Synopsis
Book by Victoroff Jeff
Extrait
CHAPTER 1
Adam's and Eve's Brains
My theory would give zest to recent & Fossil Comparative Anatomy: it would lead to a study of instincts, heredity, & mind heredity . . .
--Charles Darwin; personal notebook, 1837;his first inkling that evolution helps explain the mind and brain
Why did brains evolve? What are they for? It may not be immediately obvious how these questions relate to saving our brains, but they do, for one important reason: evolution explains aging-related brain changes. The very word evolution conjures images of misty pasts and private events, the astonishing tales of trillions of creatures over billions of years, meeting, embracing, stirring a new life into being and with it a DNA dynasty. At some point in that incredible story, a human Adam and Eve appeared, the first of their kind, with brains exactly like ours. Of course, the lucky mutations that led to humanity may have occurred several times before human beings really took hold as the world's dominant hominids; it's even possible that humans arose independently in several places and that our mental image of one Adam and Eve is a fine romance rather than a biological fact. But most likely, at some point between 100,000 and 200,000 years ago, the first few babies who had truly human brains were born. They were the founders of the world's human population--our genetic Adams and Eves.
Evolution is responsible for how human brains came to be, why they flourished, and why they are vulnerable to change with age. Indeed, the evolution of the brain--its genes, its neurons, the dazzling expansion of its cortex, the niche we evolved to fill at the edge of some long-ago forest, and even the way we have strayed from the lifestyle for which evolution prepared us--is the real explanation for why our brains are likely to get into trouble as we grow older. That story began in the molecular Garden of Eden.
Fruits from the Garden
Life at its most basic level is an organism's ability to make copies of itself, using the magnificent DNA molecule as a sort of biological copy machine. We've all seen pictures of DNA shown as a double helix that looks like a long, twisted ladder. Our genes are strung out along the DNA ladder, one after another, in a carefully determined sequence. Though scientists assumed for decades that there were more than 100,000 human genes, one of the most humbling findings of the recently completed Human Genome Project has been that it probably takes only about 40,000 genes to make a human, not many more than it takes to make a worm. Yet these genes on human chromosomes carry the basic instructions that make us us--and that make every one of us different from everyone else.
But life is not just a matter of genes. As impressive as the Human Genome Project has been, it only lets us peek at the blueprints of life. In order for these gene-blueprints to make anything useful in the body, they must first be translated into proteins. Proteins are molecules that are manufactured based upon the precise blueprint instructions from our genes. But even after a protein is manufactured, it's still not ready for work; it must first be folded into an amazingly complex shape if it's ever going to do its job. A misfolded protein works about as well as a crumpled paper airplane--it falls quickly with a disappointing thud. It can even act as a poison. As we will see very soon, misfolded proteins are probably the secret of many aging-related problems in the brain, including Alzheimer's.
Contrary to the popular vision of DNA as the fixed and stable essence of life, genes are changing all the time. Such changes are called mutations; they can affect everything from eye color to height, brain size to behavior, and they occur spontaneously, randomly, sheerly by chance. Mutations are one of the clever ways that nature changes our DNA in a busy and continuous trial-and-error form of genetic engineering. Only a few mutations turn out to be beneficial; most don't. Those rare changes in DNA that help the organism survive and reproduce tend to be carried on, because they increase the chances that the organism will make copies of itself. This is the essence of natural selection.
But organisms pay a price for DNA's changeability: mortality. No matter how many useful mutations have arisen over the course of the last four billion years, one thing has remained inescapable: every molecule of DNA is destined to break down. This is a crucial point in understanding why we and our brains cannot live forever.
Fortunately, the breakdown of DNA does not mean the end of life on earth, because there is reproduction.
Thanks to reproduction, our DNA has a shot at immortality, even if we as individuals do not. At first, reproduction took place in just one simple way: an organism split in half. But about 900 million years ago, nature developed a new strategy: two organisms encounter one another, mix their genes, and create offspring. This is what we call sexual reproduction, or sex. Like mutation, sex is a great way to make new combinations of genes.
