The Dark Side of Cultural Evolution
Cultural evolution is what created the – in many ways – wonderful societies that we live in. It created the potential to free our lives from hunger and early death, and made possible the pursuit of science and art. But cultural evolution also has a dark side, in fact, many ‘dark sides.’
Clearly domestication of plants and animals is what made our civilization possible. All sufficiently complex societies are possible only on the basis of agriculture. But we have paid, and continue paying a huge price for this advance of human knowledge and technology. This idea was brought home to me as a result of several conversations I had with Michael Rose during the Consilience Conference at St. Louis, which I talked about in my previous blog.
Michael is an evolutionary biologist at the university of California at Irvine, who studies aging from the evolutionary perspective. I actually read his book Evolutionary Biology of Aging some twenty years ago, but never met him until two weeks ago.
One way people talk about the price of civilization is in terms of evolutionary mismatch (which is one of the focus areas at the Evolution Institute). The idea is that our bodies and minds evolved during the Pleistocene, when we lived in small groups of hunter-gatherers. Now we live in a dramatically different environment, and that causes all kinds of problems. The psychological aspect of the problem was recently discussed by Robin Dunbar and commentators on his Focus Article. The physiological problems include rampant obesity, heart disease, and diabetes.
There is currently no consensus on the role of changing diet and other aspects of lifestyle, most notably exercise, in causing modern-day health problems. Some people argue that our Plestocene bodies are not adapted to high-calorie diets and sedentary life-styles of today.
On the other hand, agriculture was invented roughly 10,000 years ago. 400 generations is not an insignificant length of time for evolution to do its thing. Some anthropologists (including another participant of the Consilience conference,Henry Harpending) argue that humans evolved very intensively during this period. One famous example is the evolution of lactose tolerance, that is, ability to digest milk.
Michael Rose develops a more subtle and sophisticated argument, which is explained at length in his 55 theses-a New Context for Health. There is a sophisticated mathematical model underlying his argument, but the basic logic of it is actually quite simple.
Michael Rose 发展了更加精致灵巧的论证，并在他的网站（55theses.org，副标题“健康新语境”）上做出了详尽的解析。虽然他的论证建立在一个复杂的数学模型之上，但基本逻辑其实十分简单。
We think of people having ‘traits,’ but actually we change quite dramatically as we age. The key ‘trick’ is to realize that people have a suite of traits, and they can be quite different, depending on what stage in life we are talking about.
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. So reproductive ability is actually a trait that varies quite a lot with age.
Another example is hair color. One man can have red hair and another blond hair. However, this will be true only while they are relatively young. Older men become grey, and many become bald. So by the time our two men turned 60, they may have the same hair color (grey), or no hair at all (bald).
By the way, it is likely that the reason is not simple ‘degradation,’ reduced function due to aging, but that greyness and baldness evolved to signal maturity and wisdom. To really describe the phenotype of an individual we need to specify at what age it is expressed.
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).
Now clearly traits expressed at different ages are not completely independent of each other. An ability to digest milk sugar as an adult depends on the presence of an enzyme that evolved in order for babies to digest their mother’s milk. So traits at different ages can be correlated, either positively, or negatively. An example of negative correlation is the reproductive ability – in many animals, putting a lot of effort in reproducing early reduces the reproductive ability later in life.
So the sophisticated mathematical framework for dealing with age-dependent traits has to take into account all kinds of possible correlations, both between the same trait at different ages and between different traits. For example, most individuals have dark eye and dark hair color, or light eye and light hair color, with dark/light and light/dark combinations a relative rarity.
We can now get to the crux of the matter. 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.
The immediate corollary is that one thing they can do to improve their health is to shift to something known as the ‘Paleolithic diet,’ or paleo diet, for short. In the simplest form, this means eliminating from your diet any cereals (wheat, rice, etc), legumes such as beans and peas, and any dairy products (e.g., cheese).
It is striking that this is almost precisely the opposite of the popular Mediterranean diet, which emphasizes wheat products (bread, pasta), cheese, and legumes (as in the Italian bean soup, in pasta fagioli, and in hummus).
We are now getting to something I have a personal (rather than a scientific) interest in. I am about to turn 55, and although I am generally in good health, various worrying indicators – cholesterol, sugar – have been inexorably inching up.
A couple of years ago I read Ray Kurzweil Fantastic Voyage, but I was unpersuaded by his prescriptions to better health and longevity. Kurzweil’s prescription is, at basis, a calorie-restricted diet. Like the great majority of human beings, I find it extremely difficult to starve myself.
More generally, his approach to human health and longevity is that of an engineer – you turn that dial down, another one up, and get the result you want (according to his book, he spends one day a week connected to a machine that removes bad things from his blood and adds good things).
I am very doubtful that such an approach will work on an evolved system with multiple nonlinear feedbacks, which is the human body. So changing one variable (e.g., reducing the cholesterol level in the blood) may have unintended – and usually negative – consequences elsewhere (perhaps increasing the risk of cancer).
To conclude, the paleo diet is the first diet, of the ones I heard of, that has a sound evolutionary basis going for it. This was a deciding factor in persuading me to try it out, which I did, starting about two weeks ago. It apparently takes about six months to see its full effects, so stay tuned for progress reports.
May 17, 2012