue. During the expansionary phase it seems plausible that farmers could supplement their cereal based diet with wild game. But once they hit the stationary phase they would face the trade-off between quantity and quality in terms of their foodstuffs. Hunter-gathering is relatively inefficient, and can’t extract as many calories per unit out of an acre (at least an order of magnitude less), but the diet tends to be relatively balanced, rich in micronutrients, and often fats and protein as well. The initial shock to the physiology would be great, but over time adaptations would emerge to buffer farmers somewhat from the ill effects of their deficiencies. This is one hypothesis for the emergence of light skin, as a way to synthesize vitamin D endogenously, as well as greater production of enzymes such as amylase and persistence of lactase, which break down nutrients which dominate the diet of agriculturalists.
Once societies reached a stationary state it would take great shocks to push them to a position where becoming hunter-gatherers again might be an option. A population drop of 50%, not uncommon due to plague or political collapse, would still not be low enough so that the remaining individuals would be able to subsist upon game and non-cultivated plant material. Additionally the ecology would surely have been radically altered so that many of the large game animals which might have been the ideal sources of sustenance in the past would be locally extinct. The collapse of the Western Roman Empire and the highland Maya city-states seems to have resulted in lower population densities and reduced social complexity, but in both regions agriculture remained dominant. On the other hand, there is some evidence that the Mississippian societies might have experienced die offs on the order of 90% due to contact with Spanish explorers, and later ethnography by European settlers suggests much simpler tribal societies than what the Spaniards had encountered. Though these tribal groupings, such as the Creeks, still knew how to farm, it seems that judging from the conflicts which emerged due to European encroachment on hunting grounds that this population drop was great enough to allow for a greater reversion to the pre-agricultural lifestyle than was able to occur elsewhere. But then the pre-Columbian exchange and the exposure of native populations to the 10,000 years of Eurasian pathogen evolution was to some extent a sui generis event.