| SOUL SEARCH (what is this?) A Scientist Explores the Afterlife David Darling 
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If you can work on your mind with your mind, how can you avoid an immense confusion?
– Seng Ts'an, Chinese philosopher
Where did “you” come from? What led up to this extravagant arrangement of stardust – yourself – that can imagine and cast its thoughts far into the past or the as-yet-unknowable future, that can foresee its own end and even speculate on what may lie beyond the frontier of death?
The stuff of which we are all made traces its origin back to the Big Bang itself. In this unique event, cosmologists believe, all the raw material the universe would ever contain burst out of a point far smaller than the period at the end of this sentence. It was the ultimate conjuring trick. Out of that tiny genesis seed grew the whole cosmos we see today, teeming with a hundred billion galaxies and ten billion trillion stars.
Only the two lightest elements, hydrogen and helium, were made in the immediate aftermath of the Big Bang. All the rest, up to and including iron, had to wait to be cooked deep inside the hot cores of giant stars. These big suns then blew apart spectacularly as supernovas (creating small quantities of still heavier nuclei) and hurled their remains far into space. After billions of years, some of this scattered debris, laced with all the naturally occurring heavier elements up to uranium, found its way into growing clouds of gas and dust from which new stars would eventually form – one of them, the sun.
From what was left of the dusty cloud that spun around the infant sun came the planets of the solar system. And from the atoms of one of these worlds, in time, came ourselves. No wonder we sometimes look up at the night sky and imagine our future out there, among the stars, crossing the light-years. We have made that journey before.
Yet it was here, on earth, that we first became aware of our ancestry – and our inheritance. Here life began, we believe, as unshielded far-ultraviolet rays from the sun and powerful lightning discharges set off complex reactions in the rich chemical broth of earth’s primordial seas.
Origins fascinate us. Our minds have an incurable habit of seeking out decisive moments. When did the universe begin? When did life first appear? We are like the character in Robert Frost’s poem:
You’re searching JoeWe’re forever dividing up the world and labeling it. We’re always on the lookout for what we think might be crises or transitions so we can claim something new has appeared–and then stick a name tag on it. And so we talk about “events” like the “origin of life.” But we need to shake ourselves sometimes and remember that our labels are totally, unreservedly artificial. “Life” and “nonlife” are made-up categories like all the others we impose on the world. There are no such distinctions in reality.
For things that don’t exist.
I mean beginnings
Ends and beginnings
Ends and beginnings – there are
no such things
There are only middles.
Living things, as we conventionally think of them, grow, respire, take in food, put out waste. More important, they reproduce. To be alive, we say, an organism has to be able to make faithful copies of itself, generation after generation. It seems to be a definition that works fine for zebras and honeybees and humans. But the question of what lives and what doesn’t starts to get more problematic as soon as we move away from the “middles.”
Take stars, for instance. They grow as they form. They ingest whatever falls onto their surface. They excrete stellar winds and flares. They even reproduce in the sense that the stuff of which they’re made is recycled into new stars. So, why isn’t a star alive?
Perhaps it is. If we all agreed tomorrow that stars were alive they would be alive. “Life” is our invention, so we can do with it as we please. We only have to make changes in our worldview by global consent for living stars to become part of our invented reality. If you grew up in a culture which taught that stars (and maybe also planets and rocks and atoms) were alive, then that’s exactly what you would believe. Whatever cosmic perspective, whatever labeling system we are brought up to accept as “true” defines for us what nature is like.
But it is all a fiction. Life, death, stars, objects and events of every kind are convenient lies – received but false wisdom. What to us seem like certain facts are merely agreements between ourselves within a framework of interpretation. Change, impermanence, and undividedness are the true qualities of the universe. And the sooner we get deeply acquainted with that, the sooner we can understand better what we really are and where we are heading.
Things abound in the human universe. The way our brains have evolved has made them compulsive analyzers and classifiers. We see objects everywhere. And we see boundaries everywhere, because a boundary defines the object within.
If there is a boundary, as we perceive it, then there is an individual. So we talk about an individual star or planet or rock or microbe. Why is it, though, that of these only the microbe is considered to be alive? Forget details like genes and chromosomes – we can imagine life with a different physical basis. The real reason we say the microbe is alive is that it seems to us to act purposefully. It behaves so, not in the sense that it thinks, but in the more limited sense that it controls what passes through the boundary between itself and the outside world. And notice that in talking about life in this way, another label has crept in – self.
