Evolution And Women
We may live in both the best of times and the worst of times, although Charles Dickens would beg to differ. For those of us who have the fortune to live in health-rich nations like the United States, Canada, Japan, and those of Western Europe, we enjoy a level of health that was unimaginable when Dickens was writing. ‘‘Seventy is the new fifty’’ and not only are we living longer, we are doing so in a state of fairly good health. But for people in health-poor nations, the scenes that Dickens described of London and Paris in the 18th century would be all too familiar. Globalization, which seems to improve standards of living for some people, has had profoundly negative effects in many parts of the world. Political and social scientists tackle some of the big issues that arise from increasing health and economic inequality, and even in the health-rich nations we know we can do better to improve overall health and quality of life. Scientists who study evolutionary medicine attempt to determine, by better understanding evolutionary processes, how some of the health problems we face today arose. This approach will not solve all of our entrenched and deep-rooted health challenges, but it does offer a new, more holistic way of viewing individual and population health from both research and practice standpoints. One common refrain of evolutionary medicine is that our biological selves are not well matched with our contemporary lives, and the result of this ‘‘discordance’’ is poor health, especially with regard to chronic and degenerative diseases and disorders such as type 2 diabetes, cardiovascular disease, cancers, and hypertension. If we could return to the lifestyles of the Stone Age, the argument runs, we would quickly become healthier. Certainly there are practices from earlier times that would probably improve our health, most notably dietary and exercise modifications, but with more than 6 billion people on the planet, the reality of ‘‘returning’’ to prehistoric ways of living is out of reach for the vast majority of us. Furthermore, one of the most obvious mismatches between our evolved bodies and today’s environments results from newly created biochemical that are in our foods, water, clothing, furniture, and the air we breathe and for which our bodies have not evolved adaptions. There is not much of an evolutionary take on this except to state the obvious (that is, all of these chemicals are products of modern technology), and there is little chance that we will be able to return to the Stone Age as regards several decades of ‘‘better living through chemistry.’’The best that evolutionary medicine can offer is new ways of thinking about old diseases and disorders, particularly those for which medical research has been unable to find a cure or other highly successful treatments. Evolutionary medicine offers a broader and more inclusive approach to medicine and health and urges us to ask new questions about both the immediate and developmental causes and explanations of poor health. Most medical research is highly specialized and focuses on fairly narrow windows into diseases and disorders. This is understandable and necessary because of the continuing expansion of knowledge about disease processes at the molecular, cellular, and organ system levels. Social and behavioural scientists urge concern for behaviour, psychology, and the socio-political environments when considering not only causes of diseases but treatments as well. Evolutionary medicine steps back further and takes into consideration the entire species and its evolutionary history.
So what does evolution have to do with women’s health?
Plain and simple, evolution is about reproduction. Whether we reproduce or not, our evolutionary history and our current biology were shaped by natural selection operating to increase reproductive success. This means that almost anything women have to say about women’s health from an evolutionary perspective will involve reproduction in one way or another. On the other hand, reproduction and virtually every other aspect of human life take place in dense social and cultural contexts that also shape and define them. For example, menarche is not just the first instance of menstruation; in most cultures of the world, it comes with new social roles and status for girls who from that point on may be eligible for marriage, be required to adopt new clothing or hair styles, or be seen as women rather than girls. If we talk only about the biological aspects of menarche, we will lose track of what it means to be human. This is no less true for other aspects of reproduction and women’s biology. As noted, one of the common approaches of evolutionary medicine argues that contemporary health problems to some extent result from a mismatch between our evolved bodies and our present lifestyles (culture). What is usually meant by ‘‘evolved bodies’’ is the physical human form that resulted from millions of years of evolutionary processes at work on our ancestors from the origin of the primate order (approximately 60 million years ago) to the origin of food production (agriculture) approximately 10,000 years ago. This is not to imply that evolutionary processes stopped 10,000 years ago, but it does highlight the extraordinarily short time during which the pace of cultural evolution has far outstripped the pace of biological evolution. In this view, 10,000 years ago, a mere blip in the time span of evolution, marked the beginning of a veritable explosion of culture and technology. For most of the span of human existence, individuals were born into and lived their lives in pretty much the same environment. Furthermore, they experienced life in environments not very different from those of their recent ancestors. Thus, it was not necessary for them to respond to significant environmental changes within one or several lifetimes. Today, however, with mass migrations, major environmental challenges, and unavoidable socio-political changes occurring within single lifetimes, many humans are stressed to their limits (and often beyond) to adapt physically, emotionally, and materially, and to do so in ways that often result in considerable challenges to health, especially reproductive health. Not only are we facing a ‘‘third epidemiological transition’’ with the re-emergence of infectious disease but women are also facing an epidemiological collision as the health challenges of the past (high infant mortality, malnutrition, infectious disease) meet those of the present (obesity, cardiovascular diseases, cancer) in ways that will severely test our abilities to respond.