Sex also favored one more step in evolution: the advent of multicellular beings. Now, multicellular beings obviously need some way for their cells to communicate with one another. Throughout nature, they communicate by chemical signals. One way to do this is by hormones, or chemicals that pass through the bloodstream to stimulate cell activity. But hormonal signals are pretty slow. So another step in evolution was the development of neurons, the fast-signaling cells of the body. Neurons work best if they are close enough together to coordinate their actions. So eventually nature evolved neurons in clusters--in other words, the brain.
To put the whole story in its purest biological terms: since DNA is destined to break down, evolution produced an amazing array of beings that serve as DNA copiers. A central nervous system is a great way to keep all the parts of the body in touch with one another, which helps an animal survive and reproduce. Brains--in the final analysis--are clusters of signaling cells that make animals good vessels for the immortality of DNA.
Hopeful Monsters
Brains have been a feature of earth life for 600 million years, and they all do pretty much the same thing: help a creature to survive. But they do this job using two very different strategies, the small-brain strategy or the big-brain strategy.
Ants, for instance, are born with tiny, highly efficient nervous systems. Such tiny-brained creatures devote most of their energy to running their muscles; their nervous systems require little power, since they provide nearly automatic programs for living and breeding. At the other end of the spectrum, evolution has produced some species that have huge brains; these are the toothed whales, including the dolphins, and the great apes, including us humans. But it's not just size that matters; animals with big brains are also unusual because their brains tend to burn a disproportionate amount of energy. As big as it is, the human brain constitutes only 2 percent of our body weight, but it uses nearly 20 percent of our energy--a heck of a lot for only a few pounds of tissue. Our huge, energy-guzzling central nervous system might be considered the Chevy Suburban of animal brains.
Of course, we shouldn't assume that big brains are "better" than small brains. Evolution is not striving to reach some pinnacle of cerebral perfection; it's just experimenting, trying different strategies to see what helps the reproduction of DNA. (At a picnic, it's sobering to think that the small, energy-efficient-brained ants coming to join us might be considered the real evolutionary winners: insects make up 90 percent of the earth's land-based biomass!) Nonetheless, as natural selection worked on the big-brain theme, it came up with a particularly remarkable group of animals, the hominoids (the superfamily that includes gorillas, chimpanzees, humans, and their ancestors) who together represent a true brain revolution.
The extraordinary thing about hominoid brain evolution is that so much happened so fast. Over the last six million years--a tiny fragment of evolutionary time--the hominid brain grew from just 400 cc, the size of a small orange, to 1350 cc, the size of the modern human brain. Why did the brain grow so fast? Probably because any new ape that has both a good memory and the ability to make clever associations has an astonishing capacity to learn about his world. The result was a positive feedback loop in which a fast-learning ape with a slightly bigger brain could manage more sophisticated social behavior, and sophisticated social behavior--as one might imagine--favored reproductive success.
What happened next was a snowball effect: new genetic flukes brought about further brain changes that supported incredibly sophisticated behaviors such as self-awareness and abstract thinking. Natural selection kept one old brain feature, the limbic system, a deep and ancient brain part that still works beautifully to support learning and emotion. But evolution kept piling on more and more neocortex (new cortex), the outer shell of the brain, where memories are stored and blended to make an incredibly adaptable self. The result was a brain that ballooned in size from 850 cc to 1280 cc in a remarkably short evolutionary time, until--in an event lost in the mists of prehistory--the first children were born with brains significantly better than any before. Those children would come to be called Homo erectus (erect man). And this explosive expansion in the size of the brain implied more than just cleverness and insight; it also had remarkable implications for the way earth's newest apes related to one another.
This was the problem a million years ago: evolving big brains had major advantages, but the revolutionary new apes could not be born with their brains fully formed--their heads would simply be too big to fit down the birth canal. One possible solution would be to expand the birth canal, but this would interfere with the female's hips to the point of jeopardizing her ability to walk and run. The other solution was for the baby to be born with only a partly developed brain. That's exactly what happened. The brains of Homo erectus were born halfway built--a plan that has been passed down to us. And that's why a human newborn's brain continues to grow at a fantastic pace right through the first twelve months of postnatal life. So, in a sense, we are all born a year premature! Our brains need the nine months of pregnancy plus the twelve months after birth just to reach the maturity we'd expect in a newborn chimp.
