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Sex, Time, and Power: How Women's Sexuality Shaped Human Evolutionby Leonard Shlain
There is a female human nature and a male human nature, and these natures are extraordinarily different....Men and women differ in their sexual natures because throughout the immensely long hunting and gathering phase of human evolutionary history the sexual desires and dispositions that were adaptive for either sex were for the other tickets to reproductive oblivion.
Error is the inevitable by-product of daring.
óStephen Jay Gould2
Juxtaposing the words ìironî and ìsexî creates an odd couple. The two rarely have occasion to appear together in the same sentence, much less find themselves standing side by side with so little editorial support. In the following pages, I will propose that the first word fundamentally influenced economic matters between men and women and, as a result, profoundly affected the politics of the second word. Along the way, I will present a scenario for how the kaleidoscopic, maddening, exciting, enchanting, and baffling man-woman dance, more commonly referred to as ìa relationship,î evolved.
This book arose out of a question I posed to a professor when I was a second-year medical student, making rounds on patients in a large ward. Although the incident occurred over forty years ago, I had never really forgotten or accepted his answer.
The sophomore year of medical school represents a major transition for students. They leave the cadavers of the freshman year behind and begin having contact with respiring, perspiring patients. On this particular day, we were being taught how to interpret laboratory results.
Shifting from bedside to bedside, our knot of students listened intently to the professor. Every patient admitted to the ward, he explained, had three basic laboratory tests: a chemistry-26 panel, a urinalysis, and a complete blood count (CBC). The first measured the concentration of twenty-six constituents floating about within the patientís bloodstream.
The amount of a patientís circulating sodium or potassium, for example, provided a snapshot of the health of various internal organs, such as the heart and kidneys. The measurement of every one of the twenty-six constituents was like having a miniature finger figuratively take the pulse of some very important cellular function, which in turn reflected on the state of one or more of the bodyís organs. The lab reported each value on a slip that was placed in the patientís chart. Neatly aligned, parallel to the patientís results, was a column stating the expected normal ranges for each particular variable. What caught my eye that morning was the list of normals. Of the twenty-six numbers on the chem panel, none distinguished between the values for a man and a woman. And why should there be any? After all, sex has nothing to do with the way a lung or a stomach goes about its business, so I would not have expected any variation between the sexes. The same held true for the normal values reported on the urinalysis.
The CBC, however, was distinctly different in this regard. A complete blood count measures several different parameters of the red and white cells circulating within the bodyís miles of vascular tunnels. Although the white-cell numbers were the same for the two sexes, I noticed that the red-cell normals for men and women were surprisingly askew. I thought that was very strange.
A red cellís chief function is to pick up oxygen in the lungs, transport it through the blood vessels, and deliver it to every organ in the body. For all complex creatures, oxygen is the staff of life. Deprive them of this most precious element and they will rapidly die of asphyxiation. Yet a man normally has a 15 percent higher concentration of circulating red cells than a healthy woman has. I puzzled over this discrepancy (even after taking into account that, generally, a woman is smaller in stature and has less muscle mass than a man), and it prompted the question that lies at the heart of this book. Why would a woman need less of bloodís critical essence than a man?
I raised my hand and asked the professor to explain the reason for this vital disparity. He replied, in a way that suggested he thought the answer was obvious and my question, had I given it more thought, was unnecessary:
ìWomen bleed and men donít.î
Somewhat annoyed that I had interrupted his train of thought, he then returned to the subject on which he had been discoursing. I reddened.
I remember thinking at the time that his curt answer could not be the whole story. Later, as I turned over his reply in my head, I could not think of a plausible explanation for this difference. Why would a species evolve whose femalesóbut not malesóoperated routinely on less than a full complement of tankers to ferry a fuel as crucial as oxygen? (One could argue that the female has a normal amount and the male has a 15 percent excess capacity. Either way, the inequality begs for an explanation.)
The magic behind a red cellís seamless performance is the protein called ìhemoglobin.î And, like a set of nested Russian babushka dolls, at the core of hemoglobin lies the element iron. Iron and oxygen, because of the salutary arrangement of their outer electron shells, eagerly seek out each otherís embrace. Once merged, the two form a molecule called iron oxide. Everyone is familiar with this substance. We call it ìrust.î
Rust has a signature red color. Iron oxide in rock lends to the Grand Canyon its spectacular chromatic array. It colors the fields in Kenya and the plantations of Hawaii, and rustís distinctive hue made the earth of Scarlett OíHaraís beloved Georgian Tara red. The hemoglobin molecule, with its precious complement of iron atoms, makes blood red and white skin pink, and it tints medium-rare steaks with a distinctive ruby sheen.
