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http://blog.wired.com/wiredscience/2008/01/dirt-its-whats.html

Dirt: It's What's for Dinner, at Least for Chimps

By Brandon Keim

New research on chimpanzees may provide clues to the seemingly bizarre
human habit of eating dirt.

In a study published in Naturwissenschaften, scientists from the
French Museum of Natural History observed chimps in Uganda's Kibale
National Park consuming soil just before or after eating plants known
to contain malaria-killing compounds.

The scientists took leaf samples and mixed them, both alone and with
sampled soil, in the field equivalent of a digestive tract. On their
own, the leaves displayed no pharmaceutical power, but soil and leaves
together were powerfully antimalarial.

This could explain the human tendency towards geophagy, traditionally
practiced in many non-modern cultures but now considered bizarre.

(As an aside, I once knew someone who liked to eat dirt; the first
time I saw them grab and gulp a handful from a construction slag pile
I thought they were kidding, but they weren't. They'd picked up the
habit as a kid, as part of a local tradition. Later they even ate clay
purchased at an arts-and-crafts store. I came to accept this, but
never quite got over the weirdness.)

To further investigate the overlap between chimpanzee and human
behaviors, the researchers tested the composition of soil used by a
local healer to treat diarrhea. Both that and the chimps' soil were
high in kaolinite -- itself a once-common anti-diarrheal whose origins
are still evident in the brand name Kaopectate.

So how did the chimp geophagy start? Did some member of a colony
realize that it made defecation a whole lot tidier? Or was that a
beneficial side effect of a certain taste for earthily mixed greens, a
taste that pleased not just the palate but the physiology of its
devotees and thus spread through chimp society -- and then, perhaps,
into the societies of our own distant ancestors?