...this is the first chapter
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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 atNew MexicoStateUniversity. "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,Rosenbergsays. 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,Rosenbergsays, 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,"Rosenberganswers. 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,"Rosenbergsays. "It works, but only marginally. Its 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."