Hemoglobin transforms ironís strong affinity for oxygen into a delicate ìgrasp and releaseî maneuver, allowing oxygen to be easily acquired in the lungs and readily relinquished farther down the line to the cells. The felicitous combination of iron, oxygen, and hemoglobin is central to the process that facilitates the neurons in your brain to extract meaning from the sequentially aligned squiggles you are reading on this page.
A few weeks after I had posed my red-cell question on rounds, a second incident piqued my interest in the subject. During a rotation on the obstetrics-gynecology service, I was assigned to the prenatal clinic, where wise, experienced nurses instructed me how to care for pregnant women. Detroitís Harper Hospital in 1958 served a large population of poor people who had emigrated from other states, hoping to improve their lives in the cityís then booming auto industry. Unfortunately, most women we served were unaware of the prenatal program and did not enter it, if at all, until late in their pregnancy. Playing catch-up, I counseled these soon-to-be moms about the importance of maintaining a healthy diet, for both themselves and their unborn children.
A key element of my care was to monitor each womanís weight, blood pressure, CBC, and blood-sugar levels. My goal was to keep these four cornerstones of prenatal health within a normal range. Specifically, I tried to maintain their hemoglobin level in the 11ñ12 range, using iron pills to supplement their dietary intake if necessary.* A hemoglobin level below 10 heightened my concern, because it confirmed the diagnosis of anemia.
Consider my consternation and sense of helplessness when I discovered that it was exceptional to find one among the late-term women visiting my clinic in possession of a normal hemoglobin count. More often, they ranged from mild (8ñ10) to severe (5ñ7) anemia.* My mentors sadly shook their heads, explaining to me that fetal brains would most likely not develop properly if the mother was anemic; at worst the babies would suffer mental retardation, and at best they would fail to attain their full intellectual potential.
Outwardly, these very pregnant women seemed normal, but I soon learned to discern their anemiaís telltale signs of apathy and lethargy. Paradoxically, they appeared well fed, and, even more puzzling, many were actually overweight. A peculiar side effect of iron deficiency is the dulling of the taste buds that sense sweets. To compensate, an iron- deficient woman craves carbohydrates.
I was often disconcerted to observe a pregnant woman bringing a carton of cornstarch into my examining cubicle, eating the white powder right out of the box during my interviewóso great was her drive to satisfy her carbohydrate hunger. Meanwhile, she was unaware that the real culprit was a lack of a vital mineral in her diet. Increased intake of calories contributed to her weight gain, giving her a deceptively healthy appearance; cornstarch, which in 1958 contained negligible amounts of iron, only contributed to masking the problem.
In the years that followed, I reflected many times on my frustrating experience in that metropolitan prenatal clinic. After I had immersed myself in evolutionary theory, I wondered why Mother Nature would favor the evolution of mothers whose internal circuit breakers did not protect them from endangering their babies and themselves. Humans, like all animals, have sensitive trip wires alerting us whenever salt, water, oxygen, or carbohydrate levels enter the red zone. Why were women in general and pregnant women in particular so blissfully unaware of their susceptibility to iron deficiency?
Human intelligence is measured, for want of a better standard, in IQ points. In the long march from anthropoid hominid toward Homo sapiens, the piling up of one IQ point upon another eventually propelled our species to the top of the IQ heap. Low maternal iron depots ultimately subtract IQ points from offspring, thus subverting the entire thrust of human evolution. Our speciesí females, it would appear, had traded away sufficient maternal hemoglobin levels, but for what? Where, I wondered, was the beneficial offset that could possibly justify such a Faustian bargain?
Curious about this matter, I sought the answer in my authoritative physiology textbook. The explanation proffered was that males led a more strenuous life than females and therefore needed a more robust aerobic circulatory system. Although initially I accepted this answer, as I grew older I increasingly suspected that it could not be the complete story. I doubted that any man engages in a more metabolically taxing activity than trying to form a seven-pound infant over a nine-month period from scratch.* Also, the drain upon the energy stores of a breast-feeding mother are of the same order of magnitude as the demands made upon an Olympic shot-putter during his rigorous training.Ü
A similar disparity appears in the CBC in some other animal species, such as chimpanzees and gorillas. Hematologists suspect that this differential has something to do with the red-cell-enhancing effect of testosterone and the red-cell-depressive effect of estrogen. (Men treated with estrogen for prostate cancer are at risk to develop anemia.) Compared with these other animals, however, the differences between a manís and a womanís CBC are deeper and wider. This variation gains in significance because the other species that manifest an inequality in the number of red cells between the sexes do not have a brain as large as humans do, and as we shall see, the human brain is very sensitive to any factor that diminishes its ability to gain access to oxygen. Also, anemia is quite rare among pregnant primates, yet it commonly occurs in the human line. Many years later, after learning more, reading more, and experiencing more, I began to harbor a growing suspicion that an important missing piece of the human evolutionary puzzle was hiding somewhere behind the innocuous-looking numbers I had noticed that morning so long ago on a routine CBC slip.