In fact, even though our ancestors may not have had to face major environmental changes in their lifetimes, natural selection has favoured in the human species a level of adaptability (also known as plasticity) that is unusual in the mammalian world. For example, although we are omnivores (that is, we can eat all kinds of things and lack a specialized digestive system), we can also adapt to diets very high in animal foods (such as a traditional Arctic Inuit diet) or become exclusively vegetarian, depending on habitat and habits. Of course, there are limits to our adaptability as would be quickly seen for someone whose chosen or imposed diet excluded essential nutrients like ascorbic acid (vitamin C) or vitamin B-12. Perhaps few aspects of human life have changed as much as diet, and few aspects have as great an impact on health as diet. And as we will see, reproduction has a lot to do with food. The context of human lives changed considerably with the beginnings of food production (agriculture and animal domestication) less than 10,000 years ago. Human reproductive biology was shaped during the long period from the origin of the human lineage about 5 to 7 million years ago until the present, but almost all the time during which natural selection operated on reproduction occurred before the advent of agriculture. This period is often referred to as the environment of evolutionary adaptation or EEA, and when we discuss the concept of mismatched bodies and lives, we usually mean the body that was shaped during that period. For example, studies of our closest living relatives—chimpanzees, gorillas, and orang-utans—reveal that females in captivity reach puberty earlier, give birth first at younger ages, and have lower birth intervals than those in the wild. In most cases, captive great apes have more reliable and more abundant food and they are less active, mirroring in some ways the changes that occurred when humans began producing their own food and living in settled communities. (Perhaps it can be said that captive apes and food-producing humans have become ‘‘domesticated,’’ and that they experienced changes in reproductive biology similar to those experienced by other domesticated animals.) Thus, to the extent that there are similarities in the reproductive physiology of apes and humans, comparative research on wild and captive populations can provide insights into ancestral human biology and behaviour and the changes that occurred with domestication. Of course, chimpanzees and gorillas are not ancestral humans. Neither are the foraging populations of Africa and South America that have often been used as ‘‘proxies’’ for our ancestors because their ways of living are presumably somewhat like those of our ancestors, at least in comparison to the lives of 21st-century men and women in industrialized nations. But these are only rough proxies and much of the discussion of ‘‘ancestral bodies’’ must remain at the level of conjecture, albeit conjecture based on pretty good reasoning. Often when one uses the term conjecture to refer to an idea, it has a derogatory connotation that suggests that the idea is just somebody’s wild, unfounded notion. For those of us who provide evolutionary explanations, however, it is an admission that we can never know for certain that we are right, we can never ‘‘prove’’ anything about human thought and behaviour that occurred far in the past. (Actually, proof is never a goal of science; support or refutation of a hypothesis is the most one can strive for because the scientific method requires that every explanation be open to rejection with new data or new interpretations of existing data.) But that does not mean that we are not good scientists. As with any scientific explanation, we put forth a possible scenario and it stands or falls based on how well it explains what it proposes to explain. New fossil evidence, new observations about nonhuman primates, new data based on comparative DNA analysis, even new ‘‘wild ideas’’ can alter or outright reject any scenario. This is how science works and how knowledge proceeds, including aspects of science that explore ancestral biology and behaviours.
