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.


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.