Monday, January 18, 2010

The evolution of costly traits: A challenge to a strict paleo diet orientation

The fundamental principle of the paleo diet movement is that we should model our diet on the diet of our ancestors. In other words, for optimal health, our diet should be as close to the diet of our ancestors as possible. Following this principle generally makes sense, but there are a number of problems with trying to follow it too strictly.

Some of those problems will have to wait for other posts. Examples are: our limited knowledge about what our ancestors really ate (some say: lean meat; others say: fatty meat); the fact that evolution can happen fast under certain circumstances (a few thousand years, not millions of years, thus recent and divergent adaptations are a possibility); the fact that among our ancestors some, like Homo erectus, were big meat eaters, but others, like Australopithecus afarensis, were vegetarians … Just to name a few problems.

The focus of this post is on traits that evolved in spite of being survival handicaps. These counterintuitive traits are often called costly traits, or Zahavian traits (in animal signalling contexts), in honor of the evolutionary biologist Amotz Zahavi (Zahavi & Zahavi, 1997). The implication for dieting is that our ancestors might have evolved some eating habits that are bad for human survival, and moved away from others that are good for survival. And I am not only talking about survival among modern humans; I am talking about survival among our human ancestors too.

Here is the most interesting aspect of these types of traits. Our ancestors may have acquired them through genetic mutation and selection (as opposed to genetic drift, which may lead some traits to evolve by chance). That is, they emerged not in spite, but because of evolutionary pressures.

The simple reason is that evolution maximizes reproductive success, not survival. If that were not the case, mice species, as well as other species that specialize in fast reproduction within relatively short lifespans, would never have evolved.

In fact, excessive longevity is akin to quasi-cloning through asexual reproduction, from an evolutionary perspective. It is bad because species need genetic diversity to exist in a constantly changing environment, and genetic diversity is significantly increased by sexual reproduction; the more, the better. Without plenty of death to match that, overpopulation would ensue.

Death is one of evolution’s main allies.

Genes code for the expression of phenotypic traits, such as behavioral (e.g., aggressiveness) and morphological (e.g., opposing thumbs) traits. Costly traits are phenotypic traits that evolved in spite of imposing a fitness cost, often in the form of a survival handicap.

In non-human animals, the classic example of costly trait is the peacock’s train, used by males to signal good health to females. This trait is usually referred to, wrongly, as the male peacock’s tail. Both males and females have tails, but only the males have the large trains, which are actually tail appendages.

What about humans?

One example is the evolution of testosterone markers in human males. Testosterone markers (facial masculinity) have been hypothesized to be handicaps evolved in part by human males to signal to females that they are healthy, essentially because testosterone suppresses the immune system. This apparently bizarre idea is known as the immunocompetence-handicap hypothesis (Rhodes et al., 2003).

This idea will sound bizarre to some, because of the notion that testosterone helps build muscle mass (which it does, together with other hormones, such as insulin), and arguably muscle mass helped our ancestors hunt and fight off predators. Yet, consider the following questions: If muscularity was so useful for hunting and fighting, why are humans so weak compared with other animals of similar size? Why are not females as muscular as males? Why is it so hard to gain muscle mass, compared to fat mass?

Another example is the evolution of oral speech in humans. The evolution of oral speech is one of the most important landmarks in the evolution of the human species, having happened relatively recently in our evolutionary history. However, the new larynx design required for oral speech also significantly increased our ancestors’ chances of death by choking during ingestion of food and liquids, and of suffering from various aerodigestive tract diseases such as gastroesophageal reflux, among other survival-related problems.

Yet, oral speech evolved because it enhanced overall reproductive success, in part by enabling knowledge communication (Kock, 2009), and also due to sexual selection (Miller, 2000). As Miller put it in his book The Mating Mind, ancestral women could gauge a man’s overall health by his ability to speak intelligently, in addition to other traits, such as testosterone markers.

Most of the sexual selection pressure during human evolution was placed by females on males, not the other way around. Ancestral women were more selective than men about who they had sex with; so are modern women, Sex and the City notwithstanding.

Now let us look at the connection with strict paleo dieting.

