This explanation also turns out to be unsatisfactory, for at least two reasons. First, even if fifty follicles (the high end of the range) are activated every single month and a woman never misses a cycle due to irregular menses or pregnancy, by the time she’s in her sixties, she will still have used less than thirty thousand follicles, or one-sixth of what she started with. Second, hormone-based contraceptives interfere with both ovulation and the monthly activation of follicles, and thus every year of use should delay a woman’s menopause by almost as long. However, women who have used hormonal birth control for decades experience only a modest delay in menopause, if any.
If menopause is not caused by a woman using up all her follicles, what does cause it? It turns out that eventually follicles cease to produce estrogen and progesterone. There are plenty of follicles left; they just sort of peter out. They stop making hormones, and they stop maturing. The symptoms associated with menopause are due to this drop in hormone levels and can thus be treated with hormone-replacement pills. However, this doesn’t prevent menopause itself. Sometime in a woman’s late forties or early fifties, the ovaries fail to respond to hormonal cues, cease secreting their own hormones, and basically give up.
The precise mechanism of menopause seems to be a timed, age-related decrease in the expression of DNA-repair enzymes in the cells surrounding the egg within each follicle. Without the action of these repair enzymes, DNA damage and mutations accumulate, which accelerates the aging process, and the cells eventually enter a state called senescence. They don’t die; they just stop dividing and renewing as follicular cells are supposed to. The ovaries go into a coma of sorts. They are very much alive, but they are permanently idle.
This may sound like a normal, unavoidable, age-related breakdown, like skin losing elasticity or bones becoming brittle—but it’s really not. Those and other age-related processes are the result of accumulated protein and DNA damage despite the tissues doing everything they can to repair damage as it inevitably occurs; eventually, time wins, and the repair machinery itself becomes damaged and the death spiral of aging sets in. In the case of the ovaries’ follicles, however, the genes for the DNA-repair enzymes simply switch off. The aging that happens in the follicle is not slow and cumulative; it is timed and sudden.
This brings us to the evolutionary purpose of menopause. It’s easy to conceive that mutations could appear that turn off the ovary’s DNA-repair machinery late in life, but why did natural selection favor these mutations instead of eliminating them? The most interesting explanation for menopause is that it allows older women to redirect their efforts toward the success of their children’s children. For this reason, this explanation is called the grandmother hypothesis. This hypothesis has been enormously popular—I suspect as much for its explanatory power as for how well it matches up with our cultural notions of doting grandparents spoiling their grandchildren. But the reasoning is actually more complicated than it sounds, because any consideration of the evolutionary value of a certain phenomenon must balance pros and cons.
If an older animal ceases her own reproduction and instead aids her existing children in caring for her grandchildren, it seems obvious that those grandchildren would benefit from that help and be more likely to thrive and succeed. Thus, contributing to grandchildren has obvious natural-selective advantages. However, by forgoing her own reproduction later in life, a menopausal grandmother is reducing the total number of children she can produce. A competitor grandmother who doesn’t become menopausal will have more children, and those children will in turn have more children. While they won’t have a grandmother’s help, they will outnumber those who do. Especially if they show strong kin-cooperative behaviors (as humans do), they might be just fine without the help of a doting grandmother.
So the question is, does a grandmother’s contribution to her grandchildren bring so much selective advantage that it is worth the cost of a lower reproductive rate? This thorny question has kept some biologists from endorsing the grandmother hypothesis, and there is one very large and obvious piece of evidence against this explanation: the lack of programmed menopause in other species. If grandparental investment is so great, then those benefits should be seen across many social species, not just humans, yet none of them really do the menopause thing.
One explanation for why the grandmother hypothesis may apply almost exclusively to humans is that the structure of our social groups was, and still is, quite peculiar. All research indicates that, over the past seven million years, our ancestors lived in small, close-knit communities that were highly mobile and socially intricate. There was probably a great deal of experimentation with different lifestyles over that time, as evidenced by the interesting mosaic of anatomical features found in various hominin species. None of that is unique to humans, but one thing may have been: an elaborate division of labor.
As our ancient ancestors became increasingly intelligent and socially sophisticated, they began to amplify the complexity of their already complex primate lifestyles. Tool fashioning, organized hunts, and communal parenting bring a great deal of efficiency to the work of staying alive and free up some individuals to explore and innovate. Before long, early humans were reshaping their world by constructing shelters and fabricating complex tools and manipulating the plants and animals around them. Individuals began teaching skills to one another and dividing labor among the members of the group. It was this environment of communal living that might have provided just the right milieu for the grandmother effect to evolve.
In a highly social group, each member pulls his or her weight in a different way. There is much work to be done. At any one time, some hunt, some gather, some build, some watch out for predators or competitors, some fashion tools, some nurse children, and so on. But just because the individuals live together doesn’t mean that they don’t also compete with one another. Cooperation helps the group compete better against other groups, but within the group, competition will also be found. In the end, natural selection operates through the success or failure of individuals.