To be a “self,” as we see it, is to know what is good for you. Recognizing food, avoiding danger and knowing the difference between what is part of you and what is not are elementary skills that any aspiring life-form must have. To be alive is, in the very way we define it, to have a degree of selfhood. The “knowledge” of self may be expressed at a very low level by the physical and chemical processes that go on inside a primitive, one-celled animal. But that is knowledge enough. Long before brains and nervous systems evolved, there were organisms looking out for their own interests. Even as life began, so too did self.
And consciousness? That seems to be a much loftier property. We generally think of it as marking out humans and other “higher” animals from lesser forms of life. Even such a marvelously complex thing as a fly isn’t usually considered to be in any way conscious. And yet wasn’t the springboard for consciousness surely just the ability of an organism to sense some aspects of its surroundings and react in a way that improves its survival chances? Every living thing has to have this ability or it quickly dies before it can pass on its genes. Life, apparently, implies some degree of selfhood, which, in turn, implies some degree of consciousness: all three must have grown up together.
On the face of it, a microbe’s “consciousness” isn’t much. At best it seems to encompass a low sensitivity and inclination to react to what happens in the environment. But doesn’t an electron also “sense” and “react” when struck by another particle? Couldn’t we therefore say that an electron – one of the smallest particles in nature – was also vaguely conscious? And if so, doesn’t this imply that every single bit of the universe is somehow aware?
A very different picture of reality begins to emerge, then, as we challenge some of the categorizations of orthodox science. In fact, there is a deeper truth beyond science, beyond any form of rationalization, that has been known to the human race for a very long time. It is an intuitive, direct form of knowledge and not one that can or needs to be proven. As Max Planck said: “Science cannot solve the ultimate mystery in Nature. And it is because in the last analysis we ourselves are part of the mystery we are trying to solve.”
Fifteen hundred years earlier Buddha had voiced a similar thought:
“In the search for truth there are certain questions that are not important. Of what material is the universe constructed? Is the universe eternal? Are there limits or not to the universe?... If a man were to postpone his search for Enlightenment until such questions were solved, he would die before he found the path.”We live our lives entirely inside an illusion – a virtual reality far more convincing than any yet created by computer. So mesmerized are we by it that we find the greatest difficulty in imagining that the world could be any other way. Everywhere we look are objects and events and phenomena, from the most trivial happenings in our everyday lives to the creation of the universe. Yet it is all a mirage, a fabulous invention. And the most extraordinary, convincing part of it is ourselves. Where exactly did “we” come from?
Life started, in the conventional view, as a happy product of molecules bumping into one another and occasionally sticking together. Eventually, inevitably perhaps, one particular group of molecules came together that had the unusual property of being able to make exact copies of itself. These copies spawned more of their kind, and so it went on. Soon, the ancient waters of earth were swarming with elementary, self-replicating “life-forms.” Changes to the environment, some brought about by the presence of new life, others not, spurred the development of further types of organism. Competition began. There were survival advantages in having, among other qualities, improved senses.
So, in time, creatures emerged with primitive eyes and ears and other organs with which to better perceive their environment. Century by century, the developments were insignificantly small. But over many millions of years, through a combination of environmental pressure and random genetic variation, the growth of more advanced life-forms was encouraged. Bundles of nerve fibers organized themselves into crude biological switchboards, which in turn became the prototypes of the first rudimentary brains.
Some researchers, such as Richard Dawkins, choose to see evolution as a battle for supremacy between rival genes. Organisms, according to this view, are merely the unwitting agencies through which genes express themselves, compete with others of their kind and vie to secure their transmission to new hosts. The “selfish gene” paradigm offers a refreshingly novel insight into the way biological complexity and diversity may have come about. But we can look at the unfolding of nature from many different levels and perspectives.
At the far end of the scale from genes, we can see evolution as being a general, unpremeditated drift toward higher and higher levels of self-awareness. The more clearly an organism can see itself as an agent in the internal world it builds, the better are its chances of outwitting and outguessing its competitors. This is not to suggest that every creature is bent upon becoming smarter. Once a species has adapted fully to a particular niche, it doesn’t evolve further unless fresh demands are placed on it. There will always be jellyfish, and they will always be stupid. Good brains are needed only by animals competing for certain types of complex, unspecialized niches – niches that opened up only after those at a more elementary level had been occupied. And yet it is these brainy creatures that, from our biased human viewpoint, appear to define evolution’s leading edge.
