We think of people as having traits, Peter Turchin says, when we need to realize that people have different traits at different ages:
Because abilities to do something at the age of 10, 30, 50, etc. are separate (even if correlated) traits, they evolve relatively independently of each other. When grains became a large part of the diet, the ability of children to digest them (and detoxify the chemical compounds plants put into seeds to protect them against predators such as us) became critical. If you don’t have genes to help you deal with this new diet, you don’t survive to adulthood and don’t leave descendants. In other words, evolution worked very hard to adapt the young to the new diet. On the other hand, the intensity of selection on the old (e.g., 55 years old) was much less – in large part, because most people did not live to the age of 55 until very recently. Additionally, once an animal gets past its reproductive age, the evolution largely ceases to have an effect (in humans, presence of older individuals was somewhat important for the survival of their genes in their children and grandchildren, so evolution did not entirely cease, but was greatly slowed down).
What this means is that evolution caused rapid proliferation of genes that enabled children and young adults to easily digest novel foods and detoxify whatever harmful substances were in them. Genes and gene combinations that did the same for older people also increased, but at a much, much slower rate. This may sound puzzling – if we have the detoxifying genes that work for young adults, why shouldn’t they work for older adults? The reason is that one gene-one action model is wrong; it’s not how our bodies work. Most functions are regulated not by a single gene, but by whole networks of them. As we age, some genes come on, and others go off, and the network changes, often in very subtle and nonlinear ways. That’s why we need the ‘trick’ with which I started, to consider functions at different ages as separate traits. During the last 10,000 years evolution worked very hard to optimize the gene network operating during earlier ages to deal with novel foods. But the gene network during later ages was under much less selection to become optimized in this way.
The striking conclusion from this argument is that older people, even those coming from populations that have practiced agriculture for millennia, may suffer adverse health effects from the agricultural diet, despite having no problems when they were younger.