In everyday life, evolution explains phenomena that nothing else can—and not just in biological specialties. It is the master key to much that has been locked in mystery. If you seek real understanding in almost any field, you must not reject evolutionary theory. It has never been my bread and butter, as I practiced medicine for some sixty years. But I can cite example after example in which the understanding of evolutionary biology has enhanced my education, not only in medical school but also throughout my life. I hope that these details of my personal “curriculum” in evolutionary biology will help all students, including lifelong learners like myself.
How I Became an Evolutionary Biologist at an Early Age
In my sophomore year in high school, a friend told me what an evil man Darwin was, after which I made a thorough study of The Origin of Species. Don’t give me too much credit—Darwin’s book is unique in being the only groundbreaking scientific treatise that was written for the general public—but reading The Origin of Species made me an evolutionary biologist almost overnight.
I began surmising about the biological basis for characteristics of humans and other animals, both physical and behavioral. (“Biological basis” refers to the survival value of an anatomical structure, ability, or behavior, and its likelihood of undergoing natural selection and inheritance.) I lead off with two simple examples of evolutionary thinking from medicine (and personal experience).
- Adaptation to darkness. Why does it take thirty minutes for human eyes to get used to the dark? This happens to be the duration of tropical twilight, the most rapid onset of darkness that occurs in nature. Prior to artificial lighting, adaptation in less than thirty minutes had no appreciable survival value. (Solar eclipses are too infrequent to present selection pressure.)
- Acclimatization to altitude. Why does it take several days for the mammalian respiratory system to acclimatize to high altitude? The process takes about as long as it takes for a mammal to reach a high altitude on its own power, such as by walking up a mountain. (Birds, with their ability to reach high altitudes rapidly, have a more powerful adaptation: a countercurrent blood gas exchange system in their lungs.)
Now for some more complicated examples of evolution’s explanatory power.
- Lack of sweetness in lactose (milk sugar). If milk contained as much of any other ordinary sugar as it does lactose, it would be almost as sweet as watermelon. All mammals create lactose in their milk glands as the result of sophisticated biochemical manipulations. Lactose also requires special enzymes for digestion in all mammalian young. This biochemical complexity would require some selective advantage in order to have occurred at all. My take: it would be harder to wean the young from sweet milk at the appropriate time, resulting in fewer offspring per such mother during her reproductive life—a substantial selective advantage for mothers producing milk containing lactose instead of a sweeter sugar. Perhaps sweeter milk in early mammalian progenitors lacking lactose chemistry resulted in the entire lineage having a sweet tooth throughout life, to the detriment of adult nutrition. The evolution of the lactose biochemistry thus may have resulted in return to a more varied and nutritious diet. The innovation is as old as the mammalian class of vertebrates itself: the duck-billed platypus, the most primitive living mammal, has lactose as the carbohydrate in its milk. This kind of thinking may help future physicians steer our communities toward appropriate policies to cooperate with nature’s long-term effort to limit the damages of the desire for sweets.
- Whale communication. In his remarkable book, Among Whales, Roger Payne describes how much money has been spent by whaling countries in attempts to discover the breeding grounds of blue whales, so far without success. There is probably no breeding ground as such, since the blue whale’s auditory and vocal equipment permits communication across ocean basins. Still, this equipment is no more powerful—or sensitive—than it needs to be in order to function across the world’s largest basin, the Pacific. Thus, the whales have no problem locating one another and do not need to be near each other to locate mating partners. Note that there was no biological selection for this kind of fitness beyond what the whales need.
- Sickle-cell anemia. This disease, common in people of African lineage, shows how even seemingly deleterious characteristics can have survival value under special circumstances. Homozygotes—people who inherit the sickle-cell gene from both parents—die of the disease before reaching reproductive age. Heterozygotes, who have one gene from an unaffected parent, enjoy a pronounced survival advantage: resistance to malaria. In malarious regions of Africa, mathematicians tell us that the observed incidence of the abnormal gene is just enough to maximize the benefit to the population as a whole.