Throughout the ascent of life on earth, “self” has become an increasingly important factor. But it is only quite recently that its development has so dramatically accelerated. By 300 million years ago, self-awareness was still at a fairly low level. The first land vertebrates, primitive reptiles, had just completed their escape from the oceans; and a reptile’s brain is a meager, unversatile affair. Of the three main regions that comprise every vertebrate’s brain – hindbrain, midbrain, and forebrain – a reptile is endowed significantly with only the first two. In human terms, it has little more than a brain stem.
The reptile’s sensory world was (and is) centered mainly on vision. But unlike our own visual system, which allows us to interpret and manipulate and conjecture about what we see, a reptile’s ability to detect and process visual information is largely hard-wired into the circuitry of its eyes and its midbrain. Thus a reptile is a slave, rather than a master, of its environment – a sophisticated biological automaton.
By 200 million years ago, however, there had come a major neurological breakthrough. The first mammals had appeared, with brains four to five times bigger relative to body weight than those of their reptilian counterparts. Almost all the increase was due to the dramatic appearance of the cerebral cortex, a thin “thinking cap” of gray cells atop the forebrain that gave its owner an unprecedented new faculty for building internal world models. But why did the brain evolve so rapidly at this time?
What is clear is that the first mammals, which were nocturnal, would have been poorly served by reptilian sight. Being small and warm-blooded, they needed constantly to refuel themselves, so they scavenged for food virtually the whole time they were awake. This meant having to actively seek out insects and other elusive prey during the hours of darkness. Sight alone was not enough for them. They needed improved senses of smell and hearing and thus larger and more sophisticated olfactory and auditory systems. Evolving such systems forced in turn a radical restructuring of the nervous system. Reptiles could get by with the equivalent, in computer terms, of ROM-based handling of visual data – the hard-wired approach. But the far trickier task of extracting accurate 3-D spatial and temporal information from sounds and smells called for machinery in the brain capable of processing sensory input at a much higher level. Simply tacking on a mass of nerve cells at the periphery of the nervous system, as reptiles had done with their eyes, was out of the question; there was not enough room.
This sharp contrast with the packaging demands of the reptilian visual system was, according to Harry Jerison of the University of California at Los Angeles, one of two main factors that prompted the growth in relative brain size of the early mammals. The second stemmed from a further benefit of having a more centralized processing system: the opportunity to integrate the sensory signals from sight, sound, smell, and touch to create a more detailed and artificial mental picture of the world.
With their new cortices, the early mammals were far better equipped to generate their own internal reality than were the reptiles. They were more perceptive and more able to respond pliantly to the world they saw.
Once established, the mammalian brain plateaued in relative size for at least 100 million years. Then came another explosive period of growth. Following the sudden demise of the dinosaurs, about 65 million years ago, modern mammals began to evolve at prodigious speed. Over the next 30 million years their brains ballooned four- to five-fold, with the biggest gains coinciding with the appearance of the ungulates (hoofed mammals), carnivores, and primates. Most of this new growth, Jerison argues, was probably due to the mammals invading daytime niches left vacant by the dinosaurs and their relatives. Having adapted to a nocturnal way of life, mammals had to re-evolve diurnal sight. As it was impossible to go back to the old reptilian arrangement, the revamped visual system had to be incorporated into the forebrain along with new connections to the nerve centers handling hearing and smell. In consequence, the cortex once again expanded enormously, and with it the capacity to fashion a still more lucid model of the world.
In one particular group of mammals, the primates, the brain-to-body size ratio, or encephalization quotient (EQ), became especially large. For their size, monkeys and apes have brains two to three times as big as that of an average modern mammal, while a human being has an EQ roughly three times as big as a chimpanzee’s. Why is it that our brains have grown so much?
There are no simple answers, but two theories are popular. Some researchers, like John Allman of the California Institute of Technology, point to a link between increased brain size and the evolution of better strategies for ensuring a stable supply of food and other resources. Man’s ancestors almost certainly had to exploit a wide home base, make inventive use of whatever was locally at hand in their search for food, and broaden their diet. This, in turn, says Allman, demanded improved cognitive skills.
