Hitch's comment about there being nowhere in the world where people practised slavery except those refering to a holy book is not correct. I believe it happened in Africa, and certainly did amongst the Maoris here in New Zealand. They kept slaves and had no written language prior to the arrival of Europeans. They were weaned off both slavery and cannibalism by Christian missionaries. Some have suggested they did have a supernatural/religious basis for misuse of others as they could assume the "mauri" or spirit and mana of those whose brains they had eaten. Others say it was mainly to humiliate them.

An interesting parallel has been made where the practise of consuming a communion wafer was grafted on to existing ideas of consuming mana from another being. Maybe this meant they were ripe for Christanity.