In my view, the rapid rate at which human females become fertile again after giving birth is an accidental byproduct of the evolution of concealed ovulation. Continual and hidden ovulation led to more sex, since neither the males nor the females knew when females were fertile, and thus promoted family cohesion and paternal investment. However, more sex also led to more conception. This happy accident worked out because of the climbing rate of infant mortality due to the aforementioned increase in fetal skull size. Since the babies of humans died far more often than the babies of other apes, the higher birthrate compensated for that loss.
No matter how it evolved, reduced birth spacing with high infant mortality is incredibly poor planning by whatever force designed our species’ reproductive system. That shouldn’t surprise us, however, because evolution doesn’t make plans. It’s random, sloppy, imprecise—and heartless.
Deadly Delivery
Of course, human infants aren’t the only ones at risk during childbirth; mothers can and do die from it too. Once again, modern medicine has very effectively managed this risk; in the United States in 2008, for instance, there were only 24 maternal deaths per 100,000 live births. (Shockingly, this was up from 20 in 2004 and 9.1 in 1984, due in large part to the aforementioned overuse of C-sections.) However, in developing countries, the figures are much higher. In Somalia in 2010, there were 1,000 maternal deaths per 100,000 live births. That’s 1 percent of all births. When you factor in the much higher birthrate in developing countries, the cumulative lifetime risk for a woman to die in childbirth there is about 1 in 16. Most Somalis will lose several women in their lives to childbirth.
There is great debate regarding the maternal death rate in centuries past, not to mention in the classical periods, prehistory, and preagriculture. It seems that 1 to 2 percent is the lowest possible estimate when you consider that several countries (including Somalia) currently have that rate now. Thus, in ages past (as in some places today), childbirth was an incredibly dangerous experience. It is no exaggeration to say that, for most of the time our species has existed, being born was the leading cause of death. For women, giving birth was the next biggest threat.
This, too, sets humans apart. Childbirth is safer for primate mothers in the wild than it is for human mothers, and that’s without primates having the benefits of medical intervention. Mothers dying in childbirth is unheard of in chimpanzees, bonobos, gorillas, and all of our other primate cousins. This is a purely human peril.
Especially dangerous for mothers are breech births, when the baby comes out feet-first instead of head-first. It is, of course, possible to deliver a baby in the breech position, but it is much more difficult. Without the aid of medical care, mortality for both baby and mother is higher with breech deliveries. (Estimates vary widely, but all agree that the risk of death or harm in a breech delivery is at least three times higher for the mother and five times higher for the baby. This is the heightened risk in today’s world, where women have access to postpartum care and modern medicine.) Much of the risk to baby is due to a tenfold higher risk of constriction of the umbilical cord, which deprives the baby of oxygen. This can last many hours, given the prolonged nature of a breech delivery. For this reason, in these cases physicians almost always opt for delivery by cesarean section.
According to legend, the cesarean section was first performed on Julius Caesar’s mother when it was discovered that the infant was presenting in the breech position. This legend is now widely believed to be false, but it is true that the cesarean was well known in the ancient world, where a breech birth meant a very good chance of losing both the mother and the baby. Tales of cesarean births of humans or demigods can be found in ancient Indian, Celtic, Chinese, and Roman mythologies. In fact, long before Julius Caesar, there was a Roman law that stated that when a pregnant woman died, a cesarean had to be performed in an attempt to rescue the fetus. (This probably started as a public-health policy and then apparently morphed into a superstition that unborn infants would experience a ghoulish resurrection if they were buried in their mothers’ wombs—giving mourning family members an added incentive to carve into the bellies of the dearly departed.)
There’s no more compelling evidence for the riskiness of breech delivery in the ancient world than the fact that, unequipped and terrified, people resorted to slicing women open. In the days before hygienic practices and antiseptic operating rooms, this almost always killed the mother, though it may have occasionally saved the child. All this because of shortcomings in our species’ gestational design.