Robin Dunbar, professor of biological anthropology at University College, London, is among those in the rival theorists’ camp. He champions complex social behavior of primates as the main driving force behind bigger brains. In 1992, he completed a survey of thirty-eight genera of primates, including gorillas, chimpanzees, and humans, and found that those species dwelling in large social groups, such as chimps and baboons, boast proportionately larger cortices. Bigger primate groups, Dunbar concludes, have a need for greater social cohesion and hence more advanced skills for communicating and keeping track of group relationships. This might explain, among other things, our obsession with social tittle-tattle in the tabloid press and why gossip about relationships takes up so much of our conversation.
To find out just how important gossip is, Dunbar and his colleagues monitored conversations in a university cafeteria, scoring the topic every half-minute. Even in such a supposedly academic environment, talk about social relationships and personal experiences took up nearly 70 percent of the conversations, with half of this devoted to gossip about people not present. Males, however, tended to focus more on their own relationships and experiences, whereas females talked mostly about other people’s.
Could this mean, as Dunbar has speculated, that language evolved mainly as a vehicle by which females circulated news within their group – social chitchat that was vital to the group’s stability? The anthropological party line is that it grew up in the context of male-male relationships, as a means, for example, of coordinating hunting or defense. But the idea that interfemale social exchanges may have been the main spur to linguistic development ties in well with another observation – that in nonhuman-primate societies, female-female relationships are all-important. At any rate, it gives the modern male something to think about. The next time he complains about his partner’s propensity for gossip, he might consider that without it he could himself have been left speechless.
Doubtless a whole raft of interwoven factors, environmental and social, helped the primate brain grow. But in the ancestors of our own species this development just went on and on. In less than 3 million years, the brain tripled in size and evolved a cortex that, in modern man, accounts for an astonishing 70 to 80 percent of brain volume.
An average human brain contains 10 billion to 15 billion neurons with up to ten times as many glial, or connecting, cells. The possible number of ways of joining all these cells together is much greater than all the atoms in the universe. But the strange fact is, it seems as if we have a far bigger brain than we strictly need to think and behave as we do.
Cerebral size alone is no sure guide to intelligence. Some severely retarded people have larger-than-average brains, while among those of unusual brilliance brain size can vary by almost a factor of two. Not many eminent individuals, as might be expected, have had their brains weighed, but of those that have the current record holder is the Russian writer Ivan Turgenev, whose brain tipped the scales at 2,012 grams, or more than four pounds. (A typical adult male brain weighs three pounds.) By contrast, the French novelist Anatole France struggled by (winning the 1921 Nobel Prize for Literature) with a puny brain of just 1,017 grams (barely two pounds). Other gifted neural lightweights include Franz Gall, ironically the founder of phrenology (the study of cranial bumps), and Walt Whitman, whose poetic genius emanated from a brain of just 1,282 grams.
One might suppose there is a limit to how small a brain can be before the effects start to show. And this is generally true. So-called microcephalics have very small brains and correspondingly low intelligence. Most extreme of all are anencephalics – babies with empty skulls, who die shortly after birth.
However, the very-small-brain/very-low-intelligence rule doesn’t always hold. In the mid-1960s, the world learned of (and has since largely forgotten) the remarkable case of certain hydranencephalics. The news broke in the form of a paper in the journal Developmental Medicine and Child Neurology by John Lorber of the University of Sheffield, England. It described two children with water not “on the brain” but in place of a brain. They had fluid where their cerebrums should have been. A light shone into their skulls would have revealed the disturbing phenomenon of transillumination – the rays would have passed cleanly through from one side to the other. Yet, what was so astonishing was that although neither child showed any evidence of having a cerebral cortex, the mental development of each appeared perfectly normal. One child subsequently died at three months. The other was still healthy and continuing to develop like a normal child a year later.
Lorber’s paper was reported, briefly, without much fuss, in popular science magazines at the time, and then faded from view. Why? Perhaps because it raised too many problems or was too far off the beaten track of conventional science. In any event, the work quietly continued and other externally normal, gross hydranencephalics were found. One was a man with an IQ of 126 who had graduated from the University of Sheffield with a first-class honors degree in mathematics. He was bright, conventional in appearance and behavior, but had no detectable brain.
