Once upon a time, the jawless vertebrates ruled the world. OK, technically they ruled the seas, but 400 million years ago, the seas were the world. Jawless vertebrates were fast and strong. The soft-bodied creatures and the hard-shelled creatures rightly feared them.
But then we came along, the jawed vertebrates (or gnathostomates). Wherever the jawless vertebrates were, we took over. Whatever they did, we could do it better. We could swim as fast and agilely as they could, but we could also catch prey in our teeth and crush it in our jaws. Whatever ecological niche the jawless vertebrates had, we took from them. As soon as we could eat and breed enough, the jawless vertebrates were wiped out.
But not quite wiped out. Two kinds of jawless vertebrates survive, the lampreys and the hagfishes. How did they survive? By finding new niches. But where could poor, primitive jawless vertebrates find new niches? In us, the jawed vertebrates. They eat our insides. Lampreys attach to jawed fish, rip through their flesh, and suck out their blood. Hagfish slip into dead jawed fish and eat them from the inside out.
If you can't beat 'em, eat 'em.
Attack of the Gut-Monsters: another evolution story.