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Basing all your behavior on a pre-programmed set of instructions written in the ACGT language places no undue demands—as long as the environment is the one you were evolved for. But no pre-programmed set of instructions, no matter how elaborate, no matter how successful in the past, can guarantee continuing survival in the face of rapid environmental change. Evolution through natural selection involves only the most remote, generalized, almost metaphorical kind of learning from experience. Something else is needed. When you hunt food; when mobility is high and organisms can roam among very different environments; when social relations with your own kind as well as predator/prey interactions become intricate; when you’re required to process enormous amounts of information about the external world—at such times, especially, it pays to have a brain. With a brain you can remember past experiences and relate them to your present predicament. You can recognize the bully who picks on you and the weakling you can pick on, the warm burrow or protected rock crevice to which you have safely fled before. Opportunistic scenarios for gathering food, or hunting, or escaping may occur to you at a critical moment. Neural circuitry develops for data processing, pattern recognition, and contingency planning. There are premonitions of forethought.

The style of evolution of brains—and much else—is not usually a matter of steady progression. Instead, the fossil record speaks of short periods of rapid and radical evolution, punctuating immense periods of time in which the sizes of brains hardly change at all. This seems true from the evolution of the earliest mammals to the evolution of our own species.9 It’s as if there’s a rare concatenation of events—perhaps changes in the DNA sequence and the external environment together—that provides an adaptive opportunity. The new environmental niches are quickly filled, and for a long time subsequent evolution is devoted to consolidating the gains. Major advances in neural architecture—in the brain’s ability to process data, to combine information from different senses, to improve its model of the nature of the outside world, and to think things through—may be very expensive. For many animals these are such broad-gauge talents, requiring so many separate evolutionary steps, that the major benefits may come only in the far future, while evolution is transfixed by the here-and-now. Nevertheless, even tiny advances in thinking are adaptive. Spurts in brain size have happened sufficiently often in the history of life for us to conclude, from this fact alone, that brains are useful to have around.

Feeling, in mammals at least, is mainly controlled by lower, more ancient parts of the brain, and thinking by the higher, more recently evolved outer layers.10 A rudimentary ability to think was superimposed on the pre-existing, genetically programmed behavioral repertoires—each of which probably corresponded to some interior state, perceived as an emotion. So when unexpectedly it is confronted with a predator, before anything like a thought wells up, the potential prey experiences an internal state that alerts it to its danger. That anxious, even panicky state comprises a familiar complex of sensations, including, for humans, sweaty palms, increased heartbeat and muscle tension, shortened breath, hairs standing on end, a queasiness in the belly, an urgent need to urinate and defecate, and a strong impulse either for combat or retreat.* Since in many mammals fear is produced by the same adrenaline-like molecule, it may feel pretty much the same in all of them. That’s at least a reasonable first guess. The more adrenaline in the bloodstream, up to a certain limit, the more fear the animal feels. It’s a telling fact that you can artificially be made to feel this precise set of sensations just by being injected with some adrenaline—as sometimes happens at the dentist’s (to shorten the clotting time of your blood, another useful adaptation when you’re confronting a predator. Of course you may also be generating some of your own adrenaline at the dentist’s.) Fear has to have an emotion tone about it. It has to be unpleasant.

If the predator’s eye/retina/brain combination is geared especially to detect motion, the prey often have, in their repertoire of defenses, the tactic of standing frozen, stock-still, for long periods of time. It’s not that squirrels, say, or deer understand the physiology of their enemies’ visual systems; but a beautiful resonance between the strategies of predator and prey has been established by natural selection. The prey animal may run; play dead; exaggerate its size; erect its hairs and shout; produce foul-smelling or acrid excretions; threaten to counterattack; or try a variety of other, useful survival strategies—all without conscious thought. Only then may it notice an escape route or otherwise bring into play whatever mental agility it possesses. There are two nearly simultaneous responses: one, the ancient, all-purpose, tried-and-true, but limited and unsubtle hereditary repertoire; and the other, the brand-new, generally untried intellectual apparatus—which can, however, devise wholly unprecedented solutions to urgent current problems. But large brains are new. When “the heart” counsels one action and “the head” another, most organisms opt for heart. The ones with the biggest brains more often opt for head. In either case, there are no guarantees.

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Obliged to accommodate to every twist and turn in the environment they depend upon, living things evolve to keep up. By painstaking, small steps, through the passage of immense vistas of geological time, via the deaths of innumerable slightly maladapted organisms, uncomplaining and unlamented, life—in its interior chemistry, external form, and menu of available behavior—became increasingly complex and capable. These changes, of course, are reflected in (indeed, caused by) a corresponding elaboration and sophistication of the messages written in the ACGT code, down there at the level of the gene. When some splendid new invention comes along—bony cartilage as body armor, say, or the ability to breathe oxygen—the genetic messages responsible proliferate across the biological landscape as the generations pass. At first no one has these particular sequences of genetic instructions. Later, large numbers of beings all over the Earth live by them.

It’s not hard to imagine that what’s really going on is an evolution of genetic instructions, battles between the genetic instructions of competing organisms, genetic instructions calling the shots—with the plants and animals little more, or maybe nothing more, than automata. The genes arrange for their own continuance. As always, the “arranging” is done with no forethought; it’s merely that those beautifully coordinated genetic instructions that, by chance, give superior orders to the living thing they inhabit make more living things motivated by the same instructions.

Think again of the changes in our behavior caused by the incursion of a rabies or an influenza virus (made of nucleic acids wearing a coat of protein). Surely much more profound control over us is exercised by our own nucleic acids. When you strip away the fur and feathers, the physiological and behavioral particularities, life is revealed to be the preferential replication of some ACGT messages rather than other, competing messages; a conflict of genetic recipes; a war of words.

In this perspective,11 it’s the genetic instructions that are being selected and that are evolving. Or you might with nearly equal justice say it’s the individual organisms, under the tight control of the genetic instructions, that are being selected and that are evolving. There is no room here for group selection—the natural and attractive idea that species are in competition with one another, and that individual organisms work together to preserve their species as citizens work together to preserve their nation. Acts of apparent altruism are instead attributed chiefly to kin selection. The mother bird slowly flutters from the fox, one wing bent as if broken, in order to lead the predator away from her brood. She may lose her life, but multiple copies of very similar genetic instructions will survive in the DNA of her chicks. A cost-benefit analysis has been made. The genes dictate to the outer world of flesh and blood with wholly selfish motives, and real altruism—self-sacrifice for a non-relative—is deemed a sentimental illusion.12