May 26 2009
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.
Diseases can have good effects along with the bad. The cystic fibrosis gene may protect us from cholera, the sickle cell gene from malaria. Harboring worms affects our immune system in ways that tend to reduce our risk of allergies and Crohn’s disease. In antiparasite campaigns it may be better to reduce the worm burden rather than attempting to eradicate the worms completely.
She discusses recent research on the factors that affect the virulence of infectious agents, the relationship between sex and disease, the way parasites can change an animal’s behavior, how testosterone increases vulnerability to disease, and how even ladybugs and flowers are subject to sexually transmitted diseases. She tells us fascinating tidbits of zoological trivia. Did you know when bedbugs mate, the male pierces the female’s abdomen, making his own hole to deposit the sperm in? Did you know when ground crickets mate, the female chews off a spur on the male’s hind leg and sucks his blood? Did you know vultures use dung for cosmetic purposes?
It seems there are always trade-offs. Zinc and sufficient calorie intake support the kind of T cells that defend against viral infection but reduce resistance to worms; vitamin A boosts worm resistance but decreases cell-entry defenses.
She discusses the evidence for self-medication by animals with an appropriate dose of skepticism. Accounts of apparent animal intelligence fascinate us, but no one comments on the far more common stupid things animals do. She describes tantalizing evidence that our personalities and behavior may be influenced by our diseases. Even schizophrenia has been linked to maternal toxoplasma infection.
I only noticed two things that bothered me. She says that the idea that Helicobacter pylori caused ulcers “took decades to catch on” and was strongly resisted. As Kimball Atwood explained in his article in Skeptical Inquirer, it only took eight years and was readily accepted because the evidence was so strong. Also, she stresses disease as the reason we benefit from the assortment of genes from sexual reproduction. I would argue that having a larger variety of genes also benefits us in many other ways unrelated to disease. Disease may deserve a large chunk of the credit, but not all of it.
Other than those small quibbles, Zuk’s book is enlightening, a good read, and far more credible than that other book. She doesn’t prescribe health guidelines based on Stone Age lifestyles; she just gives us food for thought from an evolutionary perspective with an appropriate dose of skepticism.
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