Paleo man may have consumed certain types of food to help with his testosterone handicap, increasing his reproductive success. As far as evolution is concerned, this is fine – the genes are selfish, and could not care less about the host (Burt & Trivers, 2006; Dawkins, 1990). The guy will mate, but will not live as long as he would like, past reproductive age. Given this possibility, does eating exactly like paleo man make sense for a 50 year old married male today? That is where too much of a focus on a paleo diet may be a problem.

Of course "paleo man" is really a metaphor. There was no "one" paleo man. There are at least three hominid species in the Paleolithic period that differed significantly from each other: Homo sapiens, Homo erectus, and Homo habilis. If you go back in time a little further, we encounter other hominid species, such Australopithecus afarensis and Australopithecus africanus, who were mostly, if not strictly, vegetarians.

Evolution is very useful as a unifying principle to help us understand what is healthy today and what is not. But it cannot completely replace empirical research on nutrition. Some of that research will undoubtedly uncover nutrition habits that increase longevity and improve health today, even though they were not practiced by our paleo ancestors.

We know that highly refined carbs (e.g., white bread with no fiber) and sugars (e.g., table sugar) are too recent an addition to the human diet for us to have evolved to use them optimally for nutrition. So their association with the metabolic syndrome makes sense, from an evolutionary perspective. But there are very gray areas where paleo nutrition speculations cannot tell us much, and what they tell us may be misleading.

References:

Burt, A. & Trivers, R. (2006). Genes in conflict: The biology of selfish genetic elements. Cambridge, MA: Harvard University Press.

Dawkins, R. (1990). The selfish gene. Oxford, UK: Oxford University Press.

Kock, N. (2009). The evolution of costly traits through selection and the importance of oral speech in e-collaboration. Electronic Markets, 19(4), 221-232.

Miller, G.F. (2000). The mating mind: How sexual choice shaped the evolution of human nature. New York, NY: Doubleday.

Rhodes, G., Chan, J., Zebrowitz, L.A., & Simmons, L.W. (2003). Does sexual dimorphism in human faces signal health? Proceedings of the Royal Society of London: Biology Letters, 270(S1), S93-S95.

Zahavi, A. & Zahavi, A. (1997). The Handicap Principle: A missing piece of Darwin’s puzzle. Oxford, England: Oxford University Press.

10 comments:

Scott W said...

Great post. Thanks for the thoughtful research and time that went into this. I'll be reading it again to absorb it more fully.

Ned Kock said...

Thanks for stopping by Scott. Take care, Ned

Scott W said...

Well, I've come back to read this two or three times now and it is still making me think. Reading the rest of your posts, too...thanks for your work on this. (Not sure I'll be munching smelts or sardines any time soon, but maybe...).

Anyway, I have a thought about the oft-mentioned "highly refined carbs" as mentioned in this post. We hear this a lot and think "white bread" or "white rice" which are recent additions to the human diet. But there are starchy tubers that have been around for eons, and reportedly eaten by man, that would be just as "refined" in their tendency to break down quickly to glucose. So for some populations, refined carbs would not be a recent addition at all and should, hypothetically, not be lumped in with indisputably bad stuff, such as refined vegetable oil.

And starch is starch, by and large, regarding its processing by the human digestive system, as long as it doesn't contain lectins and gluten, etc. So by extrapolation, white rice should be just fine for humans who have previously adapted to eating tubers, even though it is a product of the neolithic revolution and thus fairly recent.

Now, excessive quantities of fructose, also a carb, would be another recent addition to the diet and would meet the "too-recent-to-be-adapted-to" criteria, as evidenced by the impact on the liver and on (I understand) triglyceride levels.

I am tending to think that using the word "carbohydrate" as a catch-all can be too imprecise for discussions of human nutrition. Distinguishing between starch and fructose and fiber would be more precise, just as we have learned to distinguish between different types of fat.

Anyway, my comment kind of lost its way, perhaps, but your thoughts would be appreciated.

Thanks.

Scott W

Ned Kock said...

I agree that we need to differentiate between natural and artificial high carb foods, such as sweet potatoes and white bread. They may lead to a similar elevation in glucose, but there are a number of differences.

Sweet potatoes are packed with nutrients, particularly vitamin A. They are one of the staples of the Okinawan’s diet, and the Okinawans are known for their longevity.

White bread has little more than carbs; mostly empty calories, so to speak.

