Principles of Evolutionary Medicine
Since this blog promises to use evolutionary reasoning to look at human movement and health, I thought it would be a good idea to outline some of these principles. This blogpost will make use of the pioneering work of George Williams and Randolph Nesse.
If you would like to read more on evolutionary medicine, make sure to visit Nesse’s homepage, where you can find a wealth of papers on this topic. Also, their classic book ‘Why we get sick: the new science of Darwinian medicine’ is well worth reading. By the way, they very convincingly argue that evolution should be in every medical curriculum. Also check the evolution and medicine review website.
Human animals are products of evolution by natural selection. I do not want to elaborate on what natural selection is, but I’ll give you a very good but short definition by Richard Dawkins: “non-random survival of randomly varying replicators.”
The thing I do want to explain a bit more is why natural selection makes us vulnerable to disease. Darwinian medicine sheds light on why the human organism is the way it is, and why it fails and has diseases. Evolution has produced all these wonderful living things, but it leaves us with infectious disease, cancer, heart disease, chronic low back pain, mood disorders, …
Nesse provides 6 evolutionary reasons:
Evolutionary reasons for vulnerability to disease
Pathogens evolve faster than hosts, and coevolution arms races shape protective defenses that can harm hosts
Mismatch between our bodies and the modern environment
Tradeoffs that have net benefits despite substantial costs
Constraints on what natural selection can shape
Selection does not shaped health and longevity, but maximal reproductive success
Protective responses can seem like diseases, but they are actually useful defences
“Every disease needs an evolutionary explanation that draws on some combination of these. Notice that more than one explanation can apply”
Let’s examine these principles a bit closer.
The first explains why we get infectious diseases. People interested in this just will need to read the original paper, and a lot more, because this principle has less immediate implications for this blog.
Point 2 is probably the most known evolutionary explanation for a lot of current health issues: there is a mismatch between our current way of life and the one evolution made us for. Our lifestyle is out of tune with the ancestral way of life, the hunter-gatherer way. Because this has so much implications, we’ll discuss this point further in an next post. But let’s consider the following plausable example: we have a lot of back pain, because we don’t move enough. Our body is made for a hunter-gatherer lifestyle, not for a sedentary lifestyle.
The third principle is very interesting. “No aspect of the body can be perfect, because making one thing better will make something else worse. […] Making bones thicker will make them less prone to breaking, but at a cost of a heavier, slower body.”
Another often cited example is the trade-off between the width of the human birth-canal and the female pelvis. Human birth is very difficult because the big head of the baby needs to pass the very narrow canal of its mother. Why did evolution not take care of this? Well evolution did make the pelvis of women wider than men, but it stopped at a certain width, probably because to make it more wide would make the pelvis less suited for walking and running. So there’s a trade-off.
(As an aside, please note my use of ‘probably’, because we should be very careful to make up evolutionary stories. Although they can be very logical, they can also be very wrong!)
Could we apply this principle to explain the high prevalence of low back pain? Sure we can, at least, we can think of hypotheses. One trade-off could be between stability and mobility. More mechanical stability could make our spines less prone to injury, but it would leave us stiff and it would compromise a lot of important functions.
Now principle 4 is about constraints on what natural selection can do. Evolution changes things bit by bit, and it has to work with the current model to make the next model. It would be like making a jet airplane out of a propeller airplane by adding or removing one piece at a time and seeing how it works. Very difficult, and not how engineers would do it. Evolution can not go back and start from scratch.
A fine example of how evolution works would be the recurrent laryngeal nerve. In fish this nerve had a direct route to the goal tissue (from the brain past the heart to the gills), but because of evolutionary processes, this nerve still takes the same route, but has to make a detour in humans (from the brain to the heart, under the aortic arch and then to the voice box). It gets extreme if you consider that in giraffes, the nerve has to make a detour of about 5 meter, in some dinosaurs a detour of 28 meter!!!
If we apply this hypothesis to our lower back, we could say that evolution had to work with the more ape like spine of our tree dwelling ancestors, and that could make our spine vulnerable to pain. Again, notice the ‘could’. I hope to address this principle for back pain later on.
The next principle is very important: evolution favours reproduction, and not necessarily health and happiness. Or said in another way: health, happiness and longevity are only important if they increase your reproductive success. Most of the time health is important for reproduction. You could see this as a special form of trade-off. Some traits can be beneficial for reproduction, but bad for health. These traits seem maladaptive only because we are self-centered. We are survival machines for our genes.
Diseases that have a genetic component and that occur only later in life, are less visible to natural selection. You have had the chance to pass them to your offspring before you got sick. Again an example of evolution favouring reproduction.
If I try to apply this to low back pain, we could hypothesise that our backs are made only to work great for a few decades. “The tradeoff between repairing tissues and competing for resources and mates gives advantages to the genes of men who invest relatively more in competitive ability than tissue repair.”
The last point is about diseases that are not diseases. Pain, fever, coughing and anxiety are not diseases, but protective measures that are adaptive to our health. These symptoms are not the cause, but the consequence, and our organism activates these for protection. So do we need to suppress these symptoms with medication?
“A simplistic approach suggests that we should simply respect the wisdom of the body and thus minimise the use of such medications. This is naïve. Doctors need to learn how natural selection shapes the mechanisms that regulate such defences so they can make good decisions for each individual patient. The foundations for such decisions are found in signal detection theory. It offers ways to calculate exactly when expressing a response is worth it. If the response is inexpensive, such as a single panic attack or a bout of coughing, or fever, and the danger is extremely expensive, such as some potential for death or serious injury, it is optimal to express the defence whenever there is even a slight chance that the danger is present (Nesse, 2005). This is called the ‘smoke detector principle’.”
In low back pain, we see a lot of defenses: pain, muscle tension, fear of movement, low mood, fatigue. All these are protective measures that can help our health, especially in an acute injury. But we can have false alarms and overprotections that are maladaptive on the long run. It is not easy to make the distinction between a defense and a defect. The smoke detector principle could help clinicians making decisions, or providing treatment strategies. Stuff to explore later on…
Let’s round it up for this blog: Evolution can teach clinician quite a bit, and provides them a framework to better understand our biology and our health. Darwinian medicine talks about more than the mismatch between current and original environment. It is about constraints, trade-offs, reproductive success and aversive defences.
My understanding of human health has tremendously changed, deepened, broadened, and improved because of these evolutionary principles. I hope it will make you think too.
All quotations (unless mentioned differently) are from the following papers:
Evolution, a basic science for medicine (Randolph Nesse) in Pragmatic Evolution: Applications of Evolutionary Theory, ed. Aldo Poiani. Published by Cambridge University Press 2012.
Evolution, medicine’s most basic science (Randolph M. Nesse & Richard Dawkins) in Oxford Textbook of Medicine, 5th edition (pp. 12-15). D. A. Warrell, T. M. Cox, J. D. Firth & E. J. J. Benz (Eds.), Oxford: Oxford University Press 2010.