Defined simply, evolutionary or Darwinian medicine is the application of principles of evolutionary theory to human health and disease (and, we hope, to medical practice and research). Although most recent discussions of the field trace its origin to works in the 1990s by evolutionary biologists Randy Nesse and George Williams, the usefulness of an evolutionary lens for viewing human health has been recognized for at least a hundred years. Unfortunately, that does not mean that an evolutionary perspective is currently being embraced by medical practice, which would be a key measure of its usefulness. Nesse and Stephen Stearns, another evolutionary biologist, cite a number of reasons that evolutionary theory is not incorporated into medical research and practice, including the scarcity of evolutionary biologists on medical school faculties, the very little exposure that medical students get to evolutionary thinking, and the fact that many medical students (in the United States, at least) do not accept evolution as the basis of biological sciences. Given the complexity and burden of the current medical curriculum, it is unlikely that courses in evolutionary medicine will be added any time soon. This is unfortunate, because it actually might lighten their burden if students were armed with understanding of theories rather than the endless details that are deemed necessary to pass medical boards. As Nesse and Stearns note, evolutionary biology ‘‘can help make medical education more coherent by giving students a framework for organizing the required 10,000 facts.’’ Another point at which medicine and evolutionary biology are sometimes at odds is in their views of the body. A common metaphor found in medicine is that the body is a machine designed for certain functions and when things go wrong, it can be treated in much the same way that an auto mechanic treats a poorly functioning car. Add a little oil here, tighten the belts, clean the carburettor, replace the spark plugs, adjust the tire pressure, or remove the possum from the engine compartment. The machine was designed by an engineer using blueprints that can be consulted when making the repairs. The human body, however, is a bundle of ‘‘compromises shaped by natural selection in small increments to maximize reproduction, not health.’’ Nesse and Stearns suggest that if medicine would give up the idea that the body is a machine, physicians would find it easier to place the discipline on firmer biological foundations. Evolutionary medicine attempts to move medical research and practice in that direction. For example, an evolutionary view of infectious disease can add a great deal to our understanding of newly emerging diseases and may enable epidemiologists to predict where an outbreak will occur and how it will progress. The evolutionary view warns us that trying to treat bacterial infections with ever more powerful antibiotics will ultimately backfire as the bacteria evolve resistance to our medical interventions. It may also help us deal with major health crises of our time such as the human immunodeficiency virus (HIV) by figuring out a way for evolution to work for us in directing the course of microbial evolution from greater to less virulence, rather than the usual arms race we employ that results in greater and greater virulence. We may be able to develop successful vaccines against a number of diseases if we have a clear picture of how they have evolved. Because both clinical medicine and evolutionary medicine have as their goals the control of diseases and the minimization of misery, the evolutionary perspective has hope of slowly becoming part of research and practice. The money, time, and cost of mistakes makes attempting to completely eradicate a disease a poor use of resources and one that will likely fail. Leading pathogens to a state of benign coexistence through evolutionary mechanisms would be a better use of resources and intellectual talents and would promise much greater success. This means we would have a nice barnyard of domesticated critters rather than a forest of dangerous ones with the potential to kill. As in many other modern health movements, consumers are often the engine that drives change by requesting that their physicians consider, for example, gender, inequality, or lifestyles in their treatments and health recommendations. Feminist writing and persuasion introduced into medicine new ways of viewing childbirth, reproductive technologies, human subjects in medical research, and reproductive cancers. The language used in medical discourse has undergone change (perhaps not enough) in the past few decades because of feminist critiques. Anthropology has attempted to make medicine aware of the great amount of variability that exists in the human species, including variability in ‘‘normal health.’’ Scholars writing in the field have so far had limited success with facilitating acceptance of evolutionary principles in medicine.
