Last Ape Standing: The Seven-Million-Year Story of How and Why We Survived (9 page)

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.

How the Epigenome Changes Your Brain

So while the codes in our DNA are set for life depending on what our parents pass along to us, we still have plenty of room to deviate from the precise commands of those genes. Thanks to the epigenome, events, and the physical and psychological environments in which we live during our childhoods, can modify the expression of some genes that affect brain development. Some of these amendments can be temporary; others can change us for the rest of our lives.

Study after study, for example, has found that children exposed to high stress are more likely to suffer mental illnesses later in life, including generalized anxiety and serious depression. High childhood stress has also been shown to modify how a person later handles adversity in adolescence and adulthood. When we are frightened, our adrenal glands release adrenaline, which focuses our attention, increases our heart rate, and prepares our bodies to either fight or flee, handy reactions when your life is on the line. But chronic fear and stress—the kind that continues relentlessly—can corrode us because intense, ongoing awareness of the flight–or–fight kind wears us out. In children an epigenome exposed to constant stress tends to make
those children more sensitive to even minimal stress throughout their lives, and more likely to feel anxious when others might not feel the least bit nervous. Poor nutrition or toxic substances can affect epigenomes related to brain development during childhood in ways that blunt brain function later. Together these forces can gang up to have a kind of psychological domino effect that spills into our physical health to make us more susceptible to ailments like asthma, hypertension, heart disease, and diabetes.

On the other hand, positive experiences—warmth, stability, security, love, and the joy that comes from play—can create equally powerful, but entirely positive, results. Your genes write the basic blueprint of what is personally possible, or impossible. They set the boundaries of who you are physically, psychologically, socially, and intellectually, but your epigenome etches the finer details of your personality—the ways you handle others, your fears, joys your intellectual and emotional prowess, personal talents, confidence, proclivities for optimism or pessimism, and your annoying (not to mention altogether charming) quirks. They influence whether, when, and how your personal set of genes build the capacity for thought, emotional control, and a whole bushel of other future skills. Exactly what route the timing and depth of their effect takes depends on the infinitely complex molecular interactions that constitute your world and your “self.” No matter what, the result is that you come out of it all as unique as a snowflake.

In case the connection has eluded you, it’s our neotenous nature, our long childhoods, that makes our epigenome so inclined to the influences of our personal experience during the first seven years of life. Because we are born early and since we have extended our brain development well beyond the womb, neuronal networks that in other animals would never have been susceptible to change remain open and flexible, like the branches of a sapling. Although other primates enjoy these “sensitive periods,” too, they pass rapidly, and their circuits become “hardwired” by age one, leaving them far less touched by the experiences of their youth. This epigenetic difference helps explain how chimpanzees, remarkable as they are, can have 99 percent of our DNA, but nothing like the same level of intellect, creativity, or complexity.
⁶

As productive and interesting as all the goofy openness and flexibility of our toddlerhood is, it also creates a problem. It’s not sustainable. Unless we hope to be a race of primates suffering from terminal cases
of attention deficit disorder, the time eventually comes for our lively cerebral growth to be curbed. It’s the biological equivalent of fishing or cutting bait. We can’t afford to record, some way or another, every experience throughout our lives. The costs are too high. Our minds would grow so flexible they would become floppy, and so cluttered they would be incapable of focus. Besides, not every new experience is useful (sitting in traffic, for example). There are also physical limits to how big a brain can grow, though we
Homo sapiens
have certainly pushed the boundaries. Finally, brains, being greedy organs, devour immense quantities of energy for their size, especially in childhood. A growing toddler’s cerebral appetite gobbles up as much as 85 percent of all the energy that its body requires each day. Over a lifetime that would be insupportable.