Thesis #47 – This novel agricultural lifestyle initially depressed adaptation and health, leading to intense natural selection for adaptations to the digestion of foods derived from grasses and milk, which has since produced adaptation to agricultural conditions at early ages.
There are a variety of ways to understand what has happened to agricultural populations over the last tens of thousands of years. I will focus on two.
Paleoanthropology supplies us with a long historical sequence of human skeletons to study. Before the advent of agriculture, our preserved skeletons show that we grew to a fair height, men averaging about 5 feet 5 inches to 5 feet 10 inches, women perhaps four or five inches shorter than this, on average. Close inspection of such skeletons indicates fair nutrition, although orthopedic injury was common.
With the advent of agriculture, the skeletons suggest an abrupt reduction in average height. This is also sometimes associated with evidence of chronic disease and poor nutrition. In Egyptian mummies, for example, there is evidence of chronic cardiovascular disease.
But all that evidence is indirect, and at least somewhat arguable. Fortunately, we have still better evidence of the kind assembled by Lindeberg in his “Food and Western Disease,” already mentioned. For we have clinical medical data on the effects of adopting Western foods and lifestyles on twentieth century relics of hunter-gatherer populations. And those clinical data are damning: switching from a hunter-gatherer lifestyle to an agricultural and sedentary one produces a wide spectrum of chronic pathophysiologies, especially the early and devastating onset of age-associated chronic diseases.
The first conclusion to be derived from these findings is that the original switch to agriculture must have produced significant biological challenges, impinging on both functional capacity and the kinds of robustness that would affect sexual selection. Thus, natural selection would not have been relaxed by the advent of agriculture. Rather, it would have significantly changed direction.
The second conclusion to be derived is that over the hundreds of generations of selection for adaptation to agriculture, there must have been a wide array of genetic changes. Probably not many complete “selective sweeps,” in the language of population genetics, but certainly many gene frequency changes across the genome, from what we now know of the genomics of experimental evolution. Furthermore, among long-agricultural human populations, these extensive evolutionary genetic changes probably provided us with extensive re-tuning of our functional adaptations, enabling us to thrive as young people growing up with foods derived from seeds and milk, despite the daily back-breaking labor required to cultivate, harvest, and process the plants and animals which give us these dietary staples.
Thus modern humans with predominantly agricultural ancestry are not like cats who will sicken on cereal diets. Nor are we like those with strictly hunter-gatherer ancestry, who show much more acute onset of chronic diseases on Western diets. Thus much of the “paleo-diet” literature is overdrawn and invalid, for most of the present-day populations of our species.