Sunday, April 29, 2007

Let Us Now Praise The Evolution of Disease: When Survival of the Sickest first arrived in the mail, I had trouble finding it. I would find myself with spare time to read in the evenings but couldn't locate the book. It had been confiscated first by my 10 year old, then by my 12 year old. The book, written by Sharon Moalem a physiologyt/neurogenetics/evolutionary medicine doctorate with the aid of former Clinton speechwriter Jonathon Prince, is at a level easily understood by the general population - despite it's subject. (Perhaps that's why a former political speechwriter had a hand in its authorship.) When I asked the kids to describe the theme of the book, both characterized it as how diseases have helped people survive. And so it is, but it's also much more than that.

We've long known about diseases that seem to confer an advantage against certain other diseases. G6PD deficiency, thalassemia, and sickle cell disease (or at least trait) all confer an advantage over malaria. But what abour hemochromatosis and the plague? Or cystic fibrosis and tuberculosis? How about juvenile diabetes and the Ice Age? Moalem makes persuasive arguments for each.

But there's another aspect of evolution that often goes overlooked in discussions of disease - and that's the role we have playd in the evolution of other organisms. Sometimes the role is benign and mutually beneifical, such as our relationship to the bacteria that inhabit our guts, or the possible viruses that have taken up residence in our genome and facilitated our evolution. But sometimes, the results aren't so beneficial. Cholera causes diarrhea, which in turn flushes the bacteria back into the world where it can find more human hosts. Rabies makes animals aggressive and agitated - more likely to bite- and thus more likely to spread the virus to other mammals. It is in this context that he introduces the interesting theory of evolutionary biologist Paul Ewald. Perhaps, instead of focusing on eradicating disease the traditional way - killing the bacteria or virus with drugs, or preventing it with immunizations - we should consider using evolution to our advantage. Clean water and good sanitation make the spread of cholera more difficult. From an evolutionary perspective, that should put pressure on the organism to evolve into a less virulent form. Causing bad diarrhea no longer gives it a survival advantage in that situation. It's an interesting idea, and one worth considering - espeically since so many of our disease vectors have proven adept at evolving around our treatments.

Moalem and Prince have written an engaging and accessible review of the current theories of the relatively new field of evolutionary medicine. It's to their credit that they've managed to make an often dry and turgid topic understandable to even a 10 year old.

 

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