With this in-group competition in mind, imagine a small community with children of all ages. Mortality rates are high throughout childhood, and the children compete for access to food as well as parental care and protection. When a woman is young, her evolutionary interests are probably best served by having as many children as possible to compete against other children for access to resources. Communal parenting means that the burden of childcare is borne by all, so she’d want to try to consume the biggest possible piece of that pie.
As she ages, however, and the number of her offspring rises, her calculus shifts. Her children will end up competing against one another, and her ability to help them all will become compromised by her age and growing infirmity. Success for one of her children may come at the expense of another one of her children rather than at the expense of someone else’s child, making it a zero-sum game for her. Continuing to have children may contribute very little to her reproductive potential. In fact, it could actually hurt her potential, given how dangerous childbirth is to human mothers. Under these circumstances, shifting her focus toward taking better care of the children she already has rather than making more of them may be a better use of her energy and resources. By this time, of course, her children may have children themselves.
So goes the grandmother hypothesis. It seems a little too neat, but it does fit with common cultural experience and also with some of the unique aspects of human beings: communal living with division of labor, high infant and maternal mortality rates, and long lifespans. This may have been the perfect cocktail of biological factors that rewarded the spontaneous mutations that gave us menopause.
Back to the whales. Researchers analyzed thirty-five years of data, including thousands of hours of video, detailing the movements and activities of orcas living off the coast of British Columbia. What they found was that, as orcas hunt for food in small foraging groups, leading the pack is often an older, menopausal female. In fact
, hunting groups frequently consist of an elderly matriarch and her sons. Adult male orcas spend much more of their time hunting and foraging with their mothers than with any other whales, including their fathers.
Even more dramatically, the tendency for hunting groups to be led by menopausal females is most pronounced during periods of famine. When times are tough, orcas turn to their matriarchs, usually their own mothers, to lead them through the darkness. An older orca has been hunting and foraging for many decades and, since whales have impressive memories, she possesses a lifetime of ecological knowledge regarding where to find seals and otters, when the salmon begin their spawning runs, and so on. This knowledge is especially important when food is scarce. It’s not clear why older male orcas don’t similarly share their knowledge, but the older females definitely do.
Menopause aside, most of the reproductive quirks that humans experience do not seem to be adaptive or shared by other animals. From the late onset of maturity to the eventual arrival of menopause in females, human beings have markedly error-prone and even deadly reproductive systems. These stark reproductive flaws would normally be such a handicap to the success of a species that if the needed fixes didn’t evolve, the species would go extinct.
But humans persevered despite these flaws. As we did for our other flaws, we used our big brains to create fixes to circumvent these evolutionary problems. In a way, rather than waiting for nature to do it, we took charge of our own evolutionary destiny. Our creative thinking and collaborative social living helped us scrape by during the earliest years of our species, and then the emergence of language allowed us to accumulate wisdom through the ages and teach the clever tricks to our children. And who among us are the repositories of all that accumulated social knowledge? The menopausal matriarchs we call grandmas.
Humans domesticated plants and animals, invented engineering, and built cities. The advantages that came with these innovations offset our species’ low reproductive rates and high child and maternal mortality rates, and eventually this collective knowledge surged exponentially with the dawn of the enlightened scientific era, freeing people (for the most part) from the deadly paradox that reproduction had foisted upon them for so long.
Ultimately, this intelligence allowed us to overcome the limits of biology. Modern medicine has tamed many of the beasts that killed our ancestors so early and so often. Accordingly, as medical standards of care began improving in the mid-nineteenth century, the human population exploded. Along with this explosion came the sinister handmaidens of success: resource scarcity, war, and environmental degradation unlike any our species has ever seen.
So we now have the opposite problem: too many people, rather than too few. Nothing says “poor design” quite like uncontrolled and unsustainable population growth. So maybe all those reproductive limitations weren’t so bad after all?
5
Why God Invented Doctors
Why humans’ immune systems so often attack their own bodies; how developmental errors can wreak havoc with our circulation; why cancer is inevitable; and more
We humans are a sickly bunch. You may recall from the first chapter of this book that we get head colds far more often than other mammals, thanks to our peculiar sinus-cavity drainage. But that’s just the tip of the iceberg. Our species is plagued by quite a few other illnesses as well, many of which are particular to us, and many of which have causes that are much less straightforward than a misplaced drainage hole in the sinuses.
For instance, humans get gastroenteritis a lot, a deeply unpleasant condition better known (in the United States, at least) as stomach flu. Gastroenteritis is an umbrella term for any infection or inflammation in the digestive tract that leads to some combination of nausea, vomiting, diarrhea, lack of energy and appetite, and inability to digest or even ingest food.
