The point is emergent properties in themselves aren't explained by evolution. If a property emerges that is because biochemistry (in the case of digestion) allowed a property to arise.
If I understand what you're saying, you're right, but I'm not entirely sure what your point is.
If an organism changes the composition of an chemical which it uses to digest food; and the new acid extracts more energy from the food; natural selection explains why this innovation would spread in a population. It doesn't tell you anything about why the altered chemical is more efficient at breaking down food though - that would be a matter of the chemistry.
Is this all you're saying, or am I missing something?
Incidentally, I don't think the selective advantages of consciousness are in any sense obvious. I don't think we will understand them unless and until we understand how consciousness occurs.
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