- Homosexuality. This may be another instance where a trait that seems to disadvantage the individual’s capacity to pass genes on to offspring may enhance the survival of the population as a whole. You may well ask, “How can evolution select for a trait that does not reproduce?” The mechanism involved is trickier even than that which accounts for the prevalence of sickle-cell anemia. I puzzled about it for decades, until I stumbled on an article about comparative primatology that described homosexuality in a number of primate species. Its thesis was that homosexuality motivated nondominant males to remain with the troop, strengthening the group and enhancing the survival of the individual’s other relatives (siblings, nieces, and nephews) who will pass on that male’s genetic attributes. I am sure that these extra males also help with tribal territorial warfare—a prehuman precedent for gays in the military, if you will. The case of homosexuality furnishes a salient example of the power of evolutionary biology in shaping our interpretations. When the American Psychiatric Association decided some years ago that homosexuality would no longer be defined as a psychiatric illness, instead of being upset or surprised, I promptly gave them credit for being ahead of the curve of new knowledge from Jane Goodall and her ilk.
- Obesity. A somewhat modern insight into the biological background of obesity comes from the study of the Pima Indians of the southwestern United States. The Pimas have the highest incidence of obesity of any ethnic group worldwide. They descended from ancient inhabitants of very arid canyons of the four corners area, such as Mesa Verde in Colorado and Grand Gulch in Utah. Their ancestors had survived drought and famine in endless sequence, so they had been preferentially selected for the ability to gain weight promptly whenever food was plentiful, thus increasing their chance of survival in the next famine. We are all a little like that, though to a lesser degree. If we understand how we got the way we are, there may be opportunities to adjust our behavior to blunt undesirable consequences—another example of the benefit of scientific literacy, including knowledge of evolution.
- Megavitamins. In light of the current enthusiasm for megadoses of vitamins, consider that we humans are among very few animal species that do not synthesize vitamin C in the body. We synthesize many other vital substances, such as lecithin—lecithin deficiency has never been observed in humans—but not vitamin C. What conclusions can we draw from this?
First, we are descended from organisms that did once make their own vitamin C. Vitamin C synthesis is a capacity that was abandoned in the course of human evolution. We still have all but one of the enzymes needed to synthesize vitamin C. So the foods available to our ancestors in their primeval environment must have provided ample vitamin C, such that when our early or prehuman ancestors started to lose the ability to create it, that posed no great handicap. The same is true for other essential substances: those that were available in profusion did not need to be synthesized, while if others were present in suboptimal quantities, there would have been substantial evolutionary pressure enhancing the ability to conserve these nutrients. It follows from this that a varied diet is very likely to contain adequate amounts of all nutrients, assuming that total calories are also sufficient. A corollary to all this is that hunter-gatherers should be less subject to malnutrition than settled agriculturalists because of their ability to move on quickly in case of a local food shortage. A case in point: the recommended supplementary dose of folic acid during pregnancy to prevent birth defects approximates the increased amount of folic acid most likely to have been present in a typical paleolithic diet—roots, nuts, and vegetables instead of the cereal in a modern diet. Concepts like these are baffling in the absence of evolutionary thinking.
- Recognition of friend and foe. The instant recognition of faces is a hardwired ability that confers obvious survival value in terms of recognizing friend or foe. This ability has been well studied by students of animal behavior. I was especially intrigued by recent research on face recognition using sheep as the study subjects. Twenty sheep were trained to recognize pictures of sheep or human faces that were placed at junctions in a maze with food rewards for correct choices. The sheep were very adept at recognition. If sheep can do it, who can’t? The evolutionary implication is that humans and sheep must have had a common ancestor some tens of millions of years ago already in possession of this brain circuitry. Voice recognition is also part of recognition of friend or foe and may be the underlying ability that enables us to appreciate music—something that puzzled Darwin himself: “As neither the enjoyment nor the capacity of producing musical notes are faculties of the least use to man . . . they must be ranked amongst the most mysterious with which he is endowed.”
