At its most basic, the story of life on earth is the competition among species to capture and store as much energy as possible—either directly from the sun, in the case of plants, or, in the case of animals, by eating plants and plant eaters. The energy is stored in the form of carbon molecules and measured in calories: The calories we eat, whether in an ear of corn or a steak, represent packets of energy once captured by a plant. The C-4 trick helps explain the corn plant’s success in this competition: Few plants can manufacture quite as much organic matter (and calories) from the same quantities of sunlight and water and basic elements as corn. (Ninety-seven percent of what a corn plant is comes from the air, three percent from the ground.)
The trick doesn’t yet, however, explain how a scientist could tell that a given carbon atom in a human bone owes its presence there to a photosynthetic event that occurred in the leaf of one kind of plant and not another—in corn, say, instead of lettuce or wheat. That’s because all carbon is not created equal. Some carbon atoms called isotopes, have more than the usual complement of six protons and six neutrons, giving them a slightly different atomic weight. C-13, for examples, has six protons and seven neutrons. (Hence “C-13.”) For whatever reason, when a C-4 plant goes scavenging for its four-packs of carbon, it takes in
more carbon 13 than ordinary—C-3—plants, which exhibit a marked preference for the more common carbon 12. Greedy for carbon, C-4 plants can’t afford to discriminate among isotopes, and so end up with relatively more carbon 13.The higher the ratio of carbon 13 to carbon 12 in a person’s flesh, the more corn has been in his diet—or in the diet of the animals he or she ate. (As far as we’re concerned, it makes little difference whether we consume relatively more or less carbon 13.)
One would expect to find a comparatively great deal of carbon 13 in the flesh of people whose staple food of choice is corn—Mexicans, most famously. Americans eat much more wheat than corn—114 pounds of wheat flour per person per year, compared to 11 pounds of corn flour. The Europeans who colonized America regarded themselves as wheat people, in contrast to the native corn people they encountered; wheat in the West has always been considered the more refined, or civilized, grain. If asked to choose, most of us would probably still consider ourselves wheat people (except perhaps the proud corn-fed Midwesterners, and they don’t know the half of it), though by now the whole idea of identifying with a plant at all strikes us as a little old fashioned. Beef people sounds more like it, though nowadays chicken people, which sounds not nearly so good, is probably closer to the truth of the matter. But carbon 13 doesn’t lie, and researchers who have compared the isotopes in the flesh or hair of North Americans to those in the same tissues of Mexicans report that it is now we in the North who are the true people of corn. “When you look at the isotope ratios,” Todd Dawson, a Berkeley biologist who’s done this sort of research, told me, “we North Americans look like corn chips with legs.” Compared to us, Mexicans today consume a far more varied diet: the animals they eat still eat grass (until recently, Mexicans regarded feeding corn to livestock as a sacrilege); much of their protein comes from legumes; and they still sweeten their beverages with cane sugar.
So that’s us: processed corn, walking.