Part 3 (1/2)
II.
Cambridge, Ma.s.sachusetts
”h.o.m.o sapiens is the only animal that ...”
How many flattering clauses have philosophers tacked on to that cherished construction, only to watch them eventually crumble? One by one, the faculties on which we thought we could stake the flag of our specialness science has shown belong to other animals as well. Suffering? Reason? Language? Counting? Laughter? Self-consciousness? All have been proposed as human monopolies, and all have fallen before science's deepening understanding of the animal brain and behavior. James Boswell's nomination of cooking as the defining human ability seems more durable than most, though perhaps an even st.u.r.dier candidate would be this: ”Humans are the only species that feels compelled to identify faculties that it alone possesses.”
But here's why cooking may stand a better-than-average chance of surviving this silly game: Only the control of fire and consequent invention of cooking can explain the evolution of brains big and self-conscious enough to construct sentences like ”h.o.m.o sapiens is the only species that ...”
That at least is the import of ”the cooking hypothesis,” a recent contribution to evolutionary theory that throws a wonderfully ironic wrench into the scaffold of our self-regard. Cooking, according to the hypothesis, is not merely a metaphor for the creation of culture, as Levi-Strauss proposed; it is its evolutionary prerequisite and biological foundation. Had our protohuman ancestors not seized control of fire and used it to cook their food, they would never have evolved into h.o.m.o sapiens. We think of cooking as a cultural innovation that lifts us up out of nature, a manifestation of human transcendence. But the reality is much more interesting: Cooking is by now baked into our biology (as it were), something that we have no choice but to do, if we are to feed our big, energy-guzzling brains. For our species, cooking is not a turn away from nature-it is our nature, by now as obligatory as nest building is for the birds.
I first encountered the cooking hypothesis in a 1999 article in the journal Current Anthropology t.i.tled ”The Raw and the Stolen: Cooking and the Ecology of Human Origins” by Richard Wrangham, a Harvard anthropologist and primatologist, and four of his colleagues. Wrangham subsequently fleshed out the theory in a fascinating 2009 book, Catching Fire: How Cooking Made Us Human. Soon after it came out, we began corresponding by e-mail, and eventually we had the opportunity to meet, over a lunch (of raw salads) at the Harvard Faculty Club.
The hypothesis is an attempt to account for the dramatic change in primate physiology that occurred in Africa between 1.9 and 1.8 million years ago, with the emergence of h.o.m.o erectus, our evolutionary predecessor. Compared to the apelike habilines from which it evolved, h.o.m.o erectus had a smaller jaw, smaller teeth, a smaller gut-and a considerably larger brain. Standing upright and living on the ground, h.o.m.o erectus is the first primate to bear a stronger resemblance to humans than apes.
Anthropologists have long theorized that the advent of meat eating could account for the growth in the size of the primate brain, since the flesh of animals contains more energy than plant matter. But as Wrangham points out, the alimentary and digestive apparatus of h.o.m.o erectus is poorly adapted to a diet of raw meat, and even more poorly adapted to the raw plant foods that would still have been an important part of its diet, since a primate cannot live on meat alone. The chewing and digestion of raw food of any kind requires a big gut and big strong jaws and teeth-all tools that our ancestors had lost right around the time they acquired their bigger brains.
The control of fire and discovery of cooking best explain both these developments, Wrangham contends. Cooking renders food much easier to chew and digest, obviating the need for a strong jaw or substantial gut. Digestion is a metabolically expensive operation, consuming in many species as much energy as locomotion. The body must work especially hard to process raw foodstuffs, in which the strong muscle fibers and sinews in meat and the tough cellulose in the cell walls of plants must be broken down before the small intestines can absorb the amino acids, lipids, and sugars locked up in these foods. Cooking in effect takes much of the work of digestion outside the body, using the energy of fire in (partial) place of the energy of our bodies to break down complex carbohydrates and render proteins more digestible.
Applying the heat of a fire to food transforms it in several ways-some of them chemical, others physical-but all with the same result: making more energy available to the creatures that eat it. Exposure to heat ”denatures” proteins-unfolding their origami structures in such a way as to expose more surface area to the action of our digestive enzymes. Given enough time, heat also turns the tough collagen in the connective tissues of muscle into a soft, readily digestible jelly. In the case of plant foods, fire ”gelatinizes” starches, the first step in breaking them down into simple sugars. Many plants that are toxic eaten raw, including tubers such as ca.s.sava, are rendered harmless as well as more nutritious by heat. Other foodstuffs the cook fire purifies, by killing bacteria and parasites; it also r.e.t.a.r.ds spoilage in meat. Cooking improves texture and taste as well, making many foods more tender, and others sweeter or less bitter. Though which comes first-an inborn taste for cooked food or nearly two million years of familiarity with it-is hard to say.
True, cooking can have some negative, seemingly maladaptive, effects, too. High heat produces carcinogenic compounds in some foods, but the danger of these toxins is outweighed by the sheer increase in energy that cooking makes available to us-and life is at bottom a compet.i.tion for energy. Taken as a whole, cooking opened up vast new horizons of edibility for our ancestors, giving them an important compet.i.tive edge over other species and, not insignificantly, leaving us more time to do things besides looking for food and chewing it.
This is no small matter. Based on observations of other primates of comparable size, Wrangham estimates that before our ancestors learned to cook their food they would have had to devote fully half their waking hours simply to the act of chewing it. Chimps like to eat meat and can hunt, but they have to spend so much of their time in mastication that only about eighteen minutes are left each day for hunting, not nearly enough to make meat a staple of their diets. Wrangham estimates that cooking our food gives our species an extra four hours a day. (This happens to be roughly the same amount of time we now devote to watching television.)