A pair of identical-twin girls with gross hydranencephalus were also studied. Both had above-average IQs. In another case, a young man who had suddenly died had an autopsy, which revealed only the most paltry rind of brain tissue. Trying to console his parents, the coroner expressed grief tempered by relief that such a profoundly retarded lad had finally found rest. Dumbfounded, the parents told the coroner that their son had been at work just two days before.
Intelligent hydranencephalics make nonsense of received neurological wisdom – another possible reason that they have been so conspicuously ignored. With cerebrums in some cases less than a fifth of an inch thick, they have brains smaller than that of a rabbit yet function like perfectly normal human beings. How? The only possible answer is that they are making the best use of what limited processing capacity they have. But this raises the question, if we only need brains this small, why has evolution given us brains that are so much larger? It seems that, as the British naturalist Alfred Russell Wallace pointed out, “an instrument has been developed in advance of the needs of its possessor.”
Like other parts of the human body, the brain apparently includes a very high degree of redundancy. We can make do with less than 10 percent of our digestive tract, a quarter of one kidney and a snippet of liver. Now we find that the brain, too, has a massive amount of spare capacity. Its great bulk looks impressive but most of it – perhaps 90 percent or more – is a safety buffer built into the cortex over millions of years of development. That is not to say we can just lose nine-tenths of our brain and carry on as normal. Even modest damage to a major structure is enough to deprive us of sight or speech or memory. But we apparently use only a small fraction of the brain’s overall potential. As writer Arthur Koestler remarked: “It is the only example of evolution providing a species with an organ it does not know how to use; a luxury organ, which will take its owner thousands of years to learn to put to proper use – if it ever does.” An early hint, perhaps, of what the human race might someday be capable of.
Our ancestors started to look vaguely human well before they could think in a human way. Genetic and fossil evidence suggests that the first protohominids appeared somewhere between 7.5 million and 5 million years ago. Partial skeletons, 3.5 million years old, have been found of a small, lightly built creature called Australopithecus afarensis who clearly walked upright – he left us footprints – but whose EQ seems to have been no bigger than that of a chimp. We were bipedal before we were brainy. A million years later, however, the cortex was once again on the move. Around this time the climate of the world started to change, becoming cooler and drier. Areas of Africa once densely forested turned to open savannah. These environmental changes triggered a complex sequence of events out of which emerged the first of our direct forebears, Homo habilis, or “Skillful Man.”
It can be no coincidence that the earliest stone tools, global cooling and the oldest remains of H. habilis all date back to more or less the same period of time. Skillful Man earns his name by having been the first significant tool-user. But was he also the instigator of human language? Could habiline man utter the rudiments of a spoken tongue, or did that development take place much later?
From looking closely into fossilized crania, Dean Falk at the State University of New York, Albany, has concluded that spoken language began to develop between 2 million and 3 million years ago with the incipience of the genus Homo. He cites a famous 1.9-million-year-old skull found east of Lake Turkana in northern Kenya. This reveals a slight bulge on the left side near the temple corresponding to what in a modern human brain would be the location of Broca’s area – a region thought to play a key role in vocalization.
Studies of the way the voice box has developed also lend credence to the idea that language emerged quite early in our evolution. Humans are unique in having a larynx low in the throat, an arrangement that leaves a bigger air space above and so extends the range of possible sounds that can be made. The position of the larynx is reflected in the shape of the underside of the skull, or basicranium. In chimps, this is relatively flat, whereas in humans it forms an arch. Unfortunately, no intact basicranium of habiline man has yet come to light. However, the remains of a 1.6-million-year-old Homo erectus (“Upright Man”) have been found with a basicranium flexed to a position midway between that of an ape and a modern human. Since H. erectus stands in direct line between us and our remote habiline forebears, it is tempting to speculate that spoken language may have started its long, slow development at least 2 million years ago.
Others see it differently. Probably the world’s foremost linguist, Noam Chomsky, of the Massachusetts Institute of Technology, insists that natural language is unique to our particular species, Homo sapiens. This would bring the origins of human speech forward to no more than 250,000 years ago. His argument is based on a comparison of the brains of contemporary humans and apes. No language-related structure in the ape cortex, he maintains, bears any resemblance to the neural basis of speech in man. However, researchers such as Steven Pinker of MIT and Paul Bloom of the University of Arizona have taken issue with Chomsky on this. They point out that our closest living relatives, the African apes, have been through at least 5 million years of independent evolution since we shared a common ancestor. Human language is so complex, they insist, that both it and the neural hardware that subtends it must have developed gradually from apelike precursors.
