Three years ago I wrote an article critical of “evolutionary medicine” as it was presented in a new book. Recently a correspondent asked me if I thought another book, Why We Get Sick: The New Science of Darwinian Medicine, by Randolph M. Nesse, MD and George C. Williams, PhD, was a more reasonable approach to the subject. It was published in 1994 and got good reviews from respected scientists like Richard Dawkins (“Buy two copies and give one to your doctor.”) and E.O. Wilson (“bringing the evolutionary vision systematically into one of the last unconquered provinces…”). I was able to obtain a copy through interlibrary loan.
The book was interesting and gave me some things to think about, but it didn’t convince me that “Darwinian medicine” is a new science, that its existence as a separate discipline is justified, or that its unique approach offers any real practical benefits for improving medical care.
Why do we get sick? A simplistic view of evolution holds that it systematically eliminates any factors that decrease fitness for survival. So why does disease persist? Why didn’t we evolve to be “fit” enough to never get sick? Because evolution is not a straightforward process.
Evolution is complicated. There are countless “design flaws” in the human body. For instance, we are subject to choking because of the crossed anatomy of our respiratory and digestive systems. Nesse and Williams point out that it would be more sensible to relocate the nostrils to somewhere on the neck, but that just ain’t gonna happen. Evolution is limited by pre-existing patterns and has to make compromises. Historical accidents result in developments that are far from optimum. A useless or even a harmful gene may be perpetuated because it is linked with a beneficial gene. We are in an arms race with pathogens: we evolve defenses and they evolve ways to overcome those defenses. Natural selection made us fit as small groups of hunter-gatherers on the plains of Africa. We are specifically adapted to Stone Age conditions. We face very different environments today where our evolved traits can be counterproductive.
Who are the fittest? Fitness doesn’t mean fitness for the individual’s welfare, but fitness for propagating the individual’s genes. After the individual has reproduced, diseases of old age don’t affect evolutionary fitness except in minor, indirect ways. (Grandparents past reproductive age can contribute to the survival of descendants by helping with childcare and providing accumulated knowledge and wisdom.)
Nesse and Williams differentiate between proximate and evolutionary explanations: heart attacks are caused by cholesterol deposits in the arteries, but they want to know why evolution shaped us to deposit cholesterol, crave fat, over-eat, etc. I want to know, too; but I’m not so certain that knowing will reduce my chances of a heart attack.
Are evolutionary explanations just pseudoscientific equivalents of Kipling’s Just-So Stories? They argue that they are not. They give the example of morning sickness. It has been hypothesized that the nausea, vomiting, and food aversions of early pregnancy are beneficial because they protect the vulnerable fetus from dietary toxins. This would predict that morning sickness preferentially results in avoidance of foods most likely to harm the fetus. This is a testable prediction and there is some evidence to support it; but there is no way to prove that this is the true explanation or the only one. They suggest that suppressing morning sickness might increase the risk of congenital defects. But there is no evidence for that. They recommend that women “respect their nausea” and remember that it may be beneficial. (It would likely decrease your survival prospects if you said that to your wife while she was throwing up for the umpteenth time!) They admit that relieving suffering is important too, but they recommend that any anti-nausea medicine should be carefully evaluated to make sure it doesn’t cause any harm. Of course, we already do that for all medications used during pregnancy. I fail to see how evolutionary thinking adds anything to the care of pregnant women. In fact, I can see how it might result in unnecessary worry and suffering.
They suggest that sexual reproduction is an advantage because the genetic variations increase survival when a population faces an infection. As supporting evidence, they cite studies showing that asexual reproduction is more frequent in species and habitats with fewer parasites. Maybe. Correlation doesn’t prove causation.
They speculate that schizophrenia must persist in our genomes because it offers some advantage that balances the severe costs. They even have a creative hypothesis about why we sleep.
Most mammals can make their own vitamin C; humans can’t. The authors tell us that when humans switched to a high fruit diet rich in vitamin C, it allowed our biochemical machinery for making it to degenerate. Maybe. If so, nice to know. But so what? We still need exogenous vitamin C.
Even the most plausible evolutionary explanations may or may not be the true story. We can base predictions on them and test those predictions, but that can only provide circumstantial evidence. We can never know for sure whether our Just So Story is true or whether some other accidental factor was responsible.