I confess that I had difficulty staying focused on this subject during medical school. An overwhelming avalanche of other facts, lessons, and procedures cascaded down upon me, and distracted me from pursuing the question. Besides, the subject of blood was not my passion in those days. As a teenager, I had stumbled onto Sigmund Freudís Civilization and Its Discontents, one of those seminal books that one discovers in youth that rock oneís beliefs. My curiosity whetted, I made subsequent explorations deep into Freudís subterranean warren. I was so intrigued by what I was learning there, I became convinced very early on that my destiny was to become a psychoanalyst.
Looking back now, I think it would be difficult for someone who had not lived through the 1950s to appreciate just how profoundly Freudís ideas permeated the intellectual climate then. A rough indicator of his influence was that over 30 percent of my 1957 medical-school freshman class had initially aspired to be psychiatrists. (Many medical students switch from their original choice of specialty, as did a lot of my classmates.) I envisioned that I would become a dauntless spelunker of the mysteries of the mind. Psychoanalysis seemed to be the ìopen sesameî to the entrance of that dark cave.
As a young boy, I had loved to build model airplanes and play sports. I enjoyed drawing and fancied myself a budding artist. When I rotated on the surgery service, I discovered, to my delight, that this field of medicine provided an outlet for the skills I was incubating. Surgery seemed a perfect fit for me. It was romantic, challenging, and intensely exciting. The drama I witnessed daily, combined with my sensing the enormous gratification surgeons seemed to enjoy, changed my mind about a career choice. I decided to specialize in general and vascular surgery. Though I remained faithful to my aspiration to be an explorer, now I decided I would navigate the interior passageways of the body.
Maintaining adequate circulating levels of red cells in my patients became a central concern of mine. As a young surgical resident at Bellevue Hospital in New York, I experienced an exhilarating frenzy while running with a group of emergency-room nurses and doctors gripping a gurney carrying a gunshot victim. Our goal: Reach the operating room before the patient ìbled out.î
Throughout my career, I have, on occasions too numerous to count, glanced up from a particularly vexing operative field to monitor anxiously a forest of IV poles, their precious transfusions hanging like ripe fruit. I have had many years during which to contemplate the nature, importance, and vitality of this substance we call blood.
These musings have convinced me that significant differences in iron levels between the sexes were the initial driving force behind many uniquely human cultural innovations. I shall trace the connections of art, calendars, marriage, mayhem, fatherhood, and homosexuality, to name but a few, back to this arcane feature of human physiology. Along the journey, I shall also explore the links between the moon and menses, sex and death, and funerals and paternity.
Throughout my life, I have maintained a lively interest in matters relating to life-forms and evolutionary theory. As a premed undergraduate learning the wondrous stages of embryology or comparing the anatomy of differing species, I recall having moments of pure rapture whenever I grasped the intricacies behind some aspect of the breathtaking beauty of life. The study of its sumptuous diversity is, for me, the quintessential melding of science and aesthetics. I still marvel over the improbability of a caterpillarís metamorphosis, the texture of a calla lilyís singular petal, or the amazing grace of a formation of pelicans skimming waves. I will try to infuse this book with my sense of awe and enthusiasm for the processes of life while I elaborate my theories on the bonding glue that holds human relationships together.
Some might wonder why a surgeon would dare to wander so far from his field of specialized expertise to enter the bramble-ridden thicket of human sexuality. The simple answer: It fascinates me, as, I suspect, it does you. And I believe I have some fresh insights to contribute to the subject.
My direct experience with matters relating to physiology, anatomy, biochemistry, and psychology, essential to my training as a physician and surgeon, has served me well in researching this book. My knowledge of anthropology, primatology, evolutionary biology, and archeology is the result of my abiding interest in these fields. However, I must admit at the outset that I am not an expert in all the fields into which I delve.
Sex, Time, and Power is intended for both generalists and specialists. Although it is based on scientific research and theories, I wish to keep to a minimum the standard academic practice of citing the pedigree of a particular idea by listing the numerous authorities who were involved in its lineage. I do not wish to diminish these innovators, but I also do not want the book to read too much like a textbook. I also will resist the temptation, whenever possible, to use arcane zoological names, or to cite daunting statistics.
Space requirements imposed by the publishing world prevent me from presenting every alternative theory to the ones I propose. This does not necessarily mean that I am unaware or dismissive of other possibilities, but, in a bow to the wordsmithís aesthetic, I will try to keep many supporting facts, counterarguments, and authoritative references caged in the footnotes and endnotes.