One interesting piece of archaeological evidence also seems to favor an early origin model for speech. It comes from a study of some of the oldest-known stone tools, carried out by Nicholas Toth of the University of Indiana. Toth has established from patterns of flaking that the earliest stone-toolmakers were mostly right-handed, in roughly the same proportion as in modern populations. Preferential handedness is unique to humans and is associated with laterality, the tendency of the brain’s two halves to concentrate on different aspects of cognition. In modern humans, control of language and fine motor movements is more heavily focused in the left lobe – an indication, perhaps, that handedness and language evolved at roughly the same time.
More circumstantial reasons exist, too, for suspecting that tool-making and speech developed together. Both are concerned with manipulating aspects of the environment, the former physically, the latter symbolically. With both tools and language we take the world apart, see its inner grain and come to regard it as a collection of objects in space and time. For our convenience, and for the purposes of our survival, we make the continuous discontinuous.
Language developed as a mutually-agreed-upon labeling system in which common features of the world outside were identified by specific sounds. These were mapped onto the brain and then mentally associated with appropriate images and other sensory impressions. At first, the most important words or intonations would probably have been those signaling various kinds of danger – as they are in many animal languages. “Look out!” is still one of the most immediately useful calls we can utter.
As soon as the rudiments of true spoken language began, selection would have come into play. Those individuals whose genes fortuitously supplied them with brains better able to recognize and produce vocalized sounds would have had a survival edge. Better brains meant bigger, more densely connected integration and association areas, notably in the prefrontal part of the brain. Growth of the prefrontal cortex added to the richness of man’s internal reconstruction of nature and to his perceptions of the interrelationship between different facets of his surroundings. This, in turn, would have spurred on the development of his linguistic abilities, his talent for further categorization and his faculty for analyzing and controlling his environment.
Through language man was able to manipulate mentally what he saw, because language transformed the world “out there” into a rich inner domain of symbolic equivalents. A physical object, such as a log, might be hard or impossible for one person to move. But the labeled concept “log” could be played with at will – and at leisure. It could be set up in new positions, envisaged as a roller or a bridge or a boat. The symbol, the mental icon, had a freedom that the object itself lacked. Furthermore, once acquired, each new word joined the rest of man’s burgeoning vocabulary so that it could be seen in juxtaposition with other symbol-equivalents abstracted from the real world.
At some point, this labeling process reached its climax. As humans increasingly learned to simulate the world in symbols, there must have come a time when the individual constructed a meaningful symbol for himself. Perhaps the switch from “wide-screen” experiential consciousness to an awareness centered on self took place swiftly, and even quite recently. One possibility is that the brain suddenly (in biological terms) flipped into a new, stable mode that gravitated around its internal symbolic representation of itself. Another scenario is that awareness of self grew only incrementally, over hundreds of thousands or even millions of years. Whichever is true, we can be sure that the feeling of being a particular individual brought with it considerable survival benefits, otherwise it would never have come about.
We are all descendants of those first “selfish” organisms that dwelt in earth’s ancient ocean and, more latterly, of the hunting-gathering hominids in whose minds the shadows of self-awareness surely began to stir. Seen in this context, the emergence of identity has been an unbroken process of breathtaking scope and complexity. The thread of continuity from your present existence stretches back through ancestral generations to the dawn of humanity – and further, to the birth of the earth, the sun and the universe itself.
Yet on a more parochial level, your development began in the ovaries and testes of your parents. Here, during their own embryonic and fetal lives in the wombs of your grandmothers, those cells destined to form eggs and spermatozoa were first set aside as a special germ line. Subsequently these cells specialized, becoming the clearly identifiable forerunners of eggs and sperm. Early in the fetal life of your mother, the chromosomes within her egg precursor cells started an intricate process of genetic rearrangement. This culminated eventually in the production of eggs, each of which had only half the usual complement of chromosomes, each half-set being unique in the pattern and combination of genes it contained. The same kind of process took place in the development of your father’s chromosomal contributions, albeit after his birth rather than before.
