Thesis #49 – In agricultural populations over the last ten thousand years, the longer-sustained effectiveness of natural selection has resulted in an age-dependent pattern of falling adaptation to agricultural conditions in which functional decline is sustained over a longer period than was the case under hunter-gatherer conditions.
Thus the reach of natural selection has been extended deeper into adulthood among the agricultural populations of the last tens of thousands years. As a result, at least under agricultural conditions, individuals with agricultural ancestry should have an aging period that lasts significantly longer than that of individuals without agricultural ancestry, at least when they follow their ancestral lifestyles.
Indeed, considering human demography compared to that of well-studied model organisms like fruit flies, we have an extremely protracted aging phase. Few people survive long enough to reach the demographic phase in which “aging stop,” as shown in some detail in our 2011 book, “Does Aging Stop? (Mueller, Rauser, and Rose; Oxford University Press). This, we believe, is the reason why gerontologists have long thought that aging is an unremitting process of physiological deterioration, because humans have been taken as the generic case of aging.
By contrast, hunter-gatherer populations should have an underlying pattern of aging more like that of the fruit flies that we have studied in our laboratories at UC Irvine. That is, they should show a relatively short period of aging that takes them to mortality and fertility plateaus on which much function is relatively long-sustained, at least if we contrive the analog of laboratory conditions for such populations. This would mean the provision of excellent conditions, with no shortages of food, protection from predators or criminals, and a relative absence of acute infection.
Now, hunter-gatherer populations do not in fact live this way. In some cases, they continue with a hardscrabble existence on the margins of the best habitats for agriculture, fighting tribal wars, and engaging in a pattern of competitive homicide that kills them off at a fair clip. The indigenous populations of the Amazonian jungle used to follow this pattern fairly extensively. And they also face death due to predators, including large felids and poisonous snakes. Accidental or violent injury readily leads to rapid death due to sepsis or hemorrhage among such populations, if there is no access to emergency medical services.
Or these populations are “semi-civilized,” consume milk and seed-derived foods, and receive more access to medical care. But then they suffer the chronic health problems which their lack of adaptation to agricultural foods and inactivity will engender. Many of the people on “reservations” live this way.
Thus we have not been able to observe large human cohorts with early cessation of aging, because those individuals who have the genomes to exhibit this pattern do not live in a manner in which it could be exhibited. Those of us who do have such environmental conditions, in the general sense of good medical care and lower levels of violent death, do not have evolutionary histories in which our aging phase was curtailed at early ages.