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It was, he wrote, humbly, “a matter that awaits the investigation of a physiologist.” But the implication of his litany was clear. Could clay allow us to eat the unpalatable, even the toxic? In a famous case history from 1581, a condemned criminal named Wendel Thumblardt had offered himself to science: in lieu of execution, he proposed to test the purported power of the terra sigillata of Lemnos by taking it with mercuric chloride. He survived and was freed.

Sweet Earth
Consider the potato. Today’s cultivated potatoes are the descendents of wild varieties domesticated in the Central Andes some eight to ten thousand years ago. Most wild potatoes still contain high levels of glycoalkaloids that render the tuber not only unpalatable but potentially toxic, damaging the gut wall and impairing nerve conduction. Humans deal with this toxicity in two principal manners: by the selective breeding of less bitter varieties (the kind found on American dinner tables) or by detoxification. The latter, still practiced in the Andes, is a complex process, involving freezing the potatoes, trampling them, leeching them in pools of moving water, then alternately drying and freezing them for storage. Or, as is also still practiced, they may be eaten with clays. That the clays cut bitterness, Laufer knew; what he didn’t know was how. More than fifty years after he wrote his monograph, those physiologists finally caught up with him, discovering that the preferred Andean clays bound glycoalkaloids, blocking them from being absorbed from the gut.

In other words, it is not earth itself that provides nutrition, but rather the other foods that earth allows us to eat.

The implications of these discoveries stretch far beyond the potato. We can imagine how ancient man chose which food to hunt, but how did he choose which foods to gather? Anthropologists speak of two competing impulses for any creature facing the bewildering array of wild plants: omnivory (our diet should be diverse) and neophobia (we fear the new). In times of plenty, there may be little tension between these: with a diverse diet, there is less pressure to try new foods. But in times of famine, we are driven to eat the unfamiliar: new roots, new leaves, new fruits, new bark. There is no time to learn the complex techniques of detoxification such as those practiced with potatoes in the Andes. So we protect ourselves with earth.

In turning to geophagy to safely consume poisons, we are hardly alone. It is found in wild parrots eating bitter berries and in laboratory rats fed toxins. Over fifty species of primates practice pica; it seems difficult to argue that humans should be exempted. And for us, such instincts might be reinforced by learning: from our parents or even (Eve-like) the example of animals. After all, “taste” is almost entirely learned. We crave sweet and avoid bitter, but otherwise most of our likes and dislikes are based on either cultural traditions or the “postingestive experience” (if something makes us sick, it won’t taste so good the next time around). In other words, we can learn to like, or dislike, almost anything. One needs only to look at the diversity of our delicacies. We eat the molding (Brie, France), the fetid (durians, Malaysia), the live (casu marzu maggot cheese, Sardinia), even the human (placentas, which according to a recent scholarly article, is particularly “Californian”). There is little reason not to eat dirt, provided it’s not contaminated by human waste, and provided we eat it with enough of something else.

By this vision, craving earth is not a bizarre culture of the Other. The slaves on the Surinam plantations described by Cragin craved earth because they were malnourished or hungry or ill, or because of a rich tradition of the medicinal use of clay. It wasn’t just a “habit of their own country,” as Humboldt wrote, but either an effective prophylaxis, or a wise cure for a body in need.

And so too with the Otomacs at times of flood and famine. And the Hopi during their droughts. And the Sardinian peasant and the mystified ice cravers who post their confessions online. Like Wendel Thumblardt, bodies seeking cures.

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