A couple of days ago I read a scientific paper by Harvard Professor Daniel E. Lieberman entitled Is Exercise Really Medicine? An Evolutionary Perspective (1). I’ve been a fan of Daniel Lieberman ever since I first read his book The Story of the Human Body: Evolution, Health, and Disease (2), which was published in 2013. I don’t agree with everything he says though. This became increasingly clear to me as I was reading the aforementioned article, in which he makes several statements that I take issue with. There’s one thing in particular I don’t like about his ideology and work: I think he severely underestimates the power of the Paleo diet/lifestyle concept and fails to fully recognize that organismal health plays a critical role in Darwinian evolution
He’s not alone in this regard. I’ve noticed that certain other well-respected evolutionary scientists also seem to be eager to point out in their scientific papers, books, and presentations that the only thing that “matters” in Darwinian evolution is how many reproducing offsprings organisms get, not their health or longevity, and that it’s therefore wrong to assume that one particular lifestyle or environment (e.g., the Paleolithic one) is optimal for Homo sapiens (or any other species for that matter) with respects to health promotion
I don’t fully agree with these assertions. In today’s article I thought I’d briefly explain why…
Healthy organisms are typically more reproductively fit than unhealthy ones
It’s certainly true that variation in reproductive success is ultimately what keeps the process of Darwinian evolution running. What’s important to note though is that organismal health and organismal fitness aren’t unrelated variables. Far from it. The health of an organism may greatly affect its reproductive success. One doesn’t have to possess intimate knowledge about evolutionary biology to understand why this is the case; all that’s required is a basic understanding of how nature works.
An animal that lives in a natural environment and is fit and free of chronic disease obviously has a better chance of surviving and reproducing than a similar animal that is weak, fragile, or sick. This is true regardless of whether the creatures in question are bears, elk, spiders, humans, or any other animals. An animal that is obese, metabolically deranged, or chronically fatigued or has poor eyesight or weak bones most likely won’t make it very long in nature.
Not only that, but the health status of an organism ties in with its libido, sexual attractiveness, and fecundity. This is something that has become increasingly clear to me over the years. People who are chronically inflamed, sick, and/or unfit are not as sexually robust as healthy folks. Not only does chronic inflammation/ill-health impair libido and sexual function, but it also undermines reproductive function (e.g., increases risk of miscarriage and other pregnancy complications) (3, 4, 5, 6, 7).
With this in mind, one can quickly understand why animals that live in the wild, including human hunter-gatherers, are lean, fit, and largely free of chronic disease.
The evolutionary validity of the Paleo diet/lifestyle concept
Unfortunately though, some people, including some scientists, seem largely oblivious to the things I talk about in the previous section. They focus all of their attention on the primary “currency” of evolution – reproductive success – and fail to fully recognize that the value and distribution of another currency – health – greatly affect the value and distribution of the former.
This is concerning, because in order to make sense of what type of diet and lifestyle that is optimal for different organisms with respects to health outcomes, we have to shed light on the health variables that affect organisms’ ability to pass on their genes. It’s also important that we acknowledge that the evolutionary pressures that act on organisms that live in the wild differ in strength, intensity, and nature from those that act on organisms that live in a manufactured environment (e.g., industrialized humans, domesticated animals).
Unlike Paleolithic hunter-gatherers, we – contemporary humans – don’t have to hunt and gather in order to get a hold of food, keep watch for dangerous animals, or bring children into life without medical assistance. Moreover, unlike our ancient forebears, we have access to drugs, eyesight-correcting devices, and modern technology. Hence, not all of the selection pressures that acted on humans in the past act on contemporary human populations, at least not to the same extent. This is very important to acknowledge, because it helps explain why we’re today in the midst of a chronic disease pandemic. Moreover, it implies that we can’t expect natural selection to eliminate disorders such as obesity, myopia, and type-1 diabetes anytime soon. Finally, perhaps most importantly, it helps us make sense of the question of how we should live our lives in order to attain the best health possible.
The primary reasons why the hunter-gatherer lifestyle is so healthful are that natural selection had ample time to adapt our ancestors to their hunter-gatherer niche and that human health and human fitness have been firmly linked throughout most of our evolution. Our ancestors had to compete with other organisms for nutritional resources and “fight to survive”. They didn’t live in comfortably heated, safe homes or have unlimited access to calorie-dense foods. In order to survive and reproduce, they had to be physically fit and healthy enough to get a hold of food, ward of infections, and otherwise survive in the physically demanding environment they found themselves in.
The bottom line
The point I’m trying to make with this article is not that health always tracks closely with reproductive success in the context of Darwinian evolution or that organismal health is the main focus of natural selection. It isn’t. The spotlight of natural selection is obviously directed on organismal fitness. Natural selection will favor traits that are beneficial fitness wise even if they are detrimental health wise. With that said, most of the time (not always), health and fitness go hand in hand. This is particularly true when talking about organisms that live in a natural environment; however, it also to some extent applies to organisms that live in manufactured ones (e.g., domesticated animals, industrialized humans), in part because health and inflammation tie in with libido, sexual attractiveness, and sexual and reproductive functions.
The recognition that we evolved to live in a natural environment, in which one has to be fairly healthy and fit in order to survive and reproduce, is extremely powerful, in the sense that it infers that chronic illness is not an unavoidable part of human life and that we can steer clear of many diseases and health problems by adjusting our environment so that it better matches our evolved biology.