One of the great pleasures I derive from writing is conjuring metaphors that can translate complex physiological and evolutionary processes into rich images, increasing an ideaís accessibility for the nonspecialist reader. A key metaphor that I will employ throughout this work is to treat the process of natural selection as if it behaved like an intelligent entity with forethought and purpose. Exercising a writerís prerogative to use poetic license, I will use the terms ìMother Nature,î ìNatural Selection,î and ìthe Red Queenî interchangeably.*
My use of these terms, however, should not be misconstrued. I am not imputing purposeful design to a supernatural entity. Natural selection is a natural process. The origin of species does not need a deus ex machina* to explain how it works.
Darwinís ideas have been put to the rigors of scientific examination for a century and a half, and although there still remain many intriguing questions, overall he has provided scientists with a powerful predictive tool. Whether or not a supernatural entity first set in motion evolutionís ingenious processes of natural selection based on random mutations of genes interacting with environmental changes is a religious question better left to each reader to answer.
While on the subject of disclaimers, let me neutralize the contentious nature-versus-nurture debate at the outset. There is no gene-controlled inheritable trait that cannot be altered by the environment. Similarly, the genetic makeup of the organism can overcome the influence of the environment. Each factor can affect and alter the other. Humans enter the world as a work-in-progress. In some cases, the culture or environment into which a person is born more strongly determines his or her responses to the vagaries of life, and sometimes responses are more influenced by the genes he or she has inherited. Nature/nurture is not an either/or duality but, rather, represents a both/and type of complementarity. I assume that the reader is reasonably familiar with the essence of Darwinís ideas, but there are two notions I feel compelled to differentiate at the outset. After Darwin electrified the intellectual world in 1859 with his theory of evolution based on natural selection, he refined it further in 1871 by emphasizing the importance of sexual selection. For, as Darwin realized, it was simply not enough that an organism survived both the rigors of competition and the hardships imposed by its environment (natural selection), it also had to reproduce successfully (sexual selection). Mate selection became a critical factor in accounting for how sexually reproducing organisms came to be the way they were. Males competed among themselves to see who won the right to mate with females. And females, by picking and choosing among myriad suitors, exerted an enormous influence on which male traits advanced in the genome. Males, therefore, tended to be what females wanted them to be. Natural Selection is about survival, and Sexual Selection concerns reproduction. I will use the term ìNatural Selectionî as the generic process moving evolution along, even when there may be elements of sexual selection combined within it.
Another caveat: Because the story I intend to tell unfolds in a linear, sequential narrative, it might appear that I am proposing that first one thing occurred and then that caused another to occur, with cause and effect clanking along in a prescribed sequence. This is not the case. The process of Natural Selection is a to-and-fro, give-and-take, nonlinear whorl. Ongoing, simultaneous feedback loops between local environments and individuals are the driving force compelling species to alter their shape, behavior, and metabolism. This ever-changing, continuous ebb and flow creates a fluid dynamic that linear narrative can never adequately convey.
Because I will be writing at length about matters relating to sex, birth, and death as well as the love between a man and a woman and between parents and children, I feel I should share something of my personal background in addition to my professional qualifications.
I was born in Detroit, Michigan, the youngest of four children, to first-generation Russian immigrants. My father was an extremely hardworking man who brought with him a distinctly Old World view of the place of men and women in society. My mother was gentle and loving, and laughed easily. Their relationship was a typical patriarchal one. They both taught me many valuable lessons worth emulating, and some that I have striven not to repeat. They remained married for sixty-five years and lived into their nineties.
I made the usual teenager and young-adult explorations into parties and dating, then, at twenty-seven, I married, after a tempestuous and passionate four-year courtship in which each of us experienced the dramatic highs and lows of young love. Following a stint in the army in France and a surgical residency in New York, we settled in northern California, where I finished my surgical training. During this time, we had three children one right after another: a daughter, Kimberly, a son, Jordan, and then another daughter, Tiffany.
After seventeen years of marriage, my wife and I divorced in the same manner as we had courted. I remained single for an equal number of years, during which time I had the opportunity to participate in the ìdanceî a second time around, but this time as an older, marginally wiser, but more observant ìdancer.î
I have often contemplated the nature of the persistent longing present in the majority of the hearts of both men and women. Persons of each sex, no matter how old, seem to strive to find their respective soul mates. Four years ago, I found mine. Ina and I married, each for the second time.
A judge and a surgeonósome combination. When we cook together, I, in the manner of my professional training, place my hand palm-up without looking away from the slicing and the dicing and bark, ìTomato!î Ina laughingly intones, ìMotion overruled.î We see ourselves engaged in a grand adventure. We have set out to illustrate that a man and a woman can love each other and mesh both our needs and identities in such a way that the sum of us together is greater than each half alone.
