by Chip Walter
It may not seem so on the surface, but toddlers accomplish prodigious amounts of work (all cleverly disguised as play) as they bowl and bawl their way through each day. By throwing a ball (or food), playing in the mud, a pool, or a sandbox, by attempting headlong runs and taking sudden tumbles, or swinging on swings or off “monkey” bars, the young are fervently familiarizing themselves with Newton’s laws of motion, Galileo’s insights into gravity, and Archimedes’ buoyancy principle, all without the burden of a single formula or mathematical term.
When a toddler smiles, cries, grimaces, gurgles, giggles, spits, bites, or hits; when she breaks free of mom or dad for a wild dash down the sidewalk; throws whatever he can grab for the sheer joy of it; dances spontaneous jigs or engages in other diabolical antics—she is learning what is socially acceptable and what is not, what is scary, what works in the way of communication and what fails and when. Food, and much that isn’t food, is tasted, licked, and baptized with slobber to investigate its texture, shape, and taste. Yet no artifice or logic is behind the tasting. It’s just another form of exploration. Objects, living or not, are bounced, swatted, hugged, flailed, closely inspected, all in a fervent effort to comprehend their nature. Unbounded and unstoppable greed for knowledge is the best way to put it.
Acquiring language is another big job in childhood. Babbling, squeals, and other noises are, as the best linguists have so far been able to ascertain, ways of figuring out the language that the other bigger, parental creatures speak around the toddlers whether it is Swahili, German, or Hindi. Later, early conversations are short, generally. “Here! Mama! Dadda! Mine! No! Please. Want!” Often, in times of acute frustration, communication is inarticulate, loud, and punctuated with acrobatic body language. In time, however, and amazingly, vocabularies grow, syntax improves, and full sentences are expressed, all with hardly an ounce of formal instruction. The acquisition of language is one of the great miracles in nature. At age one few children can say even a single word. At eighteen months they begin to learn one new word roughly every two hours they are awake. By age four they can hold remarkably insightful conversations, and by adolescence they have gathered tens of thousands of words into their vocabulary at a rate of ten to fifteen a day and often use them with lethal effect! And nearly every word was acquired simply by their listening to, and talking with, the people around them.1
Children do these apparently lunatic and astonishing things for a reason. Nature has wired their brains for survival by driving them to swallow the world up as fast as they possibly can. Pulling off this feat is easier said than done. However, if we hope to comprehend how cerebral connections this complex take place, it might first be useful to step back and consider why brains exist at all, and how we eventually came by the particular brand we have.
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By general agreement the first brain in nature belonged to a creature scientists today call planaria, known more commonly to you and me as the lowly flatworm. Flatworms are metazoans and wouldn’t seem therefore to be very brainy. But intelligence is a relative thing and planaria, when they first emerged more than seven hundred million years ago, were the geniuses of their time, creatures of unparalleled intelligence blessed with an entirely new kind of sensory cell capable of extracting marvelously valuable bits of information from their environment.
Unlike many of their contemporaries planaria were unusually sensitive to light, possessed rudimentary sets of eyes, and responded to, rather than ignored, changes in temperature—all radical innovations in their time. Even today they remain expert at sensing food, and then making their way with uncanny determination to it, while other metazoans (corals, for example) generally take a more leisurely approach to their cuisine, waiting for it to find them rather than the other way around.
Planaria—the Einstein of the Day
Among the cellular innovations that made an ancient flatworm’s brain possible was a protoneuron called a ganglion cell. These are clustered in the head of the worm and then connected to twin nerve cords that run in parallel down the length of its body so that certain experiences sensed alongside it can be transmitted to the flatworm’s brain for some metazoan cogitation. All the brains that evolution has so far contrived rest on this tiny foundation. So for the best ideas you had today you can thank the determined metazoan that looks something like a squished noodle.2
The purpose of brains generally is to organize the waves of sensory phenomena that nature’s cerebrally gifted creatures experience. Their job is to filter the world’s chaos effectively enough to avoid, for as long as possible, the disagreeable experience of death. A direct correlation exists between survival and how well a brain maps the world around it. The more accurately it can correlate, the more likely it will survive danger, discover rewards, and keep its owner among the living.
At the heart of every brain are its neurons, the specialized cells that make possible our brand of thinking, feeling, seeing, moving, and nearly everything else important to us. There are over 150 different kinds of neurons, making them the most diverse cell type in the human body. To support their greedy habit of consuming large quantities of energy, they are surrounded by clusters of glial cells, which serve as doting nannies busily shuttling nutrients and oxygen to them while fetching away debris and generally working to keep the neurons fresh and firing. Each of us carries roughly a hundred billion neurons clustered jellylike inside our skulls (coincidentally about the same number as stars cosmologists believe populate the Milky Way galaxy). Every one of them is supported by ten to fifty indulgent glial cells.