”Voracious animals ... both feed continually and as incessantly eliminate,” the Roman physician Galen of Pergamum pointed out, ”leading a life truly inimical to philosophy and music, as Plato has said, whereas n.o.bler animals neither eat nor eliminate continually.” By freeing us from the need to feed constantly, cooking enn.o.bled us, putting us on the path to philosophy and music. All those myths that trace the G.o.dlike powers of the human mind to a divine gift or theft of fire may contain a larger truth than we ever realized.
Yet having crossed this Rubicon, trading away a big gut for a big brain, we can't go back, as much as raw-food faddists would like to. Wrangham cites several studies indicating that in fact humans don't do well on raw food: They can't maintain their body weight, and half of the women on a raw-food regimen stop menstruating. Devotees of raw food rely heavily on juicers and blenders, because otherwise they would have to spend as much time chewing as the chimps do. It is difficult, if not impossible, to extract sufficient energy from unprocessed plant matter to power a body with such a big, hungry brain. (Our brains const.i.tute only 2.5 percent of our weight yet consume 20 percent of our energy when we're resting.) By now, ”humans are adapted to eating cooked food in the same essential way as cows are adapted to eating gra.s.s,” Wrangham says. ”We are tied to our adapted diet of cooked food, and the results pervade our lives, from our bodies to our minds. We humans are the cooking apes, the creatures of the flame.”
How do we know if the cooking hypothesis is true? We don't. It's just a hypothesis, and not an easy one to prove. The fossil evidence that humans were cooking when h.o.m.o erectus walked the earth is not yet there, though it has recently gotten stronger. When Wrangham first published, the oldest known fossil remains put the date for controlled fire at around 790,000 B.C., but Wrangham's hypothesis suggests cooking must have begun at least a million years earlier. In his defense, Wrangham pointed out that evidence of fires that old would be unlikely to survive. Also, cooking meat doesn't necessarily leave behind charred bones. But recently archaeologists found a hearth in a cave in South Africa that pushed the likely date for cooking back considerably further,* to one million years B.C., and the hunt for even older cook fires is on.
So far at least, Wrangham's most convincing arguments are deductive ones. Some new factor of natural selection changed the course of primate evolution about two million years ago, expanding the brain and shrinking the gut; the most plausible candidate for this new selective pressure is the availability of a new, higher-quality diet. Meat by itself could not have supplied that diet. Primates, unlike dogs, don't digest raw flesh efficiently enough to thrive on it. The only diet that could have yielded such a dramatic increase in energy is cooked food. ”We are,” he concludes, ”cooks more than carnivores.”
To demonstrate how the advent of cooking could have supplied a caloric boon sufficient to change the course of our evolution, Wrangham cites several animal-feeding studies comparing raw and cooked or otherwise processed food. When researchers switch a python's diet from raw beef to cooked hamburger, the snake's ”metabolic cost of digestion” is reduced by nearly 25 percent, leaving the animal that much more energy to put to other purposes. Mice grow faster and fatter on a diet of cooked meat than on a diet of the same meat raw.* This might explain why our pets tend toward obesity, since most modern pet food is cooked.
It would seem that all calories are not created equal, or, as a proverb quoted by Jean Anthelme Brillat-Savarin in The Physiology of Taste puts it, ”A man does not live on what he eats, an old proverb says, but on what he digests.” Cooking allows us to digest more of what we eat, and to use less energy doing it. What is curious is that animals seem instinctively to know this: Given the choice, many animals will opt for cooked food over raw. This shouldn't surprise us: ”Cooked food is better than raw,” Wrangham says, ”because life is mostly concerned with energy”-and cooked food yields more energy.
It may well be that animals are ”pre-adapted” to prefer the smells, tastes, and textures of cooked food, having evolved various sensory apparatus to steer them toward the richest sources of energy. Attractive qualities such as sweetness, softness, tenderness, and oiliness all signify abundant, easy-to-digest calories. A hardwired preference for high-energy foods would explain why our evolutionary ancestors would immediately have appreciated cooked foods. In speculating as to exactly how early humans would have discovered all the good things fire does to food, Wrangham points out that many animals scavenge burned landscapes, enjoying particularly the roasted rodents and seeds. He cites the example of chimpanzees in Senegal, who will eat the seeds of the Afzelia tree only after a fire has pa.s.sed through and toasted them. It seems likely that our ancestors would also have scavenged among the remains of forest fires, looking for tasty morsels and, perhaps occasionally, getting lucky enough to have the sort of transformative experience that Bo-bo, the swineherd's son in Charles Lamb's story, did when he first touched that bit of crackling to his tongue.
Like any such theory-indeed, like evolution itself-the cooking hypothesis is not subject to absolute scientific proof. For that reason, some will no doubt dismiss it as another ”just so” story, Prometheus in modern scientific garb. But, really, how much more can we expect when trying to account for something like the advent of ourselves? What the cooking hypothesis gives us is a compelling modern myth-one cast in the language of evolutionary biology rather than religion-locating the origins of our species in the discovery of cooking with fire. To call it a myth is not to belittle it. Like any other such story, it serves to explain how what is came to be using the most powerful vocabulary available, which in our case today happens to be that of evolutionary biology. What is striking in this instance is that cla.s.sical mythology and modern evolutionary theory both gazed into the flames of the cook fire and found there the same thing: the origins of our humanity. Perhaps that coincidence is all the confirmation we can hope for.
III.
Intermission: A Pig's Perspective