The same comparison holds for fruits. A peach is actually 90 percent water. It comes packed with nutrients, and some fiber.

Table sugar and soft drinks are basically empty calories.

If one is diabetic, however, that is another story. Most diabetics will have very elevated blood glucose levels after eating sweet potatoes. That is not good for them, so they are better off avoiding sweet potatoes.

Non diabetics do not have to eat as if they were diabetics.

So, the real problem, in my opinion, are highly refined carbs and sugars, whose source are industrialized foods.

Scott W said...

In your early post about experimenting to alter your cholesterol, you said that saturated fat did the trick, as long as it was not consumed in conjunction with refined carbs. Do you mean not consumed in conjunction time-wise, as in "don't eat french fries fried in tallow" but "OK to eat a nice strip of animal fat by itself, wait 4 hours and then eat a plain baked potato?"

Or as in don't consume refined carbs AT ALL when eating saturated fat?

Scott W

Ned Kock said...

Scott:

I didn't work for me until I completely eliminated refined carbs and sugars from the diet for several months. Only then SF worked its magic, for me at least.

I did not remove natural starches from my diet. Potatoes (Irish and sweet) were still in when the HDL went up and the LDL down.

Ken said...

"evolution maximizes reproductive success, not survival"

I disagree that there is always conflict, where the rearing of a child requires a dedicated male provider (ie monogamy) the healthier and longer lived a man the more children he can support to adulthood.

Prenatal testosteronization (low digit ratio) is related to risk of heart attack and number of children fathered, moreover fathering 4 or more children by age 30 increases a man's chances of living to age 100 by 100-200%.
Here.

But European men do not have low digit ratios nor are their faces bages of masculinity Visualization of the shape regression on 2D : 4D ratio . (averaged among both hands) within males.


"[T]he gist of their argument is always the same. Authentic human nature is represented today by indigenous tropical peoples. They are what we were. Therefore, human nature is about polygynous males who devote little time and energy to raising their progeny and a lot to seducing the limited number of females. Women are thus the ones who have been sexually selecting.

In this kind of argument, we means ‘people of non-tropical origin,’ particularly those of European descent. Yet clearly this argument is false. We were not them for a long time. Europeans have an evolutionary history going back some 35,000 years on their continent. And this was when and where they evolved their current physical appearance: the shape of their face, the color of their skin, hair, and eyes; the length and form of their head hair. To understand why Europeans look the way they do, we should understand how their environment of sexual selection differed from that of tropical humans."
Female face shape and sexual selection
" The results of these analyses suggest that selection on females is driving the differentiation in facial features among populations. (Liberton et al., 2009)"

Ned Kock said...

Hi Ken.

Indeed, the evolution of costly traits is intimately related with sexual selection. And these traits seem to be rarer than non-costly traits, which also enhance survival. Still, human evolution researchers believe that many costly traits exist.

Ultimately what evolution favors are traits that maximize reproductive success, but at the gene level. Genes seek to replicate, and sometimes genes that are in conflict with each other will evolve. The book below (link at the end of the post) provides a brilliant discussion on this:

Burt, A. & Trivers, R. (2006). Genes in conflict: The biology of selfish genetic elements. Cambridge, MA: Harvard University Press.

Ken said...

I had a look at some of Trivers' stuff on genes in confict but it is way, way over my head.

I've read that looks and physical size are mainly from the father (IGF2 gene is only fully expressed if it's inherited from father) while personality is mainly from the mother.


Steve Sailor had a post about the different effect a trait has in men or women "Why doesn't evolution get rid of ugly people?"

"any particular gene-based trait may have very different effects on males than in females. Extrapolating to humans (and oversimplifying, sorry) you might imagine that a particular shape of the nose or turn of the chin would look drop-dead hunky on a male, but horsey on a woman; dad got to mate because his looks attracted a female, but the result of their togetherness produced daughters whose pulchritude was less than obvious."

Red Deer Research on the Isle of Rum. Has a bit about a costly trait - antler size .

Also
"Recent work has also revealed that genes that produce fit sons may produce poor quality daughters (Figure 10) [Foerster et al. 2007]. Such sexually antagonistic effects could substantially slow the effects of natural selection, but this is the first time such an effect has been documented in the wild."

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