Where did cannibalism come from?

No one knows. It seems like it has always been around, even before people!

The jury is still out that cannibalism as a ritual practice exists at all - or ever existed in human history as opposed to that practiced under conditions of starvation or perversion). Although much folklore abounds, no credible sighting of cannibalism by anthropologists exist. As the late Scientific American writer, Martin Gardner noted in his book Did Adam and Eve Have Navels? Debunking Pseudoscience, that the sceptical case for why cannibalism is rare to non-existent, and rather something which emerges from the imaginations of ethnographers and archaeologists, or is rooted in scurrilous insults hurled between ethnic groups which have antagonistic relationships. He cites a 1979 book by William Arens entitled The Man-Eating Myth opened up the issue with a strong argument in favour of the belief that cannibalism has never been prevalent in any culture. Gardner notes that claims of institutionalized cannibalism have always been made by enemies, never the tribes themselves, and have usually proven hard to follow up. He refrains from taking sides but admits "My sympathies at the moment are with Arens."