Thesis #51 – Older adults from all human populations are not adequately adapted to agricultural patterns of nutrition and activity, resulting in an amplification of aging under such conditions.
Naturally, older adults from non-agricultural populations will be no better adapted to agricultural conditions than their children. Indeed, they will probably fare worse to the extent that agricultural foods are nutritionally similar to human milk, which almost all humans are able to digest well from birth.
It is when we turn to older adults with agricultural ancestry that the evolutionary analysis becomes interesting. I have argued to this point that the hundreds of generations of selection that some agricultural civilizations have imposed for that pattern of nutrition and activity will have extensively, albeit not necessarily completely, adapted children from such populations to these conditions. But such extensive adaptation applies only to the ages during which the forces of natural selection are near their maximal values.
At later adult ages, the forces of natural selection progressively fall. At these ages, there will have been less age-specific selection for adaptation to agricultural conditions, even in populations that have been subjected to such conditions for hundreds of generations. Thus, to the extent to which there is age-specificity to such selection, older adults will suffer from attenuated adaptation to the evolutionarily novel conditions of agricultural life. Such older adults will benefit from whatever age-independent adaptations to agriculture have been built by natural selection. But the very high mortality levels of post-aging humans from agricultural populations consuming agricultural diets, between 40 and 50 % per year, suggest such age-independent adaptation has been relatively limited.
In effect, older humans from agricultural lineages revert to a condition of poor adaptation to agricultural conditions which could be comparable to that of individuals with hunter-gatherer ancestry. But it is possible that they may retain some residual adaptation to hunter-gatherer conditions, if such adaptation has not been entirely corroded by selection acting at earlier ages. Given that many humans also vary in the extent to which their ancestry is agricultural, each older adult will have a specific mix of retained or compromised adaptation to hunter-gatherer diets and activity patterns. But what we can fairly sure of is that they will no longer have much adaptation to novel agricultural conditions; there has been too little evolutionary time for the later reaches of the human life-history to be fully remolded. This applies even among populations which have children that fare quite well under agricultural conditions.
This provides the possibility of enhancing the level of adaptation among older adults from agricultural lineages by switching them from agricultural diets and activity levels to those of hunter-gatherers at later adult ages. However, the extent of any such enhancement will vary among these individuals. Practically speaking, it is an empirical question with an answer specific to each person.