How did cannibalism begin?

Actually Cannibalism started in the 1400 where a group of passengers kids and adults were traveling on a plane then the plane crashed and the adults and some kids had died so since the kids had no way to survive they ate the dead bodies. And that's the truth!

Cannibalism has been around for thousands of years. Evidence of cannibalism stretches back to the Paleolithic, when Hominids, our Human ancestors were around. Archaeologists found evidence of hominid bones being systematically butchered, and discarded on piles of animal bones, which strongly suggests the flesh wash stripped off, and probably eaten. There is not a lot of evidence during this time, of ritual cannibalism, and it is currently assumed that cannibalism was a last resort caused by a lack of other food. Once humans became sedentary and practices domestication, cannibalism is more associated with cults and ceremony, and less with desperation.

So in broad terms, human cannibalism was probably initially as a response to lack of any other food sources. There is no evidence of a ritual process, so we must assume that it was not down to specific cults or religions.