Oddly enough I had a similar thought, and came up with a crazy hypothesis. Once upon a time (namely primordial Earth), a natural localised disaster cut off an area of plankton/early life/whatever from it's natural energy source (the Sun). Slowly, as available energy ran out, some of the early life did the only reasonable thing and started 'eating' the other life forms that were dead from starvation, and slowly started eating even forms that were still alive. Considering that there was no mechanism to defend against being eaten (yet), these new early predators spread and started eating other life forms that had already done the hard work in harvesting the sun's energy.
Yes, that does mean that my hypothesis is stating that all herbivores and carnivores evolved from single cell cannibals
Feel free to dismiss the idea