Cannibalism. It's the last, greatest taboo: the stuff of urban legends and ancient myths, airline crashes and Captain Cook. But while we might get a thrill at the thought of the black widow spider's gruesome mating habits or the tragic fate of the 19th-century Donner Party pioneers, today cannibalism belongs to history - or, at the very least, the realm of the weird, the rare and the very far away. Doesn't it? Here, zoologist Bill Schutt digs his teeth into the subject to find an answer that is as surprising as it is unsettling