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.3 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.4
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-park-ride 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 need help in bringing them up. In our modern society, that help often comes from a traditional source: fathers. But it also comes from gay or straight partners, adoptive mothers and fathers, foster families, grandparents and other family members, and loving caretakers of all sorts.
FOURTH TRIMESTER BRAIN DEVELOPMENT
Scientists now know that the brain continues to change and grow, allowing for a lifelong ability to reorganize neural pathways based on new experiences. That ability is called neuroplasticity.5 But while recent discoveries suggest that new neurons are produced throughout life, it doesn't happen nearly as rapidly as it does during the nine months spent in the womb. Some 100 billion neurons form during pregnancy. At birth, all those neurons are as yet incapable of communicating with each other.
But nature has made sure that the neural circuits responsible for basic body functions are up and running at birth. Infants arrive with the most basic and primitive operating equipment, under the control of the lower parts of the brain. During gestation, the basic architecture of the brain is laid down, beginning development soon after conception. That prenatal architecture eventually includes the brain stem, or lower part of the brain, regulating the central nervous system and cardiac and respiratory functions; the thalamus, two bulb-shaped masses above the brain stem that process and relay sensory information; and the cerebellum, which coordinates motor movement. Those parts direct the infant to kick, grasp, cry, sleep, root, suck, swallow, keep a heartbeat going, and manage a circulatory system. It's all primitive or immature, and the higher centers, those in charge of emotions, intelligence, planning, and motor responses, are still waiting to be formed, influenced by love, conversation, comforting touch, faces, movement, sound—in short, the world he was born into.
The work begins almost immediately. Each newborn is busy developing neural connections by laying down a network of dendrites, branched projections that receive signals of communication and pass them on with the aid of neurochemicals. The connections formed are called synapses. During the first three years of human life, there is an unprecedented pattern of rapid synapse formation. In fact, babies develop so many synapses there simply isn't room for them all, and those that aren't used go by the wayside. The ones that remain get more efficient at providing the information we need.
This is how it works. Neurons are cells specializing in sending and receiving signals. A neuron in the eye gets its signal from light; in the ear, from sound vibrations; in the nose and tongue, from molecules that bind to them; and on the skin, signals come from touch. A message travels, via electrical signal, from neuron to neuron to the part of the brain specializing in, say, seeing, tasting, or moving. Then the output side kicks in, sending an outgoing signal to the retina, or the tongue, or a muscle, complete with instructions on how to move, extend, or contract. So even as the brain is constructing a branchlike communication network, it is also beginning to pare down the number of neurons in the brain in order to ease overload, making experience key to wiring an infant's brain.
During that time, an infant's brain experiences sporadic bursts of activity that are known as exuberant periods. At the peak of one of these periods, the brain is creating 2 million new synapses every second, researchers estimate. These bursts of development happen at various times in different areas of the brain during the first months of life and continue, though at a slower pace, through adolescence.6 During infancy, the new connections allow for color vision, the ability to grasp, and a strong attachment to parents. Each baby is sculpting a brain that is becoming truly human and uniquely his own.
Neuroscience has become adept at studying the tiny but interconnected cells of the brain using brain-imaging technology. Going well beyond earlier scientific tools—such as observation, autopsies, x-rays, and EEGs—CT scans, functional MRIs, and PET scans create three-dimensional images of the brain and allow scientists to analyze its chemical composition, its electrical transmissions, and the blood flow through the brain. Through the use of such technology, we now know that when babies are born, they come equipped with more neurons than they'll ever need, and some, but not many, synapses.
The neurons are the raw material of the brain, and heredity determines their number.7 (Only recently has research begun to show that important areas of the forebrain continue to produce new neurons into adulthood.)8 But the infant brain is in a remarkably unfinished state, with its billions of neurons that are unable to communicate with each other. Those connections