I think of my life as resembling an onion. Each layer symbolizes one of the many roles that I have assumed. Son, brother, lover, husband, teacher, student, father, doctor, writer, surgeon, scholar, and lecturer constitute the main ones. The role I would place at the very core of the onion is the one I have cherished the most: that of a father.
I have known the delicious delight of carrying a freshly bathed, flannel-encased, sweet-smelling, sleepy toddler to his or her bed. I have run alongside three different bicycles each recently divested of training wheels and then...let go, to cringe in anxious anticipation of whether I had judged the moment of my release correctly. And that was just the beginning of a whole series of wincing withdrawals.
The military issues campaign ribbons to personnel who have served in various wars and skirmishes to wear on their chests so that comrades-in-arms can instantly identify each other. I recommend that similar insignia should be displayed by all parents who have survived the harrowing teenage years so that they, too, can acknowledge each otherís experience. Happy to say, our unit made it through the guerrilla warfare of those years. Those who had been temporarily missing in action are now all present and accounted for. I have lived long enough to watch proudly as my children have grown into high-spirited, accomplished, interesting people.
My children are adults now, forming their own families. Listening to them tell me about their loves and courtships has afforded me the opportunity to observe how this man-woman thing works once again, but in a different generation. The fight for love and glory, it seems, is still the same old story.
And now, let me tell you of my qualifications to write about death. At the age of thirty-seven, I was diagnosed as having a non-Hodgkinís lymphoma. I underwent extensive surgery, followed by many months of health-debilitating radiation treatments. The experience shook me to my very core and changed me as a human being. As Samuel Johnson astutely remarked, the prospect of hanging clears a manís mind wonderfully. I spent many dark, sleepless hours contemplating the meaning of life and the consequences of deathóparticularly mine.
After over a year of treatments and their inevitable complications, I recovered sufficiently to resume my surgical career. I began to receive many referrals for surgery of patients in the same dire straits that I had recently passed through. I suspect doctors believed that their patients would relate better to a surgeon who had just endured what they must now suffer.
Treatment for cancer, unfortunately, often failsómore so twenty-five years ago than at present. Many times, after I had developed a close connection with a patient, circumstances demanded that I abandon my mission as a healer and assume the role of Charon, the mythical boatman who ferried souls across the River Styx to the other side. My own personal experience and intimate contact with dying patients led me to explore many of the issues I will raise in this book.
All writing, despite authorsí best efforts to conceal themselves behind the scrim of objectivity, contains intimations of the autobiographical. Although somewhat unorthodox, I have briefly outlined my personal history so that you, the reader, may know something of the perspective I bring to this work.
I have chosen artwork and other illustrations to accompany my narrative. Please see pages 403ñ404 for art credits.
Above my writing desk hangs a quote from Franz Kafka urging writers to create books that ìcan be wielded like a pickax to shatter the frozen sea within the readerís mind.î If a book didnít change the way the reader thought about the world, then Kafka deemed it not worth writing. I have taken Kafkaís words as my credo. May this book set your mental ice floes grinding against each other.
A well-worn metaphor draws the analogy between an author finishing a book and a woman birthing a newborn. Observing my odd cube-shaped ìchildî in the form of a neat stack of freshly printed pages sleeping peacefully on my desk this fine morning, I can sense how the Old Testamentís Jochebed, the mother of Moses, must have felt.
The moment has arrived to tear my baby from my protective embrace and place it in its basket, preparatory to setting it adrift down the river. Like Jochebed, I, too, fervently hope that the result of my labor will become entangled in the bulrushes and find a hospitable home among accepting strangers.
Leonard Shlain, 2002
Mill Valley, California
-Hemoglobin is measured in grams per deciliter.
-Significant shifts occurring in red-cell and plasma concentrations in the last trimester of pregnancy somewhat confuse the issue of anemia; nevertheless, iron deficiency remains a considerable problem. A fetusís brain may develop normally within a slightly anemic motherís womb, but the crisis that is birth often sharply diminishes the oxygen supply to the fetusís brain. Even slight anemia in the mother can translate into grave consequences for the babyís brain if her delivery is difficult or prolonged.
-A woman must consume 80,000 calories above her normal maintenance requirements to accomplish the feat.3
ÜA nursing mother needs 76,000 surplus calories above her normal intake to satisfy the demands of breast- feeding.4 Behavioral ecologist Hilliard Kaplan estimates that provisioning a child until he or she reaches the age of independence requires 10 million to 13 million calories!5
-Evolutionist Leigh Van Valen suggested that evolution resembled the contest the Red Queen describes to Alice in Lewis Carrollís classic Through the Looking-Glass. She tells Alice, ìIt takes all the running you can do to stay in the same place.î Van Valen proposed that the process of natural selection works similarly. Once either a prey or a predator evolves an advantageous adaptation that foils the intentions of the other, then the other side must adapt to the new situation between them or face the possibility of extinction. After the new adaptations are in place, equilibrium is restored and both parties are generally back to where they started, just as the Red Queen described.6 Author Matt Ridley titled his very readable 1993 book concerning sexual evolution The Red Queen.
