¿Por qué la mujer casi siempre necesita ayuda en el parto y en cambio no lo necesitan hembras de otros animales?

Un famoso artículo de National Geographic contesta la pregunta: “The Downside of Upright”

Jennifer Ackerman entrevista a la famosa antropóloga Karen Rosenberg en National Geographic Magazine.

(artículo en inglés)

All those aching backs may be trying to tell us something: It’s part of the price we pay for walking on two legs.

We humans are odd creatures: tailless bipeds with sinuous spines, long limbs, arched feet, agile hands, and enormous brains. Our bodies are a mosaic of features shaped by natural selection over vast periods of time—both exquisitely capable and deeply flawed. We can stand, walk, and run with grace and endurance, but we suffer aching feet and knee injuries; we can twist and torque our spines, and yet most of us are plagued by back trouble at some point in our lives; we can give birth to babies with big brains, but only through great pain and risk. Scientists have long sought to answer the question of how our bodies came to be the way they are. Now, using new methods from a variety of disciplines, they are discovering that many of the flaws in our “design” have a common theme: They arise primarily from evolutionary compromises that came about when our ancestors stood upright—the first step in the long path to becoming human.

A tight squeeze

In Karen Rosenberg’s laboratory at the University of Delaware, a room packed with the casts of skulls and bones of chimpanzees, gibbons, and other primates, one model stands out: It’s a life-size replica of a human female pelvic skeleton mounted on a platform. There is also a fetal skull with a flexible gooseneck wire. The idea is to simulate the human birth process by manually moving the fetal head through the pelvis.

It looks easy enough.

“Go ahead, try it,” Rosenberg says.

Turn the little oval skull face-forward, and it drops neatly into the pelvic brim, the beginning of the birth canal. But then it jams against the protrusions of the ischial bones (those that bear the burden during a long car ride). More shoving and rotating, and it’s quickly apparent that the skull must traverse a passage that seems smaller than itself, cramped not only by the ischial bones but also by the coccyx, the bottom of the tailbone, which pokes into the lower pelvic cavity. Only by maneuvering the skull to face sideways in the middle of the canal and then giving it a firm push, does it move a centimeter or two—before it gets hung up again. Twist it, jostle it: The thing won’t budge. Rosenberg guides my hand to turn the skull around to face backward, and then, with a hard shove, the stubborn cranium finally exits the birth canal.

“Navigating the birth canal is probably the most gymnastic maneuver most of us will ever make in life,” says Rosenberg, chair of the university’s department of anthropology. It’s a trick all right, especially if there’s no guiding hand to twirl and ram the skull. And the neat two-piece model doesn’t even include the broad, rigid shoulders of the human infant, a legacy from our apelike ancestors who, some 20 million years ago, evolved wide clavicles that allowed them to hang suspended from branches and feed on fruit. To follow the head, a baby’s shoulders must also rotate two times to work through the birth canal; they sometimes get stuck, causing injury to part of the spinal nerves that control the arms.

Suddenly I understand as never before why it took 36 hours, two doctors, and three shifts of nurses to safely deliver my firstborn.

Birth is an ordeal for women everywhere, according to a review of birthing patterns in nearly 300 cultures around the world by Rosenberg and colleague Wenda Trevathan, an anthropologist at New Mexico State University. “Not only is labor difficult,” Rosenberg says, “but because of the design of the female pelvis, infants exit the birth canal with the back of their heads against the pubic bones, facing in the opposite direction from the mother. This makes it tough for her to reach down and guide the baby as it emerges without damaging its spine—and also inhibits her ability to clear the baby’s breathing passage or to remove the umbilical cord from around its neck. That’s why women everywhere seek assistance during labor and delivery.”

