Читать книгу Proust Was a Neuroscientist - Jonah Lehrer - Страница 20

How does our DNA inspire such indeterminacy? After all, Middlemarch had an author; she deliberately crafted an ambiguous ending. But real life doesn’t have an intelligent designer. In order to create the wiggle room necessary for individual freedom, natural selection came up with an ingenious, if unnerving, solution. Although we like to imagine life as a perfectly engineered creation (our cells like little Swiss clocks), the truth is that our parts aren’t predictable. Bob Dylan once said, “I accept chaos. I’m not sure whether it accepts me.” Molecular biology, confronted with the unruliness of life, is also forced to accept chaos. Just as physics discovered the indeterminate quantum world — a discovery that erased classical notions about the fixed reality of time and space — so biology is uncovering the unknowable mess at its core. Life is built on an edifice of randomness.

One of the first insights into the natural disorder of life arrived in 1968, when Motoo Kimura, the great Japanese geneticist, introduced evolutionary biology to his “neutral theory of molecular evolution.” This is a staid name for what many scientists consider the most interesting revision of evolutionary theory since Darwin. Kimura’s discovery began with a paradox. Starting in the early 1960s, biologists could finally measure the rate of genetic change in species undergoing natural selection. As expected, the engine of evolution was random mutation: double helices suffered from a constant barrage of editing errors. Buried in this data, however, was an uncomfortable new fact: DNA changes way too much. According to Kimura’s calculations, the average genome was changing at a hundred times the rate predicted by the equations of evolution. In fact, DNA was changing so much that there was no possible way natural selection could account for all of these so-called adaptations.

But if natural selection wasn’t driving the evolution of our genes, then what was? Kimura’s answer was simple: chaos. Pure chance. The dice of mutation and the poker of genetic drift. At the level of our DNA, evolution works mostly by accident.* Your genome is a record of random mistakes.

But perhaps that randomness is confined to our DNA. The clocklike cell must restore some sense of order, right? Certainly the translation of our genome — the expression of our actual genes — is a perfectly regulated process, with no hint of disarray. How else could we function? Although molecular biology used to assume that was the case, it isn’t. Life is slipshod. Inside our cells, shards and scraps of nucleic acid and protein float around aimlessly, waiting to interact. There is no guiding hand, no guarantee of exactness.

In a 2002 Science paper entitled “Stochastic Gene Expression in a Single Cell,” Michael Elowitz of Caltech demonstrated that biological “noise” (a scientific synonym for chaos) is inherent in gene expression. Elowitz began by inserting two separate sequences of DNA stolen from fireflies into the genome of E. coli. One gene encoded a protein that made the creatures glow neon green. The other gene made the bacteria radiate red. Elowitz knew that if the two genes were expressed equally in the E. coli (as classical biological theory predicted), the color yellow would dominate (for light waves, red plus green equals yellow). That is, if life were devoid of intrinsic noise, all the bacteria would be colored by the same neon hue.

But Elowitz discovered that when the red-and green-light genes were expressed at ordinary levels, and not overexpressed, the noise in the system suddenly became visible. Some bacteria were yellow (the orderly ones), but other cells, influenced by their intrinsic disorder, glowed a deep turquoise or orange. All the variance in color was caused by an inexplicable variance in fluorescent-protein level: the two genes were not expressed equally. The simple premise underlying every molecular biology experiment — that life follows regular rules, that it transcribes its DNA faithfully and accurately — vanished in the colorful collage of prokaryotes. Although the cells were technically the same, the randomness built into their system produced a significant amount of fluorescent variation. This disparity in bacterial hue was not reducible. The noise had no single source. It was simply there, an essential part of what makes life living.

Furthermore, this messiness inherent in gene translation percolates upward, infecting and influencing all aspects of life. Fruit flies, for example, have long hairs on their bodies that serve as sensory organs. The location and density of those hairs differ between the two sides of the fly, but not in any systematic way. After all, the two sides of the fly are encoded by the same genes and have developed in the same environment. The variation in the fly is a consequence of random atomic jostling inside its cells, what biologists call “developmental noise.” (This is also why your left hand and right hand have different fingerprints.)