At the moment of your conception, your unique genetic program was brought together, in effect, by the toss of nature’s dice, and then set running. Guided by your genes, still in the amniotic haze of preconsciousness, you developed in just forty weeks from a single, information-rich cell to a fetus containing a brain with about 100 billion neurons – effectively, the full adult complement. Brain cells are not replaced like other cells of the body, nor are they added to much after birth. However, the sylphlike dendrites that form connections between brain cells, up to a thousand of them per neuron, do continue to change and reconfigure themselves throughout our lives. These slender branches and the nerve synapses (the microscopic gaps across which chemical signals influence neighboring cells) are crucial to the formation and functioning of the brain’s internal maps – the maps that help us make sense of the world and of ourselves.
Much of the wiring of your brain took place while you were still in the womb. A typical fetal brain cell sprouts a main trunk, or axon, which then begins to grow and grope its way toward a specific target location in some other part of the brain. It does this by a kind of molecular sniffing. Each young axon has a specialized tip, called a growth cone, that can recognize a chemical trail laid down by other cells along the way. The target itself may also release chemical signals to tell the axon when it has arrived. But this is not the end of the story. Having reached the correct target, an axon still needs to find a particular “address,” otherwise its connection will be faulty. However, unlike the pathway and target selection, address selection is much more hit-and-miss. In fact, axons have to fine-tune their connections by trial and error after birth, based on exposure to signals from the outside world.
Even while you were in the womb, your genes had only a general say in how your brain became wired. The huge number of brain cells and the vastly greater number of ways in which they could join together means that precise genetic control over every axon movement and dendritic branching is impossible. Gerald Edelman, Nobel laureate and head of the Rockerfeller Neurosciences Institute in New York, has drawn an analogy between the brain and a rain forest, its vast flora of microscopic fronds, vines and arbors unrepeatably complex and unique.
Because you and I have many thousands of tiny differences between our genes, the circuit diagrams of our brains are bound to be different. But they are very much more distinct because of the unpredictable ways in which individual neurons and groups of neurons subsequently develop, before and after birth.
During your early development, the massive number of circuits and potential signals in your synapses represented a kind of catalog of all potential human skills. This catalog has been built up and made available to all of us (with variations) through the evolution of our species. And it is from this great wealth of possibilities that your own specific environment and experience made their choice. A common fallacy is to assume that brain connections start to be laid down in earnest after birth. In fact, the opposite seems to be true. There are many more connections between nerve cells in an infant than in an adult. Development is more a matter of pruning than of proliferation.
After birth, connections between your neurons that were frequently stimulated survived and grew stronger; others atrophied or were switched to other tasks. Exposed to the particular circumstances of your childhood, specific orchestrations of cells were favored out of the colossal repertoire of possibilities. Gerald Edelman sees a parallel between this process and Darwinian selection in the world at large. According to this idea, the brain looks less like a rigidly programmed computer than an ecological habitat that mimics the evolution of life itself.
Some recent research in child development backs up this controversial new idea of dynamically evolving neuronal groups. Esther Thelsen, at the University of Indiana, has investigated how babies learn to reach. She found a wide variation among infants in the way they move their arms and legs to grab hold of an object. Over several months these pattern appeared to be in competition. Finally a number of successful strategies emerged. These strategies were always unique and adapted to the individual circumstances of each child.
The same winnowing process happens with language. During its first year or so of life, a baby babbles its way through almost every sound of every language. Later on, though, it loses the ability to make sounds that are not in its native tongue. An enormous range of sound patterns is available to us at birth, just as there is a huge variety of other potential skills waiting to be developed among the myriad unpruned circuits of the pristine brain. In the end, we learn to use only a few of them. But it is fascinating to speculate what else we might be capable of, given the appropriate nurturing.
The picture emerging is that individual brains and behavior patterns are governed far less by our genes than had previously been suspected. It may be that, in different people, the fetal neural maps carry a predisposition for aptitudes such as playing a musical instrument or math or strong hand-to-eye coordination. But if our brains subsequently evolve in Darwinian style, the development of such traits will depend to a large degree on the actual life experiences of a child. Each of us, then, is very much an individual, molded more by nurture than by nature – a unique creation in a highly creative world.