Evolution and the Primitive Brain of a Newborn

Why Infants Arrive Unfinished

Blame Lucy. In the throes of labor contractions and delivery, remember that it was this 3.2-million-year-old human ancestor who first had the big idea to stand up and walk on two feet. Lucy is considered by many scientists to be the mother of humankind, and her skeletal remains, discovered in 1974, provided scientific evidence of one of the first upright walkers in our family tree. From that point onward, the human ability to walk on two feet would demand some reworking of the adult pelvis and a major overhaul of the birth canal. Those evolving alterations would, in all descendant female hominids leading up to Homo sapiens, introduce a host of inefficient twists and turns, making the process long and painful for mothers and a brutal challenge for babies.

Lucy's hypothetical offspring, no longer able to survive with the limited brainpower required to climb a tree or flee from danger on four legs, needed more time to grow a bigger brain in order to outwit predators. But a maximum of forty weeks' gestation is all that biology allowed our ancient ancestors and modern babies. They would need a fourth trimester of intense development experienced outside the uterus while remaining physically and emotionally bound to their mothers, just as they had been for the previous nine months. The tight fit through that circuitous birth canal set absolute limits on how much brain development could occur during pregnancy. The additional brain growth required to keep the species thriving would have to happen outside the uterus, an astonishing amount of it occurring during the fourth trimester.

Modern human infants are at the receiving end of millions of years of evolutionary progress, but the tradeoff for upright walking has been immature brain development at birth.

Walking on two feet has been a mixed blessing. Standing upright altered the entire skeletal structure, and the changes came with physical costs for males and females: flat feet, aching backs, and stiff necks. "Ultimately, every part of the human body had to change to adapt to bipedalism," says Dr. Wenda Trevathan, evolutionary anthropologist at the University of New Mexico.

Women and infants have suffered especially onerous consequences of upright posture. What was once a straight shot down a roomy birth canal in our four-legged ancestors has evolved into something akin to an amusement park ride through the modern female pelvis.

Once humans had only two feet for mobility, they could no longer climb as well as, or run as fast as, the local lions and tigers and bears. Since then, they've had to rely on brainpower to escape predators. The pressure for increased intelligence was on, and our ancient forebears began to grow brains far larger than ever before. In fact, the human brain had to get bigger, or our species would have died out. Those outsized brains, and resulting intelligence, began to change the world.

As the human brain has grown in size during evolution, the additional brain growth needed for survival has had to take place after birth because there simply is no extra room in the birth canal for a bigger head. After roughly nine months in the uterus, emerge babies must, ready or not. Most aspects of brain development are delayed until after birth. "And that means the baby is a little more unfinished, if you will," says Dr. Trevathan. "The evolutionary compromise is that about 75 percent of human brain development takes place after birth." That's in contrast with the rest of the animal kingdom. Most animals are born with their brains about half developed, but today human infants are born with only 26 percent to 29 percent of their brains developed. And so, with a brain only about a quarter of its necessary size, the newborn needs a fourth trimester of development, with comforts similar to those enjoyed in the womb preparing him for life in the world.

THE BIRTH RIDE

The evolutionary compromise between the need for a large brain and the confines of a narrow birth canal continues with modern-day infants. They arrive extremely neurologically immature and completely dependent on adults.

Leading, in most cases, with a head that can accommodate roughly a quarter of the brain mass she'll eventually require, the fetus is forced to negotiate a series of turns aligned with the widest parts of the pelvis. The entrance of the birth canal is widest from side to side. About halfway through, the orientation shifts about ninety degrees, and the fetus must turn her large head to make it through. So the infant starts her journey facing her mother's side. Midway she must shift her head to face her mother's back. As the fetus's head turns from facing her mother's side to facing her back, she goes through a series of rotations as she passes through the birth canal. Once the head has emerged, the shoulders must shift, so the baby turns her head to the side, rotating her shoulders so they, too, can make the tight squeeze between pubic bones and tailbone.

The average infant head is ten centimeters from front to back. It's little wonder that childbirth hurts, considering that the average woman's pelvic opening is thirteen centimeters at its largest point and ten centimeters at its smallest point.

The quadruped ancestors of modern humans, with larger birth canals and smaller brains, once might have given birth in solitude—like chimpanzees, orangutans, and gorillas can. However, because of the revised size and position of the human female pelvis, women need midwifelike assistance to give birth. If the mother reached down to assist her own baby's birth, she would risk injuring her baby by bending his back against the natural curve of the spine.

As a result, not only did human bodies change with upright walking, but society also had to change in ways that could accommodate the demands placed on the mother by the baby. First, mothers couldn't deliver in solitude. Once here, babies could not cling with hands and feet, so mothers had to use one arm to hold them and, often, the other arm to quiet them when danger lurked. With hands occupied, human females needed help. They needed fathers to stick around. One of the profound consequences of evolution, including the amusement-parkride aspect of birth, is that it has forced humans to be interdependent and social. New mothers need help in birthing their babies—whether from an obstetrician, a midwife, a father, or an unlucky cab driver—and then they...