Is Consciousness Purely an Epi Phenomenon?

I was entriggued by what you were saying about how much of our mental life is unconscious or preconscious. Of course, some people, and it goes back at least as far as thomas huxley, think that consciousness is purely an epi phenomenon, has no function. But i don't believe in this because human beings seem to have a much richer inner mental life than other creatures. And if it has evolved, it must be giving us some sort of advantage. If there is such a thing as a human who is having no conscious experience, they would not actually be able to produce all the behaviour that the rest of us can produce.