If you’ve witnessed the birth of another mammal species, you’ll know that it is usually not a dramatic affair. Cows seem to barely notice when they give birth. Gorilla mothers often continue eating or caring for other children during delivery. The difficulties we associate with childbirth are uniquely human, the product of the rapid evolution of a large cranium together with the failure of evolution to keep up with those changes.
Given enough time, natural selection would surely have sorted this out in any number of possible ways. But the chance of a natural adaptive remedy is now virtually nil, as medical intervention has largely solved the problem of childbirth and removed the negative selection of so many women and children dying in childbirth. This is a triumph of human ingenuity over human limitation. Once again, science has provided solutions to a problem caused by nature. But in the process, science has effectively short-circuited evolution, consigning humans to the faulty reproductive systems that nature gave us.
Any discussion of the mortal danger to women during pregnancy and childbirth would be incomplete without mention of ectopic pregnancies. In science, the word ectopic is used to indicate that something exists (or an event happens) in a place where it usually doesn’t. In the case of ectopic pregnancies, the location is almost always the fallopian tube. When a fertilized egg implants in a fallopian tube instead of the uterus, it is an extremely dangerous situation, and before the age of modern medicine, it usually led to the mother’s death.
When an egg is released from the ovary, it travels through one of the fallopian tubes and eventually reaches the uterus. However, unlike sperm cells, an egg has no flagellum—that whiplike tail—to propel it. Also unlike sperm cells, the egg is surrounded by hundreds of follicular cells that form a protective layer called the corona radiata. None of those cells have flagella either, so the result is that the egg and its crew drift down the fallopian tube rather slowly and aimlessly. It’s more or less like a bunch of life rafts tied together and floating in a very large sea. It can take the egg a week or more to reach the uterus from the ovary, despite the distance being a mere ten centimeters.
Each sperm cell, by comparison, is jet-propelled by its whiplike tail. Because the egg moves slowly and the sperm move quickly, conception almost always occurs in the fallopian tube; an ovulated egg is still meandering through the tube when a sperm cell rushes to meet it. (In fact, if it hasn’t been fertilized, an egg will usually perish before reaching the uterus—that’s how slow it’s moving.)
Following fertilization, a series of chemical reactions occur in the zygote as it prepares to start development. About thirty-six hours after conception, the zygote begins rapidly and repeatedly dividing in half. The one-celled zygote becomes two cells. Those two divide again to make four. Four become eight, eight become sixteen, and so on until the embryo grows into a hollow sphere of 256 cells, nine or ten days after conception. Only then is the embryo ready to begin tunneling into the uterine wall and sending out signals to the host body to prevent menstruation. Pregnancy begins. As discussed earlier, halting menstruation is the first and biggest challenge that the embryo faces, and a great many fail to do so and are lost with the monthly flow.
Ten days should be plenty of time for the embryo to make it to the uterus, but the problem is that embryos, like eggs, move along aimlessly. Occasionally, an embryo will not make it out of the fallopian tube and into the uterus by the time it reaches the 256-cell stage. When this happens, the em
bryo tunnels into the tube walls just as it would into the uterus. This is an ectopic pregnancy. Through the first eight weeks of pregnancy, embryos are incredibly tiny, and their need for nutrients and oxygen is served perfectly well by simple diffusion from the surrounding tissue. Thus, neither the embryo nor the fallopian tube will initially notice anything amiss during the early stages of an ectopic pregnancy. However, as the embryo continues to grow, the trouble starts.
The fallopian tube is in no way equipped to support a pregnancy, and the embryo becomes like a parasitic invasion. The embryo itself has no way to detect the problem, and it continues on its aggressive program of expansion and development. The fallopian tube, unlike the uterus, cannot cleanly abort the doomed and increasingly dangerous pregnancy. Eventually, the untenable situation comes to a head as the growing embryo presses against the walls of the fallopian tube; this will likely be the first time the woman senses that something is not right. The pressure will become increasingly painful, and if she does not seek medical intervention, the embryo can rupture the fallopian tube. Severe pain and internal bleeding follow, and without emergency surgery to repair the damaged tissue and seal the bleeding vessels, the woman may bleed to death, killed by her own offspring attaching where it should not.