-The Latin term refers to having to invoke a miracle to explain a feature of the natural world that confounds reason. The metaphor is based on the common practice among classical playwrights of resorting to the arrival of a god onstage by means of a mechanical contrivance. Writers used this ploy to get past a difficult transition in their plots.
Unknown Mother/African Eve
Sex endows the individual with a dumb and powerful instinct, which carries his body and soul toward another; makes it one of the dearest employments of his life to select and pursue a companion, and joins to possession the keenest pleasure, to rivalry the fiercest rage, and to solitude an eternal melancholy. What more could be needed to suffuse the world with the deepest meaning and beauty?
The reconstruction of evolutionary history is better regarded as a game than as a science, evolutionary hypotheses should be stated with varying degrees of confidence always keeping in mind that certainty cannot be achieved.
She died an agonizingly slow and painful death. She was not accorded funerary rites, nor was her corpse laid to rest in a grave. Her remains constitute but a sliver of debrisóa disconnected tooth here, a chip of a fossilized bone there, fragments lost in the strata of bygone ages. At the time of her death, she represented the latest in a line of primates called ìhominidsî that had begun their evolutionary trial run several million years earlier. If paleontologists ever find her final resting place, we should erect a memorial on the spot in recognition that she did not die in vain. An appropriate name for her marker would be ìThe Tomb of the Unknown Mother.î Her passing heralded the birth throes of a new species.
Imagine that a group of intergalactic anthropologists had been observing these primates from the beginning. When Unknown Mother died, the visitors would have exchanged knowing looks, because they could plainly see that her fate was foredoomed. The hominid line from which she arose had split away from other primates by developing two adaptations destined to collide. Hominids were the only primates to depend on a new means of moving about that required only two limbs instead of four. An upright stance allowed them to clamber down from the trees and seek a living first on the forest floor and later on the open savanna. Because their erect posture greatly increased the possibility that the first creature to stride would end up as ìcat food,î they needed a crucial second adaptation. Since they could not outrun or outfight predators, they required an enlarged brain capable of outwitting those creatures intent on devouring them.
During the last two and half million years, the hominid brain had tripled in size but the opening in the pelvic girdle through which this rapidly enlarging brain had to pass at birth did not keep pace. These two adaptationsótwo- leggedness and watermelon-sized headsówere clearly incompatible.
The new engineering imperatives of standing upright had sculpted the hominidís pelvic ring of bone into a new shape, flattening it from front to back. The bipedal pelvis, anatomically dissimilar to its counterpart in four-legged animals, also acquired a novel architectural function. It had to serve as a basin to contain the mass of intestines pressing down from above and prevent them from falling down and out through the rectum. Consequently, the bony hole in the pelvis had to remain relatively small. Only the wide, comparatively horizontal flanges of the human iliac pelvic bones, the narrowness of the pelvic inlet, and the thin sheet of muscles suspending the anus prevented this unusual primate from having the discomfiting experience of being turned inside out while out for a stroll after a particularly heavy lunchóa gravitational hazard that does not pose a problem for any other animal.*
These functional constraints prevented the channel in the femaleís pelvis from enlarging sufficiently to accommodate easily the continually growing size of her fetusís brain during childbirth. Mother Nature devised numerous ingenious sleights of hand to thread the baby through the ìeyeî of a motherís birth canal.Ü Despite these clever adaptations, hominid females began to experience increasingly difficult deliveries. The problem became especially acute around 150,000 years ago, at which point the hominid brain had completed a remarkably short burst of rapid inflation that had added one-third to its size. A disaster was in the making.
Eventually, somewhere, sometime, a healthy young hominid had growing within her a new life whose head was simply too large to negotiate the confining walls of her birth canal. During the delivery, her baby became wedged. After a prolonged labor, she died. Her baby died. Those in attendance could do nothing to help. The laws of physics superseded the strength of her uterine contractions. Unfortunately, she was the first of an avalanche of young mothers to die. For the first time in the history of any higher animal, extraordinarily high numbers of healthy females began to die in childbirth; the percentage of stillbirths rose with the number of maternal deaths.