Compared with humans, most primates have an easier time, Rosenberg says. A baby chimpanzee, for instance, is born quickly: entering, passing through, and leaving its mother’s pelvis in a straight shot and emerging faceup so that its mother can pull it forward and lift it toward her breast. In chimps and other primates, the oval birth canal is oriented the same way from beginning to end. In humans, it’s a flattened oval one way and then it shifts orientation 90 degrees so that it’s flattened the other way. To get through, the infant’s head and shoulders have to align with that shifting oval. It’s this changing cross-sectional shape of the passageway that makes human birth difficult and risky, Rosenberg says, not just for babies but also for mothers. A hundred years ago, childbirth was a leading cause of death for women of childbearing age.

Why do we possess a birth canal of such Byzantine design? “The human female pelvis is a classic example of evolutionary compromise,” Rosenberg answers. Its design reflects a trade-off between the demand for a skeletal structure that allows for habitual walking on two feet and one that permits the passage of a baby with a big brain and wide shoulders. Its unique features didn’t come about all at once, but at different times in our evolutionary history, in response to different selective pressures. “The result of these different pressures is a jerry-rigged, unsatisfactory structure,” Rosenberg says. “It works, but only marginally. It’s definitely not the type of system you would invent if you were designing it. But evolution is clearly a tinkerer, not an engineer; it has to work with yesterday’s model.”

Yesterday’s model

Humans come from a long line of ancestors, from reptile to mammal to ape, whose skeletons were built to carry their weight on all fours. Our ape ancestors probably evolved around 20 million years ago from small primates that carried themselves horizontally. Over the next several million years, some apes grew larger and began to use their arms to hold overhead branches and, perhaps, to reach for fruit. Then, six or seven million years ago, our ancestors stood up and began to move about on their hind legs. By the time the famous Lucy (Australo-pithecus afarensis) appeared in East Africa 3.2 million years ago, they had adopted walking as their chief mode of getting around.

It was a radical shift. “Bipedalism is a unique and bizarre form of locomotion,” says Craig Stanford, an anthropologist at the University of Southern California. “Of more than 250 species of primates, only one goes around on two legs.” Stanford and many other scientists consider bipedalism the key defining feature of being human. “Some may think it’s our big brain,” Stanford says, “but the rapid expansion of the human brain didn’t begin until less than two million years ago, millions of years after we got upright and began using tools. Bipedalism was the initial adaptation that paved the way for others.”

Evolutionary biologists agree that shifts in behavior often drive changes in anatomy. Standing upright launched a cascade of anatomical alterations. The biomechanics of upright walking is so drastically different from quadrupedal locomotion that bones from the neck down had to change. The skull and spine were realigned, bringing the head and torso into a vertical line over the hips and feet. To support the body’s weight and absorb the forces of upright locomotion, joints in limbs and the spine enlarged and the foot evolved an arch. As for the pelvis: It morphed from the ape’s long, thin paddle into a wide, flat saddle shape, which thrust the weight of the trunk down through the legs and accommodated the attachment of large muscles. This improved the stability of the body and the efficiency of walking upright but severely constricted the birth canal.

All of these architectural changes, seen clearly in the fossil record, did not happen overnight. They came gradually, over many generations and over long periods of time, in small steps favored by natural selection.

Upright citizens

Consider the simple human act of walking or running. At his laboratory in the anthropology department at Harvard University, Dan Lieberman does just that, using biomechanical studies to see how we use our body parts in various aspects of movement. As a volunteer subject in one of his experiments last fall, I was wired up and put through paces on a treadmill. On my feet were pressure sensors to show my heel and toe strikes. Electromyographic sensors revealed the firing of my muscles, and accelerometers and rate gyros on my head detected its pitching, rolling, and yawing movements. Small silver foam balls attached to my joints—ankle, knee, hip, elbow, shoulder—acted as reflectors for three infrared cameras mapping in three-dimensional space the location of my limb segments.

These biomechanical windows on walking and running illuminate just how astonishing a feat of balance, coordination, and efficiency is upright locomotion. The legs on a walking human body act not unlike inverted pendulums. Using a stiff leg as a point of support, the body swings up and over it in an arc, so that the potential energy gained in the rise roughly equals the kinetic energy generated in the descent. By this trick the body stores and recovers so much of the energy used with each stride that it reduces its own workload by as much as 65 percent.