In order for any characteristic to have an evolutionary basis, it must have some underlying genetic basis, it must vary, and it must be heritable. This last requirement is where reproduction comes in. The only thing that ‘‘counts’’ as far as the process of evolution is concerned is whether or not a trait is passed on to succeeding generations, and the only measure of evolutionary success is reproductive success (also known as fitness). For instance, if a woman has a genotype that results in sterility, she will not pass that trait along to her direct descendants (because she will not have any), so the trait will not evolve. In the language of evolution, the trait has a negative effect on fitness and will not persist. On the other hand, a trait that enhances reproductive success will have a positive effect on fitness and will be passed down through generations. But, as we will see, whether a trait is positive or negative for reproductive success depends almost entirely on context. Evolution has often been described as a ‘‘selfish’’ process. According to this view, individuals (actually, individual genes) will do almost anything to get their genes into the next generation. The result is intense competition between individuals to get ahead in the evolutionary race. The individual whose genes are represented in the greatest numbers several generations down the line is, at least for a time, the ‘‘winner.’’ To get those genes into succeeding generations, individuals have reproductive strategies to find the best mate (known as mating effort), provide the best care of offspring (parental effort), and help others with shared genes do the same (kin selection or inclusive fitness). The strategies lead to allocation of time, energy, and other resources toward the goal of increased reproductive success. Many of the terms used to describe behaviours related to reproduction are not ideal (like selfish, competition, strategy, goal, allocation) because they imply conscious intent, but the behaviours and traits are simply the results of whatever characteristics lead to greater numbers of surviving genes in subsequent generations. Most of us have intimate experience of this conflict in the colloquial sense if we were ever teenagers or had teenage children. But when it is used in an evolutionary sense this concept refers to the fact that the interests of two individuals are never the same, and when the needs of one compromise the health or reproductive success of the other, conflict in one form or another often results. Consider, for example, that a mother (who is related to her offspring by about 50 percent of her genes) will do everything she can (theoretically) to enhance the survival of a given child as long as it does not interfere with the survival of other young she has or can potentially have. The child is related to himself by 100 percent of his genes, so he, in turn, will do everything he can to survive, even if it compromises the health of his mother or current and future siblings. As we will see in later chapters, this comes into play in pregnancy and during breastfeeding when the needs of the developing fetus or nursing infant and of the mother are not always the same. The infant may try to prolong the free ride and nurse as long as he can, but the mother needs to terminate breastfeeding in order to reproduce again and to preserve her own energy reserves.
Proximate and Ultimate Causation
When we examine a characteristic that influences health, survival, and reproduction, it is important to consider that ‘‘causes’’ can be both proximate and ultimate. The proximate cause is usually the immediate one and the one that a clinician might write on the medical chart. The proximate cause is what is most commonly treated by drugs, surgery, or other medical procedures. A frequently cited example is that of a fever in response to an infectious agent. The cause of the fever is a virus or bacterium and a treatment would involve seeking something to wipe out the cause or reduce the symptoms. But an evolutionary, or ultimate, perspective argues that a fever is one of the best tools the body has to fight off infection, a tool that has evolved as part of a complex immune system that enhanced survival and reproductive success in the past. In fact, it may be one of the few tools at our disposal for dealing with viruses, which do not respond to antibiotics. In the language of evolutionary medicine, a fever is a defence rather than a defect. Of course, like any defence mechanism, it can get out of control and become a problem that needs medical intervention. Certainly a fever of 105F in a 3-year-old child would not be considered a healthy response to an infection and would require treatment to bring it down. Another example comes from considering why a person develops sickle cell anaemia. The proximate focus may be on the genetic anomaly that caused the anaemia: people with two alleles for sickle cell (haemoglobin S rather than the ‘‘normal’’ haemoglobin A that most people have) usually develop a severe form of anaemia that most commonly resulted in death or severely compromised health before age 20 in the past, as it often does today in the absence of medical intervention. When a person seeks medical help for anaemia, most clinicians seek the proximate cause or the