This same principle is even at work in our brain. Neuroscientist Fred Gage has found that retrotransposons — junk genes that randomly jump around the human genome — are present at unusually high numbers in neurons. In fact, these troublemaking scraps of DNA insert themselves into almost 80 percent of our brain cells, arbitrarily altering their genetic program. At first, Gage was befuddled by this data. The brain seemed intentionally destructive, bent on dismantling its own precise instructions. But then Gage had an epiphany. He realized that all these genetic interruptions created a population of perfectly unique minds, since each brain reacted to retrotransposons in its own way. In other words, chaos creates individuality. Gage’s new hypothesis is that all this mental anarchy is adaptive, as it allows our genes to generate minds of almost infinite diversity.

And diversity is a good thing, at least from the perspective of natural selection. As Darwin observed in On the Origin of Species, “The more diversified the descendants from any one species become in structure, constitution and habits, by so much will they be better enabled to seize on many and widely diversified places in the polity of nature.” Our psychology bears out this evolutionary logic. From the moment of conception onward, our nervous system is designed to be an unprecedented invention. Even identical twins with identical DNA have strikingly dissimilar brains. When sets of twins perform the same task in a functional MRI machine, different parts of each cortex become activated. If adult twin brains are dissected, the details of their cerebral cells are entirely unique. As Eliot wrote in the preface to Middlemarch, “the indefiniteness remains, and the limits of variation are really much wider than anyone would imagine.”

Like the discovery of neurogenesis and neural plasticity, the discovery that biology thrives on disorder is paradigm-shifting. The more science knows about life’s intricacies, about how DNA actually builds proteins and about how proteins actually build us, the less life resembles a Rolex. Chaos is everywhere. As Karl Popper once said, life is not a clock, it is a cloud. Like a cloud, life is “highly irregular, disorderly, and more or less unpredictable.” Clouds, crafted and carried by an infinity of currents, have inscrutable wills; they seethe and tumble in the air and are a little different with every moment in time. We are the same way. As has happened so many times before in the history of science, the idée fixe of deterministic order proved to be a mirage. We remain as mysteriously free as ever.

The lovely failure of every reductionist attempt at “solving life” has proved that George Eliot was right. As she famously wrote in 1856, “Art is the nearest thing to life; it is a mode of amplifying experience.” The sprawling realism of Eliot’s novels ended up discovering our reality. We are imprisoned by no genetic or social physics, for life is not at all like a machine. Each of us is free, for the most part, to live as we choose to, blessed and burdened by our own elastic nature. Although this means that human nature has no immutable laws, it also means that we can always improve ourselves, for we are works in progress. What we need now is a new view of life, one that reflects our indeterminacy. We are neither fully free nor fully determined. The world is full of constraints, but we are able to make our own way.

This is the complicated existence that Eliot believed in. Although her novels detail the impersonal forces that influence life, they are ultimately celebrations of self-determination. Eliot criticized all scientific theories that disrespected our freedom, and instead believed “that the relations of men to their neighbours may be settled by algebraic equations.” “But,” she wrote, “none of these diverging mistakes can co-exist with a real knowledge of the people.” What makes humans unique is that each of us is unique. This is why Eliot always argued that trying to define human nature was a useless endeavor, dangerously doomed to self-justification. “I refuse,” she wrote, “to adopt any formula which does not get itself clothed for me in some human figure and individual experience.” She knew that we inherit minds that let us escape our inheritance; we can always impose our will onto our biology. “I shall not be satisfied with your philosophy,” she wrote to a friend in 1875, “till you have conciliated Necessitarianism … with the practice of willing, of willing to will strongly, and so on.”

As Eliot anticipated, our freedom is built into us. At its most fundamental level, life is full of leeway, defined by a plasticity that defies every determinism. We are only chains of carbon, but we transcend our source. Evolution has given us the gift of infinite individuality. There is grandeur in this view of life.

* Newton himself wasn’t so naïve: “I can calculate the motion of heavenly bodies,” he wrote, “but not the madness of people.”