Should we treat fevers? Fever probably evolved as a defense mechanism: it may do something towards helping fight off the infection. Evolutionary thinking makes us ask why we developed this adaptation and whether it is wise to interfere. But do we need evolutionary thinking for this? Doctors have already questioned the need to lower a fever, recognizing that it is not the fever but the infection that needs to be treated, that fever itself doesn’t do much harm, and that lowering a fever might have adverse effects in some cases. I’ve read many discussions of those points, and nowhere did they mention wondering about why we evolved to have fevers. I don’t see that evolutionary thinking adds anything useful to the discussion. Fever is what it is, and we can study it and deal with it without speculating about how it came to be that way.
Why are wisdom teeth a problem? Modern children frequently need orthodontia and surgery to remove wisdom teeth. They propose a possible explanation: we don’t chew enough. In the Stone Age, food required more jaw exercise. Today softer foods result in deficient use of jaw muscles, which results in poorer development of jawbones so there is less room for all the teeth. They suggest that many dental problems might be avoided if children chewed more gum or engaged in prolonged vigorous biting competitions.
This is nothing but silly, far-fetched speculation. You’ve got to give them credit for imagination; but if this is the kind of thing “Darwinian medicine” produces, we can do without it.
There is a disconnect in their logic. They say
For instance, if we hypothesize that the low iron levels associated with infection are not a cause of the infection but a part of the body’s defenses, we can predict that giving a patient iron may worsen the infection — as indeed it can. Trying to determine the evolutionary origins of disease is much more than a fascinating intellectual pursuit; it is also a vital yet underused tool in our quest to understand, prevent, and treat disease.
We can ask if something acts as a defense. We can ask whether something is a cause or effect. We can ask whether something does more harm or good. We can ask all these questions about how something works without necessarily needing to ask why it evolved to work that way.
Evolutionary thinking is already an integral part of medicine and an essential element of all biology. E. O. Wilson’s description of medicine as “one of the last unconquered provinces” simply is not true. Doctors regularly think about evolution and study its effects. The evolution of drug resistance in bacteria is the best-known example but there are many others. For instance, we think that sickle-cell anemia has persisted because it only affects those who inherit the gene from both parents, while those with only one copy of the gene (heterozygotes) have an increased resistance to malaria. G6PD deficiency causes hemolytic anemia but also offers protection against malaria.
But consider this: malaria is a credible explanation, but we can’t prove that it is the real one. Some other factor that we have not considered might be the true explanation, and malaria resistance might be a coincidence. And the malaria explanation is intellectually satisfying to those who ask “why” but it has had no practical impact on diagnosis or treatment.
They want anyone faced with a problem of medical importance to ask “what is its evolutionary significance?” But they have not demonstrated to my satisfaction that systematically asking that question improves the progress of modern medicine.
So why do we get sick?
- We have genes that make us vulnerable. They may be new mutations, or genes that haven’t been eliminated because their effects occur too late in life to affect reproductive fitness, or genes that have unappreciated benefits that outweigh their costs, or genes that only have harmful effects in novel environments.
- We are exposed to factors that did not exist in the environment in which we evolved.
- Design compromises.
- The arms race with pathogens.
- Unfortunate historical legacies.
They think every discussion of a disease should ask questions based on each of these factors, starting with the question “which manifestations of the disease are actually defenses?” Is the runny nose of a cold a host defense, a means the virus uses to spread itself, or both? That’s a good question, but it can be answered without any need for evolutionary speculations.
They propose special separate funding for studies of evolutionary hypotheses, and they accuse medical science of ignoring evolutionary thinking and even showing an antipathy to it.
I’m sorry, but I just don’t “get it.” Am I missing something? Am I just a contrary curmudgeon? Evolution is already an essential part of all science. Medical scientists already understand evolution and apply its principles appropriately. I didn’t see a single example in their book of any significant practical development in medical care that would not have occurred in the general course of medical science as it is commonly practiced, without any need for a separate discipline of “Darwinian medicine.” Evolutionary explanations, whether true or speculative, may satisfy our wish to understand “why,” but I can’t see that they have much objective usefulness. Instead, they have produced at least one major annoyance: a movement that preaches to us how we ought to revert to the supposed diet of our ancestors (the Cave Man Diet, etc.).
This article was originally published in the Science-Based Medicine blog.