specific mechanism that caused the disorder in the individual and this is the focus of treatment. On the other hand, concern for the ultimate explanation for the high prevalence of the sickle cell allele would lead to assessing why the alleles are distributed in such a way that they reach high frequencies among those living in or descended from people in Africa and the Mediterranean. What selective value does the sickling allele have in these regions that would explain the high frequencies? Anthropologist Frank Livingstone, who can be considered one of the first scholars of evolutionary medicine (although it was not called that at the time he was writing), noted that the geographic distribution of the sickle cell allele coincides with the distribution of malaria. His conclusion was that those who had a singleallelefor sickle cell (referred to as ‘‘sickle cell trait’’) were less likely to develop or die from malaria than those who had two copies of the allele for ‘‘normal’’ haemoglobin A. Thus, those who were heterozygous for haemoglobin S had greater reproductive success in malarial areas, and the allele is maintained in high frequencies even today. This hypothesis, that sickle cell trait is protective against malaria, has been tested and confirmed, and although knowing this has not necessarily been useful in treating sickle cell anaemia, it has led to greater understanding of the ‘‘adaptive significance’’ of the allele and its pattern of distribution. Another way of thinking about proximate and ultimate causation is to consider that proximate causes are things that happen to an individual in a lifetime, whereas ultimate causes affect populations and species over much longer spans of time, thousands rather than dozens of years. This is a key to knowing why clinical medicine has been slow to embrace evolutionary medicine—clinicians who focus on the ultimate or evolutionary explanation (who ‘‘practice’’ evolutionary medicine) probably will not have as much impact on their clients’ health and well-being as those who focus on and treat the proximate causes. Evolutionary biologist Paul Ewald has expressed concern that when evolutionary medicine tries to develop ultimate or evolutionary explanations for diseases and disorders based on well-established proximate medical explanations, the wrong explanation may result if the proximate medical causes are themselves wrong. The risk of not being right about evolutionary explanations increases when infectious causes are not considered, and Ewald has been critical of much of evolutionary medicine for ignoring them. For example, when the assumed cause of peptic ulcers was stress interacting with a particular genotype, an evolutionary explanation based on that assumption would have been wrong once it was determined that ulcers had infectious origins. If our focus on ‘‘causes’’ of cancers is restricted to mismatches between evolved biology and 21st-century lives, we may ignore evidence that infectious agents play roles. Ewald argues that evolutionary medicine approaches to every disease and disorder should consider all three categories of explanation: genetic, infectious, and environmental/lifestyle. This is particularly important in investigations of causes when treatment and prevention are the goals of research. Clearly, treating a disease that is presumed to be caused solely by genes but is, in fact, infectious in origin, will result in failure or incomplete healing if the infectious agent is not considered. Telling people to relax and remove stress from their lives did not really cure them of ulcers caused by a bacterium (although it was probably helpful for their overall health). On the other hand, we know that tuberculosis is cause by a bacterium, but how it is manifested and how it is treated depend a great deal on both genes and environment (such as diet and hygiene). If it turns out that some severe cases of morning sickness (see Chapter 4) are caused by pathogens, making a case that the nausea is adaptive because it ‘‘protects the embryo’’ may not only backfire but could also result in the death of the mother and her fetus from severe dehydration. In this case, the watchful waiting recommended by evolutionary medicine would be the wrong prescription. If evolutionary medicine is to have any impact at all, it must live up to its claim of taking a holistic and deep-time view of human health.
The Concept of ‘‘Normal’’
Those who view human health and development through the lens of evolutionary medicine take as their beginning assumption the idea that humans are highly diverse with a biology that unfolds in a context rather than in predictable, unvarying ways. Modern medicine tends to think in terms of ‘‘normal’’ and averages, with the model for normal usually being well-nourished people in health-rich populations. Clinicians tend to treat the deviations from normal, but what is normal for women growing up in relative affluence may be very different for women in many populations today and in the ancestral past. Some of the perceived deviations from normal may actually be healthful responses to local circumstances. Evolutionary medicine recognizes that for almost any human trait, there is a broad range within which growth and development can occur and living and reproducing can be successfully maintained.
