In the past I have criticized evolutionary medicine for its tendency to rely on unverifiable “Just-So Stories,” but a new book has helped me appreciate what the best kind of evolutionary thinking can contribute to our understanding of medicine. Doves, Diplomats, and Diabetes: A Darwinian Interpretation of Type 2 Diabetes and Related Disorders by Milind Watve investigates diabetes from an evolutionary perspective, suggesting how it might have originated, why it persisted, and how it is related to survival advantages. Watve develops well-reasoned hypotheses that can be tested by examining their expected consequences. He believes it is impossible to understand metabolism without understanding behavioral ecology, and he makes a good case.
A reassessment of the evidence concerning Type II diabetes (T2D) reveals a number of paradoxes. Elevated blood glucose is the defining feature of T2D but controlling it doesn’t prevent all the complications of diabetes, and it doesn’t appear that elevated blood sugar could produce all the pathological changes of diabetes. Insulin resistance is believed to be central to a cluster of deadly diseases in humans, but in other animals it has no adverse effects on health and even increases lifespan. Studying diabetes from an evolutionary perspective can shed light on such paradoxes. (more…)
Whipworms in the intestine. Click to enlarge.
Humans evolved in an environment where they were exposed to animals, dirt, and a variety of pathogens and parasites. Our immune systems evolved to cope with that environment. Now most of us live in a different environment, with safe drinking water, flush toilets, food inspection, immunizations, and public sanitation. This means that we are far less likely than our ancestors to die of infectious diseases or to harbor intestinal worms. But it seems that the cleaner we get, the more likely we are to suffer from allergies and autoimmune diseases. One hypothesis is that our immune systems evolved to require early challenges by parasites and pathogens in order to develop properly. A hygienic environment fails to give our immune system the exercise it needs, resulting in imbalances and malfunctions.
The hygiene hypothesis was first proposed to explain observations like these:
- Hay fever and allergies were less common in large families where children were presumably exposed to more infections through their siblings.
- Polio attack rates were higher in high socioeconomic groups than in lower ones.
- Allergies and many other diseases were less common in the developing world.
Investigation of these and other phenomena is contributing to a better understanding of the immune system, which is a good thing. At the same time, it has led some people to deliberately infect themselves with intestinal worms in an attempt to cure their allergies and autoimmune diseases, which may not be such a good thing. These treatments are far from ready for prime time, are risky, and they have a high yuck factor. The very idea of deliberately infecting yourself with worms is unpalatable, and finding wiggly live creatures in your stool or passing a 20 foot tapeworm are not generally considered to be pleasant experiences. (more…)
3 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.
Last month I wrote about a book on evolutionary medicine that I could not recommend. Now I’ve found one I can recommend. Marlene Zuk, an evolutionary biologist at the University of California, Riverside, has written a delightful book entitled Riddled with Life: Friendly Worms, Ladybug Sex, and the Parasites That Make Us Who We Are. She describes how our parasites and diseases co-evolved with us: as we developed better defenses, they developed better weapons. “We do not choose to have them, but our lives are unimaginable without them, and for better or worse, they have made us who we are.” Parasites have altered our bodies in ways that science fiction filmmakers could never have imagined. She even suggests that we can thank parasites for the fact that we reproduce sexually rather than asexually.
She advocates an evolutionary medicine that “places diseases and defects in an evolutionary framework to make sense of the apparent mismatch between the way our bodies often work and the way we would like them to.” Evolution did not design our bodies for health, but to maximize reproduction.
“Just because our species evolved in a different environment does not mean …that following the ways of the past is automatically going to free us from the illnesses of modern life.” She incisively debunks the myth of the Paleolithic diet and points out that if we wanted to copy our hunter-gatherer ancestors we would have no way of deciding which ones to copy – the ones from 10,000 years ago or the ones from 100,000 years ago; the Inuits or the Kalahari Bushmen. She also points out that evolution explains why humans vary and why one diet won’t suit everyone. (more…)
I have been asked to review a pre-publication proof of a book that will be published in May 2009: Evolution Rx: A Physician’s Guide to Harnessing Our Innate Capacity for Health and Healing by William Meller, MD. It offers “a primal yet radical new view of why we act and feel the way we do, why we get sick and how we heal. This new perspective, known as evolutionary medicine, looks at how our Stone Age ancestors lived, loved, got sick and got well over millions of years, which leads to guidelines for living longer healthier and happier lives today.”
He says we are the way we are because that’s what it took to adapt and survive throughout our evolutionary history. To some extent, that’s true, but that’s not the whole story. Sometimes we are the way we are because of an accident of evolutionary history that had no bearing on survival. Sometimes we are the way we are because a useless trait was linked to a useful one and came along for the ride – what Stephen Jay Gould referred to as “spandrels.”
The problem with evolutionary explanations is that we can never know for sure if they are true. We may be inventing “Just So Stories” like Rudyard Kipling’s “How the Camel Got His Hump.” Our explanation may seem perfectly reasonable but we may not have all the information and there may be a better explanation that simply doesn’t occur to us. (more…)