The fact that earth's new apes were born with such immature brains, and were therefore highly dependent and in need of prolonged early life care, had profound implications for human behavior. It meant that the mother had to provide an extraordinary amount of time and energy to nurture and educate her helpless child. It decreased the number of offspring she could effectively rear in her lifetime. It made her more vulnerable to predators. And for all these reasons, it probably made her somewhat more dependent on a stable bond with a provision-carrying mate. As a result, males and females and babies would all benefit from a key feature of emerging human behavior--the capacity for devotion, which assures the infant of nurturing, the mother of support, and the father that his investment of provisioning energy is for the sake of his own children, which requires faith in the mother's fidelity. All of these changes favored a brain in which thoughtful planning was beautifully integrated with tender feelings. With the arrival of Homo erectus, the countless love stories of humankind began.
The snowball continued to roll down the hill of evolution. Better and better brains came into being through lucky mutations, until two new apes leaped onto the world's stage, crowned by a beautifully dome-shaped head and a brain size of 1350 cc: the Neanderthals, our close rivals, and Homo sapiens, the people we could call our genetic Adams and Eves. By luck or skill, Homo sapiens soon bested the Neanderthals to become earth's dominant ape. Something old--the limbic system--and something new--the neocortex--had evolved to make an unbeatable combination. This was the brain revolution. We are the result. Because it's risky to leap so far and fast in evolution, animals that undergo such a leap are sometimes called "hopeful monsters." Thus, the revolutionary apes, starting with Homo erectus and including both Neanderthals and Homo sapiens, could be deemed the hopeful monsters of the hominid world.
That rapid leap to a highly plastic brain--a brain that can readily change and adapt--came with a cost. Indeed, the brain revolution is probably deeply linked to our current need to save our brains. The reason: a brain that's brilliant enough to support complex social structures and cooperative hunting in Africa, farming in the Fertile Crescent, writing among the Egyptians, and the medicine of Hippocrates is a brain that can overcome many threats to mortality, making our life expectancy grow longer and longer. But the human brain was not built to last as long as we are now asking it to. As DNA-based matter, it is vulnerable to deterioration if we manage to outlive its natural limits. This may be the ultimate evolutionary irony: a brain that has thought its way into a life span beyond evolution's design. Aging-related neurodegeneration may be the price of human genius. We, at the cusp of a new millennium, are the first who will really pay this price, as modern medicine lets us live to an age at which virtually all of us will notice some of those effects on our thinking, and some of us will be devastated by them. So we--the first victims of this price of genius--had better use some of that genius to save our brains.
New research has given us remarkable insights into how we remember, why we forget, and what factors are most likely to protect our memories. To appreciate these discoveries, it will be helpful to take a quick look at the amazing modern human brain.
Brains for Behavior
Figure 1 shows the modern human brain. The neocortex, sometimes called the cortex for short, is the upper and outermost surface of the brain. It looks as wrinkled and parboiled as an overdried prune. Its wrinkliness is due to the fact that its surface is literally too big to fit into the skull unless it bends and folds over on itself during development. Fully developed, the brain takes on its familiar oblong shape, with the neocortex wrapped around the central parts of the brain like a thick convoluted shell.
When I was a child, the kids in my neighborhood really liked Tootsie Roll Pops. These lollipops have a hard candy shell surrounding a chewy chocolate center. I've used this lollipop in lectures because it's a fairly good representation of the structure of the human brain. We have a candy shell of neurons on the outside--the neocortex and all its connections. Deeper within is the chocolate center--the deep neurons of the brain, including the limbic system. Both the candy shell and the chocolate center are perched on the plain white stick--the brainstem and spinal cord.
Les informations fournies dans la section « A propos du livre » peuvent faire référence à une autre édition de ce titre.