Human Evolution and Life History
Living requires energy. Growing requires energy. Reproducing requires energy. Healing requires energy. Energy, in the form of calories from the food we eat, does not seem to be in short supply to most of us, but for animals living in the wild and for humans throughout evolutionary history and in some regions today, energy is not only in limited supply but availability also varies greatly throughout a year or a lifetime. Thus, natural selection has favoured mechanisms that work to allocate energy among bodily functions so that the end result for an individual (or kin group) is increased reproductive success. Because energy is limited, this allocation process requires trade-offs, an important concept in life history theory. In short, unless resources are abundant and the work required to obtain them is low, women cannot have well-functioning immune systems, fat babies, and healthy toddlers all at the same time. Something has to give; one has to be ‘‘traded’’ for another. Life history theory helps predict what will be traded. The concept of trade-offs also informs us that reproductive success is not just about quantity but, especially for humans, it is equally or more importantly about quality. It is not just a matter of producing lots of offspring, but producing offspring that survive and reproduce. Avian ecologists long ago demonstrated that reproductive success (via survival and next-generation reproduction) was often greater when there were fewer nestlings and that very few birds reproduce at the maximum. Imagine a woman who has 10 children but none of them survives and one who has only a single child who lives to have several grandchildren. Which has the greatest reproductive success? For humans, the quantity-quality trade-off point is impacted by offspring size, delayed growth, and social sharing of resources and child care. In fact, the practice of sharing may well be the most important factor in life-history strategies for humans. Because of sharing, humans have a higher trade-off point (more quantity) in comparison with other large primates. Here again, culture and the practice of sharing resources makes human life history hard to predict from models created for other species. If an animal is getting sufficient food to maintain life, it is in energy balance: what goes in equals what goes out. An excess of calories that can be used for something else (growth, reproduction, healing) would yield positive energy balance (what goes in is greater than what goes out), and a deficit may yield negative energy balance (what goes in is less that what goes out). In this last case, the only way that an individual can survive on fewer calories than needed to live is to draw upon energy reserves in fat, muscle, and, ultimately, organs. If negative energy balance is prolonged by ongoing deprivation, death may result. In the short term, however, other energy-hungry functions are put on hold, including growth in children and reproduction in adults. As we will see, women’s reproductive functioning seems to be more sensitive to energy balance than men’s, but sperm production is also negatively affected by prolonged negative energy balance. As noted earlier, at its most basic level, the study of reproduction is the study of food. Negative energy balance can interfere with reproduction at several points. Early on in the life course, it can delay puberty and inhibit ovulation following puberty. Lactational amenorrhea (cessation of ovulation because of breastfeeding) is prolonged with negative energy balance and shortened with positive energy balance, a phenomenon that explains why women who are well fed (and thus usually in positive energy balance) cannot rely on breastfeeding to inhibit ovulation. For example, anthropologists Claudia Valeggia and Peter Ellison report that Toba women in Argentina who are well nourished and breastfeed intensively return to positive energy balance soon after giving birth and resume ovulating in less than a year, providing support for the view that energy balance has more of an impact on ovulation than nursing frequency. Energy balance influences hormone levels during menstrual cycling and pregnancy, the timing of births, and infant birth weight. Finally, it appears that prolonged negative energy balance can result in early menopause, although there is still some controversy about this. And, of course, starving people often have decreased sex drives and may not have the energy for courtship and mating activities, further reducing reproductive success. Many aspects of life history theory can be applied in similar ways to all sexually reproducing animals, but there are a number of characteristics of human life history that make it somewhat unique, at least as we humans see them. In the language of evolution, these unique characteristics are referred to as derived traits and serve to distinguish humans from our closest relatives, the great apes. They include longer life spans, later age at first birth, longer growth period, larger maternal body size, larger and more dependent neonates, earlier age at weaning, shorter interbirth intervals, and a prolonged postreproductive life. Also, as anthropologist Gillian Bentley notes, humans today and in the recent past have the ability to ‘‘maximize reproduction through cultural interventions,’’ a characteristic not found in other animals, at least to the degree it exists in humans.