There is an even rarer, stranger, and more dangerous form of ectopic pregnancy. Very seldom, when an egg is ejected from the ovary, it does not make it into the fallopian tube at all. This is because, quite oddly, the fallopian tube is not actually connected to the ovary. Rather, the opening of the fallopian tube envelops the ovary, sort of like a too-large garden hose resting on a too-small spigot. The two are not actually attached, and sometimes an egg gets squirted out of the ovary and into the void of the abdominal cavity instead of into the fallopian tube.
When this happens, it is usually of no consequence. The egg simply dies after a few days and is resorbed by the peritoneum, the thin wall of highly vascular tissue surrounding the abdominal cavity. No problem.
However, if an egg gets squirted into the abdominal cavity and sperm bursts onto the scene within a day or so, it might find this egg and fertilize it. Once again, this is a rare event because the sperm would have to be searching throughout the lower abdomen for the egg rather than staying in the confined space of the oviduct as it generally does. But this does occasionally happen. The resulting embryo, completely unaware of how far it is from home, mindlessly begins the process of growth, division, and tunneling into whatever nearby tissue that it can find, usually the peritoneum but occasionally the outer covering of the large or small intestine, liver, or spleen.
The human female reproductive organs. Because the ovary is not physically connected to the fallopian tube, there is no guarantee that a released egg will even end up in the reproductive system.
Abdominal pregnancies pose serious risks. In developing countries, they usually result in the death of the mother. In developed countries, they are easily spotted with ultrasounds and treated with surgical intervention to remove the doomed embryo and repair any damaged tissue or bleeding.
Incredibly, in a small handful of cases, an abdominal embryo makes it into the twentieth week of gestation without killing the mother, is delivered extremely premature by surgery, and survives, though not without serious medical and developmental complications. Though they are invariably termed “miracle babies” by the popular press, these infants survive due to intense and superb medical intervention and a great deal of luck.
Perhaps the opposite of an ectopic “miracle baby” is something called a lithopedion. Occasionally, an abdominally developing embryo will make it into the second trimester before dying and, somehow, not kill or harm the mother in the process. At this point, the fetus is too large to be resorbed by the peritoneum but it obviously cannot be delivered in the normal way as miscarriages or stillbirths are. It’s stuck there. The mother’s body then reacts to the fetus the way it does to any foreign body that might cause infection: it calcifies the outer layers of the amnion and fetus, covering it in a hardened shell.
The lithopedion, commonly referred to as a stone baby, is very rare; only about three hundred cases have been documented throughout history. These lithopedions usually cause medical problems that must be resolved by surgery, but there have been reports of women carrying them safely and asymptomatically for decades. There is even a report of a woman in Chile who carried a nearly five-pound stone baby in her abdomen for fifty years. She gave birth to five children naturally during that time.
While lithopedions and abdominal pregnancies are quite rare, they are also 100 percent the result of poor design. Any reasonable plumber would have attached the fallopian tube to the ovary, thereby preventing tragic and often fatal mishaps like these. Similarly, even the most unimaginative engineer would have given egg cells some sort of means of propulsion or, at the very least, put cilia on the walls of the fallopian tubes to gently brush the fertilized egg cell into the uterus. Either of these would eliminate tubal pregnancies and both are possible with design structures that already exist elsewhere in the body.
But of course nature did not come up with solutions like these, which helps explain why ectopic pregnancies, especially the most common kind (those that take root in the fallopian tubes), are more frequent than you might think; 1 to 2 percent of conceptions result in tubal implantation. This is likely an underestimate, moreover, because at least 10 percent of tubal implantations (and possibly up to one-third) spontaneously resolve through the death of the embryo before it has implanted too deeply, meaning that many surely go unnoticed.