The number of live progeny per mother at the outset of our species was low, because prolonged childhoods forced ancestral women to space their pregnancies far apart. Moreover, one child per pregnancy was the general rule. Young children who lost their mother during a subsequent delivery experienced a catastrophe. Their prospects of surviving without her were bleak. Even a small percentage of mothers dying in childbirth in each generation, especially when combined with factors like disease, drought, or predators, could have placed great stress on a local population.
In a supreme paradox, the leading cause of death for females of the human species became birth. A cursory examination of dates on old gravestones in any cemetery prior to the twentieth century confirms the high mortality routinely associated with childbirth, a condition that does not exist for any other mammal. No female of any other species has as much difficulty bearing her young as a human. And no female of any other species routinely solicits and requires help from others to deliver her baby.*
The death of the Unknown Mother signaled the onset of an evolutionary crisis. The loss of a significant number of mothers and their newborns in childbirth was a wasteful reproductive strategy that could have been expected to toll the death knell of the line. Yet it created precisely the kind of crucible in which a species must adaptóor die.
Scientists working in the field of evolutionary biology hypothesize a mechanism to explain how a new species often seems to appear all at once in the fossil record. Imagine an isolated local population of an existing species living in harmony with its ecosystem. Suddenly, some new, harmful environmental factor impinges upon the system; large numbers of the local population begin to die. At the eleventh hour, a beneficial random mutation (or mutations) that had previously occurred in the genes of one individual increases its ownerís chances of surviving to the next generation.* The offspring of this fortunate individual inherit the gene (or genes) and it quickly spreads. Within the span of several generations, the hard-pressed local population that was on the verge of extinction surges back by evolving an innovative suite of internal metabolic adjustments, physical changes, or modified behavioral responses that allows it to adapt to its new circumstances.
The animal to emerge sometimes differs so significantly from its predecessor that it can be categorized as an entirely new species. Scientists refer to this large dying off of the many so that the few (or even one) can evolve as ìpassing through a bottleneck.î When there is a sudden discontinuity between a precursor species and a new one, some scientists propose that this evolutionary process is due to what they call ìpunctuated equilibrium.î4
Many conditions can precipitate bottlenecks. Geologic catastrophes, major volcanic eruptions, abrupt climatic changes (such as the sudden onset of ice ages), pluvials (periods of rains of Biblical proportions), and prolonged droughts can all position a species in the crosshairs of extinction. Epidemics of viruses, bacteria, or parasites can decimate food sources or attack the local population directly.
Approximately 150,000 years ago, in a small region of East Africa, around present-day Uganda, Kenya, and Tanzania, the current countries bordering Lake Victoria, one such bottleneck occurred. A local population of Homo erectus, a tool-making hominid, had been living there successfully for over a million years.Ü Then some yet-to-be- identified event occurred that affected the survival of this particular group of hominids. From this stressed band, a single female known as Mitochondrial African Eve succeeded where Unknown Mother had failed, giving birth to the new species originally classified as Homo sapiens sapiens,* the doubly wise human.
Though the exact birthdate of our species remains uncertain, the scenario that a single woman birthed the modern human species is on firmer scientific grounds thanks to the reliability of the new science of molecular biology.5 Laboratory tests performed on mitochondrial DNA can accurately measure the genetic variation that exists between members of a species and the differences existing among species. Scientists can then construct ìmolecular clocksî and calculate how long ago a particular species split away from its precursor.6 Molecular biology has proved to be the great Rosetta Stone of evolutionary changes. The existence of an African Eve is extremely likely, because the genetic material of all humans alive today is eerily similar.
The genes of chimpanzee communities inhabiting ranges only a few thousand yards apart have more genetic diversity than those of humans separated by oceans. Despite the dramatic differences in the skin pigmentation, eye color, body shapes, and hair types of people from disparate regions of the world, all humans are genetically homogeneous to an extraordinary degree. In fact, there is less than 0.1 percent difference between the gene structure of any one human and another. This suggests that each of us is a not-so-distant descendant of one fairly recent ancestral female. Since we have not had time to diverge very far genetically, our speciesí birthdate can be calculated backward in tens of thousands of years, instead of millions.
Some dire factor, condition, or event adversely affected the species that lived in the area around present-day Lake Victoria, leading to a population bottleneck. Let us call it Factor X. But what was X? What environmental challenge could have been the catalyst for the radiation of a new species? The geologic and archeological record is relatively silent. Variations in the local climate did occur, but none seems harsh enough to prompt our origin. Scientists have not identified sudden discontinuities in the areaís flora or fauna. And yet some extreme condition must have occurred, for African Eve to burst forth like Athena fully formed from the brow of Zeus. Though there are many competing scientific theories, none has managed to gain sufficient support to explain the bottleneck fully.
