Wednesday, October 25, 2006


All of the following is cut and pasted from a paper by Leda Cosmides & John Tooby which paper you can link to with the following Principle:

Principle 2. Our neural circuits were designed by natural selection to solve problems that our ancestors faced during our species' evolutionary history.

To say that the function of your brain is to generate behavior that is "appropriate" to your environmental circumstances is not saying much, unless you have some definition of what "appropriate" means. What counts as appropriate behavior?

"Appropriate" has different meanings for different organisms. You have sensory receptors that are stimulated by the sight and smell of feces—to put it more bluntly, you can see and smell dung. So can a dung fly. But on detecting the presence of feces in the environment, what counts as appropriate behavior for you differs from what is appropriate for the dung fly. On smelling feces, appropriate behavior for a female dung fly is to move toward the feces, land on them, and lay her eggs. Feces are food for a dung fly larva—therefore, appropriate behavior for a dung fly larva is to eat dung. And, because female dung flies hang out near piles of dung, appropriate behavior for a male dung fly is to buzz around these piles, trying to mate; for a male dung fly, a pile of dung is a pick-up joint.

But for you, feces are a source of contagious diseases. For you, they are not food, they are not a good place to raise your children, and they are not a good place to look for a date. Because a pile of dung is a source of contagious diseases for a human being, appropriate behavior for you is to move away from the source of the smell. Perhaps your facial muscles will form the cross-culturally universal disgust expression as well, in which your nose wrinkles to protect eyes and nose from the volatiles and the tongue protrudes slightly, as it would were you ejecting something from your mouth.

Photo: the back of the Museum of Arts and Culture in Spokane from a little ampitheater looking up.

For you, that pile of dung is "disgusting". For a female dung fly, looking for a good neighborhood and a nice house for raising her children, that pile of dung is a beautiful vision—a mansion. (Seeing a pile of dung as a mansion—that's what William James meant by making the natural seem strange).

The point is, environments do not, in and of themselves, specify what counts as "appropriate" behavior. In other words, you can't say "My environment made me do it!" and leave it at that. In principle, a computer or circuit could be designed to link any given stimulus in the environment to any kind of behavior. Which behavior a stimulus gives rise to is a function of the neural circuitry of the organism. This means that if you were a designer of brains, you could have engineered the human brain to respond in any way you wanted, to link any environmental input to any behavior—you could have made a person who licks her chops and sets the table when she smells a nice fresh pile of dung.

But what did the actual designer of the human brain do, and why? Why do we find fruit sweet and dung disgusting? In other words, how did we get the circuits that we have, rather than those that the dung fly has?

When we are talking about a home computer, the answer to this question is simple: its circuits were designed by an engineer, and the engineer designed them one way rather than another so they would solve problems that the engineer wanted them to solve; problems such as adding or subtracting or accessing a particular address in the computer's memory. Your neural circuits were also designed to solve problems. But they were not designed by an engineer. They were designed by the evolutionary process, and natural selection is the only evolutionary force that is capable of creating complexly organized machines.

Natural selection does not work "for the good of the species", as many people think. As we will discuss in more detail below, it is a process in which a phenotypic design feature causes its own spread through a population (which can happen even in cases where this leads to the extinction of the species). In the meantime (to continue our scatological examples) you can think of natural selection as the "eat dung and die" principle. All animals need neural circuits that govern what they eat—knowing what is safe to eat is a problem that all animals must solve. For humans, feces are not safe to eat—they are a source of contagious diseases. Now imagine an ancestral human who had neural circuits that made dung smell sweet—that made him want to dig in whenever he passed a smelly pile of dung. This would increase his probability of contracting a disease. If he got sick as a result, he would be too tired to find much food, too exhausted to go looking for a mate, and he might even die an untimely death. In contrast, a person with different neural circuits—ones that made him avoid feces—would get sick less often. He will therefore have more time to find food and mates and will live a longer life. The first person will eat dung and die; the second will avoid it and live. As a result, the dung-eater will have fewer children than the dung-avoider. Since the neural circuitry of children tends to resemble that of their parents, there will be fewer dung-eaters in the next generation, and more dung-avoiders. As this process continues, generation after generation, the dung-eaters will eventually disappear from the population. Why? They ate dung and died out. The only kind of people left in the population will be those like you and me—ones who are descended from the dung-avoiders. No one will be left who has neural circuits that make dung delicious.

In other words, the reason we have one set of circuits rather than another is that the circuits that we have were better at solving problems that our ancestors faced during our species' evolutionary history than alternative circuits were. The brain is a naturally constructed computational system whose function is to solve adaptive information-processing problems (such as face recognition, threat interpretation, language acquisition, or navigation). Over evolutionary time, its circuits were cumulatively added because they "reasoned" or "processed information" in a way that enhanced the adaptive regulation of behavior and physiology.

Realizing that the function of the brain is information-processing has allowed cognitive scientists to resolve (at least one version of) the mind/body problem. For cognitive scientists, brain and mind are terms that refer to the same system, which can be described in two complementary ways—either in terms of its physical properties (the brain), or in terms of its information-processing operation (the mind). The physical organization of the brain evolved because that physical organization brought about certain information-processing relationships—ones that were adaptive.

It is important to realize that our circuits weren't designed to solve just any old kind of problem. They were designed to solve adaptive problems. Adaptive problems have two defining characteristics. First, they are ones that cropped up again and again during the evolutionary history of a species. Second, they are problems whose solution affected the reproduction of individual organisms—however indirect the causal chain may be, and however small the effect on number of offspring produced. This is because differential reproduction (and not survival per se) is the engine that drives natural selection. Consider the fate of a circuit that had the effect, on average, of enhancing the reproductive rate of the organisms that sported it, but shortened their average lifespan in so doing (one that causes mothers to risk death to save their children, for example). If this effect persisted over many generations, then its frequency in the population would increase. In contrast, any circuit whose average effect was to decrease the reproductive rate of the organisms that had it would eventually disappear from the population. Most adaptive problems have to do with how an organism makes its living: what it eats, what eats it, who it mates with, who it socializes with, how it communicates, and so on. The only kind of problems that natural selection can design circuits for solving are adaptive problems.

Obviously, we are able to solve problems that no hunter-gatherer ever had to solve—we can learn math, drive cars, use computers. Our ability to solve other kinds of problems is a side-effect or by-product of circuits that were designed to solve adaptive problems. For example, when our ancestors became bipedal—when they started walking on two legs instead of four—they had to develop a very good sense of balance. And we have very intricate mechanisms in our inner ear that allow us to achieve our excellent sense of balance. Now the fact that we can balance well on two legs while moving means that we can do other things besides walk—it means we can skateboard or ride the waves on a surfboard. But our hunter-gatherer ancestors were not tunneling through curls in the primordial soup. The fact that we can surf and skateboard are mere by-products of adaptations designed for balancing while walking on two legs.

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