- Addiction. Tolerance is a mechanism permitting consumption of plants with toxic effects, and it causes drug addicts to be able to survive, even thrive, on otherwise fatal doses of the drug to which they are addicted. A corollary to this is the idea that addiction has persisted by being tied incidentally but inextricably to tolerance. Thus, seemingly harmful traits can survive natural selection if they are associated with a trait of great benefit to the species. Could drug addiction be such a harmful trait unavoidably connected with tolerance, and could its survival value lie in permitting greater consumption of a “dangerous” food? Could addiction be a vestige of an inheritance from some ancient, long-extinct ancestor who actually benefited from the trait, most likely a common ancestor of insects and vertebrates? There are modern insects whose larvae become “addicted” to toxic plants, rendering the entire species toxic to predators. An example is the common monarch butterfly, which eats poisonous milkweed and thereby avoids predation. While this strategy of becoming tolerant to a poison was evolving, addiction may have become part of the package because that assured an adequate, steady intake of the poison—stragglers who failed to become addicted quickly enough became prey and ceased to be part of the lineage. Countless other such fascinating discoveries await us especially, in the tropics. For example, butterflies of the genus Parides in Central America, as larvae, eat Aristoloquia leaves whose toxicity is similar to that of milkweed.So the following scenario may partially explain the origin of addiction. Unlike the Parides larvae, which hatch on a poisonous Aristolochia leaf, our aquatic ancestor (an acorn worm)* which “invented” addiction would have had to keep seeking the poisonous food that rendered its lineage poisonous and therefore less subject to predation. Addiction served this purpose by overcoming the temptation posed by ample alternative food choices. In our case, the ability to become addicted just happens to have persisted after its benefit became moot. Compare that with our appendix, which also persists with negative benefit. More toxic animals and plants inhabit the marine environment than our terrestrial one. Undoubtedly some of the toxic species utilize secondhand toxins much as Parides larvae do. If we can find a poisonous marine vertebrate that achieves toxicity through its diet, this may begin to confirm the hypothesis of an evolutionary origin of addiction, especially if that species bears live young so that they would be born addicted, as are human infants born to mothers currently addicted to heroin. Experiments with a captive population of such animals should easily demonstrate tolerance or even tachyphylaxis, the ability of former addicts to become readdicted extraordinarily promptly. It is a universal feature of the addiction syndrome (along with tolerance and drug-seeking behavior). Once again, evolution has explained the well-nigh inexplicable.
My personal experience with caffeine addiction is a good explanation of tachyphylaxis—significant because it was so extreme though not disabling or otherwise harmful, aside from occasional withdrawal headaches. I was never a regular user of caffeine before I lived in Afghanistan from 1968 to 1970. While there, I drank tea very frequently. By that means I could restrict my fluid intake to water that had been boiled without insulting the locals—as might happen if I insisted on boiling my drinking water. It takes much tea to fill one’s water requirement in a tropical environment—so much that the dose of its active ingredients might exceed toxic levels for a naïve user. So I cultivated a tea addiction, accustoming my system to a high consumption level. As I actually dislike tea, it was easy to wean myself from it in about a week, unless I was expecting to travel. When I traveled during my sojourn in Afghanistan, I would drink copious amounts of tea. Tachyphylaxis restored my addiction sufficiently promptly to avoid any toxic symptoms, emphasizing to me how tachyphylaxis got into the addiction package along with a withdrawal syndrome and tolerance.
My oddest personal insight into tachyphylaxis came after a subsequent decade during which I consumed almost no caffeine. One week I broke my years of abstinence with one cup of midday coffee three days in a row. Amazingly, four or five hours after the first “missed dose,” I developed a headache. My addiction, dormant for years, had been rekindled by a minuscule exposure to caffeine; now the headache was warning me that I had “missed a dose.” I have had no experience with patients experiencing such headaches if they drank less than five or six cups of coffee or tea per day. Now I drink no more than a cup or two of real coffee a month.
What little is currently known about the biochemistry of addiction is merely a discussion of the possible interactions of neurotransmitters with the mesolimbic dopamine system, where pleasure is produced, and how some of the addicting substances produced by plants are similar to (and imitate) various neurotransmitters. The literature seems to contain very little about how addiction could enhance survival. The bottom line: regard addiction as similar to having an appendix and don’t bother with the cure until threatened by some inconvenience.
When I started to write this article several years ago, my goal was to defend the teaching of modern biology and to demonstrate the power of evolutionary thinking in understanding and explaining many important and interesting phenomena. Since then, I have gradually received numerous reinforcing insights that suggest that evolution is a pillar contributing to mankind’s intellectual and cultural history. We can hope that further study will result in still deeper insights regarding the connections between evolution and social and political spheres. It took four centuries for Newton’s ideas to lead to space travel. We will need to digest Darwin more quickly if we are to achieve hoped-for benefits in time to avoid disasters like climate change and overpopulation.
In short, the proper and widespread study of evolution is a vital and integral part of a liberal education. What field is better positioned to lead the way in this endeavor than medicine? Let us do it for ourselves and for the world. I am reminded of reading in my youth a book about the Western frontier. The author stated that the frontier doctor was more likely than any other citizen to be forgiven for believing in evolution. I sense that this may still be somewhat true, in which case we are “ex officio” leaders in the effort to teach biology.