I propose that the ìbottleneckî through which our unfortunate immediate ancestors squeezed was actually a real bottleneck. Scientists scouring the landscape in search of an external Factor X may have been looking in the wrong place. The precipitating event that pushed a local population of hominids toward the edge of extinction was neither a climate change, a geological force, the arrival of predators or disappearance of prey, nor a shift in the availability of food resources. It was an internal, anatomical one.*
The unyielding walls of the birth canal, like the alignment of Scylla and Charybdis,Ü produced the bottleneck that shaped all subsequent hominid evolution. The death of the Unknown Mother and her unlucky baby, and the subsequent dying off of increasingly large numbers of hominid mothers and their newborns, was the stressful Factor X that precipitated the Homo sapiens line.
*The muscles making up the human pelvic floor formerly served the genial function of wagging animalsí tails. Natural Selection urgently pressed what was left of them into a new use in the bipedal hominid. They now served to buttress a potentially lethal defect. Some intermittently upright animalsófor example, penguinsóhave evolved similar adaptations to defend against this problem, but in no other species is the gravitational hazard as serious as it is in humans.
ÜThe soft bones of a human babyís skull resemble tectonic plates. As the infantís head wends its way down the motherís tortuous birth canal, the plates slide and bend to conform to each twist and turn. To assist this molding process, the bony circle of the motherís birth canal relaxes. Under normal conditions, the iliac, pubic, ischial, and sacral bones are welded together by dense bridges of cartilage nearly as rigid as the bones they join. During delivery, however, this tissue undergoes a remarkable transformation, akin to concrete dissolving into Silly Putty. As the fetal head progresses, the pelvic circle, in a complementary maneuver, stretches imperceptibly, its new elasticity conveniently conforming to the mush-skull pushing through it.
-There have been sightings of dolphins and whales attempting to assist pregnant females of their kind with deliveries. In one spectacular example observed in captivity, three different species of dolphins were involved. A full-term female was in trouble: Her newbornís dorsal fin was caught in her pelvis. The second dolphin pulled out the baby and assisted the mother to raise it to the surface. While this was happening, the third female delivered the afterbirth, using her teeth. A few land mammals, such as rodents and primates, may also offer limited assistance to a female in labor. Despite these isolated reports, what is clear from observations of many animal births is that no other speciesí full-term females routinely signal their need for birth assistance. Contrary to popular myths, indigenous women do not simply go into the field and bear their babies alone. In a cross-cultural study of 296 peoples, only 24 reported that a woman on occasion has her baby without assistance. In none of the cultures studied was an unassisted first birth a routine event.3
*A single geneís sequence of DNA contains the instructions for how to build a protein, which in turn can become an enzyme that further directs the building of an organism. Since there are many variations on the 3-D configuration of proteins and the timing of their entry into the building schedule, a single gene can have an enormous impact on the final form, metabolism, and responses of an organism.
ÜTo avoid a clutter of scientific terms, I will gloss over the subtle distinctions between the fossils referred to as Homo heidelbergensis, Homo ergaster, archaic Homo sapiens, and many other recent paleontological finds. Each new detail gleaned from studying these ancient bones adds incrementally to our understanding of the evolution of our species, but, unfortunately, to do the subject justice would, I believe, distract from my narrative. Also, a species can evolve in response to a positive development in its environment. A new untapped food source, for example, can prod a species into evolving novel adaptations to take advantage of the bounty. I conjecture that the dominant influence affecting our species was a negative one rather than a positive one, however.
-This term is now outmoded. With the recent identification of Neanderthal DNA, it is no longer necessary to call us Homo sapiens sapiens, except for occasional emphasis. For the rest of this book, I will use the current classification of our species, Homo sapiens.
-This is the ìobstetrical dilemmaî first described by Sherwood Washburn in 1960 and elaborated by others, particularly Wenda Trevathan and Karen Rosenberg.7 The major focus in the literature has been on the effect of difficult labor on child development. The narrowness of the human female pelvis caused infants to be born alitricialóthat is, extremely immature. According to estimates based on the size of other primate infants, the length of a human pregnancy should be eighteen months instead of nine. Bringing infants into the world long ìbefore their timeî created unique survival problems.
Helpless babies imposed immense child-rearing responsibilities on mothers, forcing a drastic division of labor between the human sexes. And it required women to enjoin men to assist them in raising their offspring, since failure in this endeavor would have fatal consequences for the entire species. It also created novel opportunities for children to have a longer period, called childhood, in which to learn. Much has been written about the consequences of the prolonged human childhood. I wish to redirect the focus away from immature infants to what I consider to be a relatively neglected aspect of the human speciesí obstetrical dilemmaónamely, maternal mortality.
ÜFrom a Greek myth, Scylla and Charybdis were two dangerous obstacles between which Odysseusí ship had to navigate during the odyssey.
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