The inhabitants of the east African island of Pemba rejoice when one of the young women there starts eating earth -- this unusual food supplement can only mean one thing: that she is expecting a baby.

"The daily portion is about 25 grams of dirt," says Sera Young, who works full-time on research into geophagy, or the practice of eating earth. The 30-year-old anthropologist is soon to transfer from Cornell University to the University of California in Berkeley.

On every continent with the exception of Antarctica, there are people who snack on chalk, loam or marl. But it's only now that Young and her colleagues are gradually beginning to understand what force brings them to do this. Whether people are eating loam from natural sources or buying "healing clay" at the drugstore and eating it, they are clearly following some ancient craving that has been shaped over the course of evolution.

It is not only humans who indulge in a bit of dirt every now and then -- parrots, cattle, rats, elephants and chimpanzees also partake. Even prehistoric man shared this passion for eating earth -- an archaeological dig in Africa uncovered powdered loam that had clearly been used as marching rations two million years ago. But the question remains: why?

In her field studies on the island of Pemba, which belongs to Tanzania, Young observed that it is mainly pregnant women who experience cravings for earth. "It's like an addiction. There is even a word for it: vileo," she says.

However, the pregnant women do not simply sweep up their earthy meal from the streets. In fact, they go to great lengths to ensure they have the right type of earth. They rake loam from specific springs or collect it from certain places outside their villages. "The dirt cannot be dirty," Young explains.

The choosiness of earth eaters was something that struck German naturalist Alexander von Humboldt 200 years ago when he spent time in what is now Venezuela. The indigenous Ottomac people, he noted, preferred those alluvial layers where "the thickest, finest-feeling earth" was to be found.

The fact that the indigenous people devoured this dirt in "tremendous quantities" and stored it for times of hardship in the form of dried clay balls, led Humboldt to infer that geophagy was used as a makeshift solution in times of food shortages. In fact, people do eat earth particularly often in leaner times, like on Haiti in 2004 when slum dwellers were given flat cakes baked from butter, salt, water and dirt.

However, this hunger hypothesis does not really explain the phenomenon fully -- earth is also on the menu for the well fed. Many researchers, therefore, think that earth works as a natural medicine. Loam, after all, contains magnesium, sodium, calcium, potassium, iron and large amounts of silicates. In cases of severe diarrhea, according to some scientists, a teaspoon of dirt could provide the body with the minerals it has lost.

The British soil researcher Peter Hooda, however, has discovered indications that, on the contrary, loam takes more away from the body than it provides. The scientist and his team came to this surprising conclusion after carrying out a laboratory simulation of the interaction between dirt and the digestive tract. They mixed loam, gastric acid and nutrients, left the resultant muddy mixture at body temperature for long enough to react fully and then analyzed the resultant compound.

A Natural Detox for the Stomach

Their results showed that many nutrients clung on tightly to microscopically small structures in the loam. This led to a significant reduction in available iron, zinc and copper in the mud bath, which is in line with one of Young's observations on Pemba: many loam-lovers were anemic and had conspicuously low levels of iron in their blood.

In certain circumstances, however, surmises the anthropologist, the leaching effect of the dirt must be an advantage. "Dirt may help to remove poisonous substances from the body." This theory is backed up by something that Young noticed after studying over 2,700 relevant cases in literature on the subject: small children and pregnant women -- people for whom poisoning could be particularly serious -- make particularly frequent use of this natural resource.

Up until now, morning sickness has been seen as an evolutionary mechanism developed to protect the unborn child from harmful substances in food. Could geophagy be an additional strategy?

In an attempt to give more substance to her theory, Young is currently having 30 loamy samples from Pemba, Kyrgyzstan, Indonesia and other areas analyzed by the Macaulay Institute in Aberdeen, Scotland, in order to understand to what extent they have the chemical potential to get rid of toxic foodstuffs.

The analyses could provide scientific proof of what many earth eaters have always said: dirt cleans the stomach.