Our huge heads make birth painful. Mother and fetus battle for nutrients. Sometimes evolution settles for ‘good enough.’
(The following appears in Jan 29, 2016 WSJ; it was slightly abbreviated for this post. As a reminder Mr. Taylor will be giving a special lecture at the University of New Mexico on February 2nd – details here)
Jan. 29, 2016 4:14 p.m. ET
Body By Darwin
By Jeremy Taylor
Chicago, 252 pages, $30
Anyone eager for a sound education in evolutionary anatomy along with a heady dose of exciting speculation would be well advised to start with “The Incredible Unlikeliness of Being” and then, if still game, graduate to Jeremy Taylor’s “Body by Darwin.” In contrast to Ms. Roberts, Mr. Taylor is often eager to demonstrate that the human body is for the most part a remarkably well-adapted structure. But he, too, acknowledges that evolution only rarely has the luxury of “designing” something from first principles; nearly always, it must work with history’s bequest.
Our (presumably adaptive) insistence on walking upright, unlike all other mammals, has some unfortunate side effects: “fractured hips, bunions, hernias, fallen arches, torn menisci, shin splints, herniated disks, fractured vertebrae, spondylolysis (damage to a delicate part of the dorsal side of the vertebra caused by over-extending or arching of the back), scoliosis, and kyphosis.” Moreover, the human foot—a perfectly good primate adaptation as a tree-climbing implement—hasn’t had nearly as much time to adapt to walking and especially running as, for example, the feet of ostriches have. They’ve had something like 25 million years. Us? Maybe three million.
Among the provocative questions raised by Mr. Taylor (to which he proposes likely, evolution-based explanations) are: Why are there epidemics of heart disease? Why are our eyes so peculiarly vulnerable to retinitis pigmentosa and wet macular degeneration? If the appendix is a useless fossil, why hasn’t it disappeared, saving many of us from appendicitis? And why do so many of us descend into the mental twilight world of Alzheimer’s disease in later life?
When it comes to explaining some of these chronic health problems today, Mr. Taylor invokes the “hygiene hypothesis”: The human immune system, having evolved in an environment with numerous antigens and allergens, is prone to misfire when—ironically—our civilized environments are too clean. “The world of our ancestors,” we learn, “was a much dirtier place than it is now. Evolution took the expedient route, since microorganisms in prehistory could not be eradicated, of allowing humans to live with them. . . . Evolution could not foresee a world where public hygiene, antibiotics, and chemicals that kill 99.9 percent of all household germs has so depleted this microbial population inside all of us that our immune systems no longer mature properly or are properly regulated.”
In a fascinating chapter titled “Absent Friends,” the author essentially suggests that our pathological obsession with hygiene has left our immune system with too much time on its hands. And those idle “hands” have induced at least some people’s immune system to get into mischief, including a likely proportion of those who suffer from celiac disease, Type 1 diabetes, hay fever, irritable bowel syndrome, even—perhaps—multiple sclerosis and autism. It is entirely likely that many parasitic worms, moreover, were part of our prehistoric “disease-scape” and that we co-evolved with them, unlike, say, the pathogens that cause typhoid, cholera, smallpox, measles and mumps, all of which appear to be relative newcomers to human experience.
Another question concerns the persistence of certain human genes that predispose us, for example, to breast cancer. Why haven’t those particular genes been selected against and thus eliminated? Mr. Taylor’s answer—shared by most evolutionary biologists—involves “antagonistic pleiotropy,” in which evolution may have traded off increased fertility earlier in reproductive life for increased risk of death later on.
“Body by Darwin” can be a heavy slog on occasion, resembling a medical tome more than a trade book. The author evidently felt it advisable to structure each chapter around the story of an identified individual suffering from an example of the phenomenon he is seeking to illuminate. There is nothing wrong with this, except that it becomes tedious long before the book’s halfway mark. Mr. Taylor also embraces some controversial and highly speculative propositions, such as the possibility that beta-amyloid neural plaques (associated with Alzheimer’s and generally thought to be causative) may actually be a potent antimicrobial agent in human brains, with the downside of sometimes causing trouble in later life. Maybe so . . . or not. But probably worth exploring.
In any event, Mr. Taylor does a fine job of raising provocative questions and pointing the reader toward the ways in which evolutionary biology has been enhancing medical science. He interprets, for instance, some of the more tragic problems of pregnancy—the hypertension known as eclampsia—as stemming from the deep evolutionary phenomenon of parent-offspring conflict over metabolic resources, and he discusses the important work on this topic by the Australian-born biologist David Haig. You needn’t be pregnant to appreciate the elegance—and sometimes the awfulness—of the fetal-maternal tug-of-war. This squabbling seems to be a major cause of gestational diabetes, with the fetus trying to get as much blood glucose as possible by secreting a hormone that makes the maternal cells less sensitive to insulin, and the mother seeking—unconsciously, to be sure—to restrain the hungry passenger by secreting more maternal insulin so as to reduce the circulating levels of glucose available to the fetus.
For many people, evolution is the province of fossils and musty museum specimens, perhaps with a dose of DNA. But if you want a tour of evolution as a living, breathing, anatomical reality, just take off your clothes and look in the mirror.
—Mr. Barash is a professor at the University of Washington. His books include the forthcoming “Out of Eden: The Surprising Consequences of Polygamy.”