This makes our brains a remarkable and mysterious place still well beyond the comprehension of the thing itself (a fascinating irony), but the cerebral cortex of a growing human child is more remarkable still. Only four weeks after a human sperm and egg successfully find one another, when we are still embryos no larger than a quarter, clusters of neurons that will eventually become our brain are replicating at the rate of 250,000 every minute, furious by any standard. Around this time, a bumpy neural tube that looks suspiciously similar to a glowworm has begun to take shape. Over the next several weeks four buds within the tube will begin developing into key areas of the brain: the olfactory forebrain and limbic system—the seat of many of our primal emotions; the visual and auditory midbrain, which governs sight, hearing and speech; the brain stem, which controls autonomic bodily functions such as breathing and heartbeat; and the spinal cord, the trunk line for brain–body communication. Two weeks later a fifth cluster of neurons begins to blossom into the frontal, parietal, occipital, and temporal lobes of the cerebral cortex, where so many exclusively human brain functions reside.3
The brain constructs itself this way, with neurons ebulliently proliferating, and then, like the rest of the cells in the embryonic body, they march off to undertake their genetically preordained duties. During this process and throughout our lives, every cell in the body communicates. It is in all cells’ DNA, not to mention our best interests, to reach out and touch one another, mostly by exchanging proteins and hormones. But neurons are especially talented communicators. This is because whenever the biological dice fell in such a way that they came into existence, they began to evolve specialized connectors—dendrites and axons—that vastly improved their exchange of information compared to other cells in the body.
Before brains came along, primitive protoneurons communicated by secreting hormones and electrical currents in no particular direction, mumbling their messages to the other cells and protoneurons in their vicinity, and not getting terribly quick results, at least compared with our current models. With the invention of dendrites and axons, however, they could form elegant, smart clusters that shared at high speed the information each of them held with the others nearby. (Planaria were among the first to accomplish this.)
The emergence of high–speed, if exceedingly minute, communications cables meant that any creature fortunate enough to inherit them could more fully and rapidly sense the world it inhabited—light and dark,
food, danger, pain and pleasure—then react to it all in a blink. Not only that, the cables could link different sectors of the brain the way highways connect cities. This meant the brain could not only improve contact with the world, but also stay in better touch with itself, not a trivial matter as brains grew larger. (This turns out to be important to consciousness, but we will visit that subject later.)
Dendrites generally conduct signals coming into a brain cell while axons do the opposite. Dendrites (also known as dendrons) are so eager to make contact that they extend treelike in multiple directions and can place one neuron in touch with thousands of its neighbors. Axons aren’t nearly as obliging as dendrites, but can still make uncounted connections as they transmit signals outward when a neuron is stimulated and reaches what is known as a threshold point, a moment that is vitally important when it comes to thinking, feeling, and sensing. At that instant an electrical impulse bolts down the axon at 270 miles an hour. When it reaches the end of the axon, a tiny pouch of chemicals bursts, sending neurotransmitters across a synaptic gap like party confetti, where they embrace the receptor sites of the next neuron like a long–lost relative and then pass along their message.
Your brain is capable of making one quadrillion (that’s a 10 with fifteen zeros behind it) connections like these. Even as you read the words in front of you, impulses are flaring out and back at high speed, a three–dimensional, electrochemical storm tirelessly at work conjuring your thoughts, assessing your feelings, ensuring your body operates according to plan, and generating your personal version of reality. It’s a busy place.
While neurons multiply at blistering rates before we are born, the business of building the brain continues even more earnestly after we enter the world. By strict decree, the twenty-five thousand genes—the “structural genome”—each of us inherits in fifty–fifty doses from our parents resolutely continue the construction of our own wetware, and its underlying neuronal infrastructure, complete with our specific talents and predispositions. Just as some of us may inherit stocky bodies and others long, slim ones, our parents can also issue brains that incline us to be gregarious or shy, a leader more than a follower, mathematically, musically, or verbally predisposed. This part of us is a genetic crapshoot, and we have no control over it.
Nevertheless, more than other forms of life, even other primates, we can be thankful that we are not immutably linked to our genetic directives. In us they are editable, able to be altered by our personal experience and environment, a phenomenon that explains why each of us is not a clone of the other, not even in the case of identical twins, who carry precise copies of their sibling’s DNA. It is impossible to overemphasize the impact this new ability had on human evolution and has each day on your life and mine. The farther down the evolutionary chain creatures fall, the less complex their brains are as a rule, and the less they are shaped by their personal experience, which is another way of saying that their day–to–day actions are largely, if not entirely, governed by their genes, rather than by anything we might call a “self.”
Moths, for example, are drawn to candle flames because they are genetically programmed to navigate by the light of the moon. Not having much of a brain, they have been known to mistake a flame for the moon and get incinerated for their trouble. This happens not simply because their brain is small, but because it is also hardwired by its genes and not readily able to learn from experience.
For hundreds of millions of years genes were a perfectly effective, if plodding and random, way of adapting to changes in environment, but it wasn’t efficient. It took a long time for evolution to get around to building a brain that could think, even a little, for itself. But once it did, those animals blessed with one tended to survive longer than those that weren’t. Brains are more resourceful than trial–by–error genetics. They map the world in real time and increase the chances that you will make a lifesaving decision on the spot rather than a deadly, DNA–dictated one that isn’t even aware you are on the spot. Not that the influence of genes versus brains is either/or. All creatures endowed with a brain lie along a continuum of cerebral, and therefore behavioral, flexibility. There are no hard boundaries. But the degree of that hardwiring in many ways marks the difference between, say, a flat–worm, and us.