Let’s not single out the poor fallopian tubes, though. The entire human reproductive system is littered with inefficiencies and poor design. To recap: Humans mature late, conceal female ovulation, have trouble making healthy sperm and eggs, create embryos that don’t implant or that have missing or extra chromosomes, initiate pregnancies that aren’t successful—and even when everything goes right, both babies and mothers die in childbirth at shockingly high rates.
In fact, of all the organ systems and physiology in the human body, the reproductive system is the most problematic—the most likely not to work. This is especially odd given how important reproduction is for, you know, the survival and success of the species. And it is especially humiliating when you consider that many of these problems are either nonexistent or at least far less common in other animals. When you consider how poorly designed our species’ reproductive systems are, it’s kind of amazing we made it to the modern era, when science could solve some of these problems for us.
Coda: The Grandmother Hypothesis
Recently, it was discovered that two species of whales, orcas and pilot whales, undergo menopause. One study found an orca that had died at the ripe old age of 105, more than forty years after she had last reproduced. So while reproduction in humans is uniquely inefficient and often uniquely deadly in many respects, when it comes to menopause, at least, we are not entirely alone.
Also called reproductive senescence, menopause is the stage of a woman’s life in which she ceases to have menstrual cycles and is no longer capable of reproducing. And while some whales seem to experience it, most female mammals go on reproducing through old age, right up until the end. The cessation of reproduction by a female, even late in her life, would seem to reduce her chances of passing on her genetic legacy, so menopause is contradictory to the way natural selection usually works. It is a conundrum that requires an explanation, yet another possible flaw in our species’ reproductive capacity. However, since evolution did leave us with menopause, it might somehow be advantageous to an older woman—or her offspring—that she stop reproducing later in life. But how?
One idea is that when older women are relieved of the burden of reproducing, they begin to more actively invest in their children and their children’s children, and in so doing, they enhance the success of their genetic legacy to a greater degree than they would by simply making more children. But before we explore that possibility, we need to say a
word about how menopause actually happens in women.
I once heard the folk wisdom that menopause is nothing more than a peculiar byproduct of the recently extended lifespan of humans, the idea being that, since life expectancy in the premodern era was only thirty or forty years, women usually didn’t live long enough to become menopausal. Only now that humans are routinely living into the seventh, eighth, and ninth decades has menopause begun to occur.
This idea is predicated on a misunderstanding of what life expectancy really means. While it is true that the average lifespan in the medieval, classical, and even prehistoric periods was just twenty or thirty years, the average age of death was low because so many died as infants or as children. Most people born during prehistory thus didn’t make it to reproductive age at all, but many of those who did enjoyed a fairly long life, even by today’s standards. We know from ancient texts and recovered skeletons that humans as old as seventy or eighty were not unheard of, even in the prehistoric period. It has been estimated that the average age of death for those who made it past adolescence was somewhere in the late fifties, with lots of people living into their sixties and a few into their seventies.
When it comes to middle age, modern medicine has really added only a decade and a half or so to the end of the human lifespan. The bigger difference has been made in the first decade, and that is what has drastically altered the average lifespan average. The point is that women have been living long enough to undergo menopause for hundreds of thousands of years. It is not a recent quirk, so we can dispense with that bit of folk wisdom.
Not long ago, it was thought that menopause occurred when women used up all of their ovaries’ egg-containing follicles. Women are born with a set number of follicles, around two hundred thousand or so in each ovary. Each of these follicles contains one egg cell, which paused its maturation process way back when the woman was just an embryo herself. Then, each month, anywhere from ten to fifty follicles are selected at random to reactivate the maturation of their eggs in a sort of race. Whichever follicle and egg complete the maturation process first “wins” and is ovulated. The losers all die and, presumably, are never replaced. We used to believe that menopause occurred when a woman simply “ran out” of these egg-containing follicles.
Human Errors Page 12