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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?

— George Santayana1

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.

— Sherwood Washburn2

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 debris a 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 "hominids" 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 "The Tomb of the Unknown Mother." 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 "cat food," 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 adaptations two- leggedness and watermelon-sized heads were clearly incompatible.

The new engineering imperatives of standing upright had sculpted the hominid's 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 lunch a gravitational hazard that does not pose a problem for any other animal.

These functional constraints prevented the channel in the female's pelvis from enlarging sufficiently to accommodate easily the continually growing size of her fetus's brain during childbirth. Mother Nature devised numerous ingenious sleights of hand to thread the baby through the "eye" of a mother's birth canal. 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 adapt or die.

© 2004 Penguin, a division of Penguin Putnam, used by permission.

Tags: Sex and Romance

About the Author

Leonard Shlain is the author of Art & Physics: Parallel Visions in Space, Time & Light, and The Alphabet Versus the Goddess: The Conflict Between Word and Image. He is the chief of laparoscopic surgery at California Medical Center in San Francisco.