The impact that the outside world can have on our brains during our childhood explains how seven billion of us can be walking the planet every day, each a thoroughly unique universe unto ourselves, distinct in personality, experience, thought, and emotion; yet similar enough that we can (more or less) relate to one another and be counted as members of the same species. What has been far less clear, and a slippery problem for scientists, has been exactly how the genetic commands we inherit from our parents are bent by the unique relationships and events in our lives. It turns out several forces are at work. Very hard at work.4
In the first three years of life the human cerebral cortex triples in size. This is like nothing else in nature. Yet it isn’t simply the growth of neurons that makes the human brain so powerful. It is also the way it feverishly links them up. Why should this matter? Think of the brain as a miniature, though considerably more complex, Internet, compressed in size and time. Each neuron is like a computer sitting on a lap or desk somewhere. Computers today are powerful, like neurons, and can by themselves accomplish a great deal. I am writing this book on one right now. But connect neurons or computers to one another, and they become amplified and add up to far more than the sum of their parts. When my computer links to the Internet, it enables me to research information I use in the book, share passages I am writing with others in a blink, and gather opinions, thoughts, and insights by engaging in any number of conversations. I can instantly track down specific bits of information I need or download facts, maps, images, even whole books and movies. By branching out and communicating in all directions, my computer becomes, in many ways, all the computers it can touch. Now multiply this by millions of sites from Facebook to the Library of Congress, billions of Web pages, and innumerable other computers, and you begin to get a feel for the benefits of interconnecting neurons in the brain. There is power in communication.
The pathways between neurons begin to radiate almost the moment nerve cells undertake their growth in the fetal brain. Yet while the proliferation of neurons begins to slow at age three, the branching of pathways between them continues more urgently than ever. So urgently that a thirty–six–month–old child’s brain is twice as active as a normal adult’s, with trillions of dendrites and axons making contact, jabbering and listening and tightening the collaborative party that makes the human mind possible. One neuron can be directly linked to as many as fifteen thousand other nerve cells, generating more connections within the brain than there are electrons and protons in every heavenly body within every one of the hundred billion galaxies in the universe. That’s a lot of communication, and it is all happening between your ears.
The culprits behind this mad construction project, the forces that create and shape these connections, are the boisterous circles of the outside world with all of its smells and sensations, sound, touches, social interactions, and dangers. In attempting to make sense of the world it lives in, the brain creates its connective architecture by smelting and hammering out a massive, riotous explosion of wetware, which is shaped by a child’s sensory conversation with the world. The trillions of connections that blossom physically and chemically represent every new, frightening, exhilarating, or surprising experience children come across, which in the case of children is almost everything. For a toddler, novelty is riot in life. Since even big brains can’t predict the future, this is nature’s way of attempting to prepare for all flavors of trouble (and pleasure) yet to come; an all–out effort to create synaptic antennae that can better sense what may be, or could be, and use whatever tools and information are at hand to the best possible advantage. If music is part of your life, then neuronal pathways and structures begin to fan out to better handle, at first, listening to music, and
then later making it. The same holds true for language, physical dexterity, sight, and social cues. Everything from the mundane to the sublime is shaped in the brain by the events around us.
You will have realized by now that this pretty much renders the old nature–versus–nurture debate irrelevant. The trillions of connections our brains make in childhood help explain why we are neither purely a product of our genes nor altogether the result of our personal experience, but both. Nevertheless, this does not represent the whole picture. The brain is like an onion. Peel back one mysterious layer and it only reveals another: a recently discovered parallel genetic system, for example, that works within each of us, and profoundly affects the person we become. This system is related to the genome, but it is not the genome. It’s something else equally as fascinating called the epigenome.5
The long and spiraled strands of DNA that vibrate within the cells of all living things dictate whether they are plant or animal, have feet or wings, lungs or gills, and explain why you and I are tall or short, blond or brunette, Asian or black, even human as opposed to a planaria. But as if that weren’t impressive enough, there is still more to our DNA. It is wrapped around proteins called histones. This two–leveled structure—the histones and the DNA—constitutes the epigenome. Scientists are a long way from fathoming the many–layered mysteries of epigenetics, but they know that when such a structure is tightly coiled around inactive genes, it renders them utterly silent and unreadable, but when it relaxes pressure the genes become more accessible and therefore more expressible. How exactly these genes are expressed depends on our personal experiences and the environment in which we live, physically, socially, and emotionally. Specific experiences can deeply affect different brain circuits during developmental stages that go by the self–descriptive term sensitive periods. Cells in different parts of the brain that affect sight, language, hearing, are sensitive at different times and for differing lengths of time in life, particularly childhood. How deeply our epigenetics change shapes the circuitry in our brains, which in turn shapes how we behave and who we are. Once a sensitive period passes, particular circuits grow set in their ways and then lie beyond the reach of new experience.