Age-dependent evolution


Jason Collins


September 10, 2012

At the Consilience Conference earlier this year I bumped into evolutionary biologist Michael Rose, whose research interests include examining ageing through the lens of evolutionary theory. In our brief conversation, Rose mentioned that he had laid out much of his thinking on the topic in 55 Theses, where Rose describes how to use the insights of evolutionary biology to improve your health.

Reading through the archives of the Social Evolution Forum over the weekend (worth adding to your feed), I came across a post by Peter Turchin recounting a conversation he had with Rose at the same conference. Turchin writes on one element of Rose’s thinking, which concerns age-dependent traits:

We think of people having ‘traits,’ but actually we change quite dramatically as we age. … As an extreme example, consider reproductive ability, something of great interest to evolution. Humans do not reproduce until they reach a fairly advanced age of maturation (puberty). Young adults are not very good mothers or fathers, but they improve with age during their twenties. After that reproductive ability declines and eventually disappears. …

Ability to digest certain foods can also be age-dependent. I have already mentioned the ability to digest lactose, the sugar present in milk. Before we domesticated animals such as cows and sheep, only very young humans had this ability. Natural selection turned this ability off in adults because they never needed it (and it would be wasteful to continue producing the enzyme lactase that aids in the digestion of milk sugar). …

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. …

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.

The conclusion that Rose draws is that young people descended from populations with a substantial history of agriculture can probably cope with an agricultural diet. However, as they age, may need to revert to a Paleo diet. Those without that agricultural history should get on a Paleo diet from the start. It is an interesting twist on the usual Paleo diet story.