These two ailments—head colds and gastroenteritis—are the most common illnesses in the developed West. While they are rarely deadly, they nevertheless are so common that they cost billions of dollars annually, mainly attributable to wages lost as workers rest and recover. Tragically, some forms of these diseases are much costlier; for instance, diarrheal diseases (types of gastroenteritis affecting the intestines and, in developing countries, usually caused by sewage-contaminated water) remain one of the biggest global killers.
Neither head colds nor stomach flu nor diarrheal diseases are common plagues among other animals. Of course, although head colds are partially attributable to evolution (owing to our species’ poorly designed nasal cavities), they are also infectious processes, just like the stomach bugs that smite us every so often. And when it comes to infectious diseases, humans should blame ourselves first and nature second. That’s because diseases such as these are at least partly due to the high population densities and other living conditions peculiar to urbanization.
Beginning in classical times, humans started living on top of one another in booming but filthy metropolises. Their livestock lived on top of one another as well (and still do) and also on top of the humans. Our ancestors’ raw and prepared food was in the mix too. These unsanitary conditions—which humankind proceeded to endure for centuries—resulted in a witches’ brew of bacteria, viruses, and parasites of all kinds. We can somewhat manage this epic pile-on now, thanks to the invention of modern plumbing. But when you think about the pestilence that our species’ way of life invites, it’s really kind of amazing that human civilization made it off the ground at all.
All of our ancestors who managed to survive childhood developed antibodies—a type of protein produced by the immune system to protect against bacteria and viruses that might otherwise be fatal. These antibodies made them immune to at least the worst of the bugs that festered in their surroundings. When the age of European exploration began, the aboriginal people with whom Europeans came in contact did not fare well. They hadn’t needed the antibodies that European children were forced to develop in order to survive. While they had no doubt developed antibody-based resistance to their own set of infectious agents, they were completely unprepared for the cocktail of pathogens that arrived with the invaders.
For humans today, the infectious ailments that are a routine part of human life were born and bred of the wretched conditions of urban life in Europe and Asia. We therefore can’t really call most infectious diseases a design flaw; as I mentioned previously, they are our own fault, not nature’s.
Yet we do have design flaws that make us sick. We are plagued by an immune system that seems to be constantly misfiring. When it isn’t mistakenly attacking our own cells and tissues in autoimmune diseases, it is overreacting to harmless proteins. Just as humans reach what should be their prime middle-age years, the cardiovascular system starts to develop weaknesses that will only get worse. Not long after, cancer strikes, usually the result of nothing more than accumulated damage occurring within cells.
While none of these conditions are unique to humans, most of them are much more pronounced and deadly in humans than they are in most other animals. We suffer from these illnesses much more than our pets do, more than zoo animals do, and far more than animals in the wild do. For reasons that defy logic, it almost seems as if we are built to be sick.
We Have Met the Enemy, and It Is Us
Of all the diseases that humans have evolved to suffer, autoimmune disorders are among the most frustrating. They do not involve bacteria that we can fight with antibiotics. There are no viruses against which we can develop antibodies. There are no tumors that can be sliced out, poisoned, or irradiated. When we track down the cause of the malady, we find only ourselves.
Autoimmune diseases are the result of mistaken identity. An individual’s immune system “forgets” (or never learned) that some protein or cell in the body is its own and not a foreign invader. Not recognizing its own cells, the immune system attacks it vigorously. It’s a tragic case of friendly fire.
Predictably, this does not end well. When a body begins attacking itself, there is little physicians can
do except give medications that suppress the immune system. This is quite dangerous, and so it must be done very carefully and with close monitoring. There are also all sorts of complications. Besides the obvious threat of infections and a higher rate of common respiratory ailments, drugs that dial down the responsive power of the immune system cause side effects such as acne, trembling, muscle weakness, nausea and vomiting, an increase in hair growth, and weight gain. Long-term use of these immunosuppressants can lead to fat deposits on the face (sometimes called moon face), kidney dysfunction, and high blood sugar levels, increasing the chance of diabetes. They also increase the risk of cancer. The treatment can be nearly as bad as the disease.
Nearly all autoimmune disorders strike women more often than men, for reasons no one understands. As if this were not cruel enough, autoimmune diseases often develop slowly and imperceptibly. Patients become accustomed to the pain and limitations and may even doubt that there is anything physically wrong with them. This is compounded when others, even their physicians, brush off their symptoms. A friend of mine has an often debilitating suite of symptoms that come from chronic fatigue syndrome and rheumatoid arthritis, two probably related autoimmune diseases. She has been told by medical professionals, “Well, none of us feel that great first thing in the morning”; “It sounds to me like you need to get out of the house more and get more physical exercise”; and the always helpful “This could just be in your head, but either way, lying around won’t help.”
Human Errors Page 13