† Napoleon fired Laplace after only six weeks. He said Laplace had “carried the idea of the infinitely small into administration.”

* Shortly after breaking Eliot’s heart, Spencer wrote two cruel essays on “Personal Beauty” in which he extolled the virtues of prettiness. In his autobiography he had the audacity to brag of his shallowness, and wrote, in a veiled reference to Eliot, that “Physical beauty is a sine qua non with me; as was once unhappily proved where the intellectual traits and the emotional traits were of the highest.”

But if Spencer was a man unable to see beyond appearances, Henry James believed Eliot was proof that personality could triumph over prettiness. James memorably described his first meeting with Eliot: “To begin with, she is magnificently ugly — deliciously hideous. She has a low forehead, a dull grey eye, a vast pendulous nose, a huge mouth full of uneven teeth and a chin and jaw bone qui n’en finessent [sic] pas … Now in this vast ugliness resides a most powerful beauty which, in a very few minutes steals forth and charms the mind, so that you end as I ended, in falling in love with her.”

* Necessitarianism is a synonym for determinism. Popular in the nineteenth century, the theory holds that human actions are “necessitated” by antecedent causes over which we have no control.

* Of course, being free also makes us accountable for our behavior. One of Eliot’s main problems with social physics was that it denied humanity any moral agency. After all, if every action has an external cause, then it seems cruel to punish cruelty. In her novels, Eliot wanted to describe a more realistic vision of human nature and thus inspire us to become better. If social physics made us callous, then art might make us compassionate.

* Darwin acknowledged the deep chanciness at the heart of natural selection. Although he never uses the adjective random, Darwin constantly asserts that variations are “undirected” and “occur in no determinate way.”

* Eliot enjoyed telling a story about her botanical expeditions to Kew Gardens with Spencer. Being a devout Darwinist, Spencer explained the structure of every flower by referencing some vague story about “the necessity of evolutionary development.” But if a flower failed to fit his neat theory, then it was “tant pis pour les fleurs” (“too bad for the flowers”).

* How did Rakic make his original mistake? There is no easy answer. Rakic is an excellent scientist, one of the finest neuroscientists of his generation. But seeing radioactive new neurons is extremely difficult. These cells are easy to ignore, especially if they shouldn’t be there. One has to be looking for them in order to see them. Furthermore, almost all lab primates live in an environment that suppresses neurogenesis. A drab-looking cage creates a drab-looking brain. Unless the primates are transferred to an enriched enclosure, their adult brains will produce few new neurons. The realization that typical laboratory conditions are debilitating for animals and produce false data has been one of the accidental discoveries of the neurogenesis field.

* Scientists are now discovering that even mental traits that have a strong genetic component — such as IQ — are incredibly sensitive to changes in the environment. A French study of troubled children adopted between the ages of four and six clearly demonstrated the way our innate human nature depends on how we are nurtured. At the time they were adopted, these young children had IQs that averaged around 77, putting them near retardation level. However, when the children retook the IQ test nine years later, all of them did significantly better. This was extremely surprising, since IQ is supposed to be essentially stable over the course of a lifetime. Furthermore, the amount that a child improved was directly related to the adopting family’s socioeconomic status. Children adopted by middle-class families had average scores of 92; those placed in upper-class homes had their IQs climb, on average, more than 20 points, to 98. In a relatively short time, their IQs had gone from significantly below average to practically normal.

* Although Kimura’s conclusions provoked a storm of controversy — some neo-Darwinists said he was just a creationist with fancy mathematics — they shouldn’t have. In fact, Darwin probably would have agreed with Kimura. In the last edition of On the Origin of Species, published in 1872, Darwin makes his own position crystal clear: “As my conclusions have lately been much misrepresented, and it has been stated that I attribute the modification of species exclusively to natural selection, I may be permitted to remark that in the first edition of this work, and subsequently, I placed in a most conspicuous position … the following words: ‘I am convinced that natural selection has been the main, but not the exclusive means of modification.’ This has been of no avail. Great is the power of steady misinterpretation” [Darwin, 1872, p. 395].