The Omnivore's Dilemma - Michael Pollan [147]
There does seem to be an evolutionary trade-off between big brains and big guts—two very different evolutionary strategies for dealing with the question of food selection. The case of the koala, one of nature’s pickiest eaters, exemplifies the small-brain strategy. You don’t need a lot of brain circuitry to figure out what’s for dinner when all you ever eat is eucalyptus leaves. As it happens, the koala’s brain is so small it doesn’t even begin to fill up its skull. Zoologists theorize that the koala once ate a more varied and mentally taxing diet than it does now, and that as it evolved toward its present, highly circumscribed concept of lunch, its underemployed brain actually shrank. (Food faddists take note.) More important to the koala than brains is a gut big enough to break down all those fibrous leaves. By the same token, the digestive tract of primates like us has grown progressively shorter as we’ve evolved to eat a more varied, higher quality diet.
Eating might be simpler as a thimble-brained monophage, but it’s also a lot more precarious, which partly explains why there are so many more rats and humans in the world than koalas. Should a disease or drought strike the eucalyptus trees in your neck of the woods, that’s it for you. But the rat and the human can live just about anywhere on earth, and when their familiar foods are in short supply, there’s always another they can try. Indeed, there is probably not a nutrient source on earth that is not eaten by some human somewhere—bugs, worms, dirt, fungi, lichens, seaweed, rotten fish; the roots, shoots, stems, bark, buds, flowers, seeds, and fruits of plants; every imaginable part of every imaginable animal, not to mention haggis, granola, and Chicken McNuggets. (The deeper mystery, only partly explained by neophobia, is why any given human group will eat so few of the numberless nutrients available to it.)
The price of this dietary flexibility is much more complex and metabolically expensive brain circuitry. For the omnivore a tremendous amount of mental wiring must be devoted to sensory and cognitive tools for figuring out which of all these questionable nutrients it is safe to eat. There’s just too much information involved in food selection to encode every potential food and poison in the genes. So instead of genes to write our menus omnivores evolved a complicated set of sensory and mental tools to help us sort everything out. Some of these tools are fairly straightforward and we share them with many other mammals; others represent impressive feats of adaptation by primates; still others straddle the blurry line between natural selection and cultural invention.
The first tool is of course our sense of taste, which performs some of the basic work screening foods for value and safety. Or as Brillat-Savarin put it in The Physiology of Taste, taste “helps us to choose, from the various substances offered us by nature, those which are proper to be consumed.” Taste in humans gets complicated, but it starts with two powerful instinctual biases, one positive, the other negative. The first bias predisposes us toward sweetness, a taste that signals a particularly rich source of carbohydrate energy in nature. Indeed, even when we’re otherwise sated, our appetite for sweet things persists, which is probably why dessert shows up in the meal when it does. A sweet tooth represents an excellent adaptation for an omnivore whose big brain demands a tremendous amount of glucose (the only type of energy the brain can use), or at least it once did, when sources of sugar were few and far between. (The adult human brain accounts for 2 percent of our body weight but consumes 18 percent of our energy, all of which must come from a carbohydrate.