Consciousness allows an organism to leverage evolutionary principles within its own lifetime.
[reading time 5 minutes]
We are all familiar with Darwinian evolution: an organism which happens to woo fitter mates or take special advantage of its environment will produce more successful offspring. The timescale for action here is no less than one generation; within that generation the cards, as they say, are already dealt. Furthermore, the mechanisms by which evolution acts apply whether the organism is conscious or not; whether baboon or bacteria.
What if an organism could apply the principles of evolution within its own lifetime? What a spectacular advantage this would be. Indeed, this advantage is what we call consciousness. That is, consciousness evolved precisely because it imparts flexibility over and above the capabilities of non-conscious lifeforms.
This does not tell us how consciousness is produced by a nervous system (the "easy" problem of consciousness - see my post on Cognition as Maps-of-Maps), nor does this solve the "hard" problems of consciousness (why subjective qualia are so unique and personal). But this does tell us why consciousness came to be, what it is useful for, and how it sets conscious beings apart from non-conscious ones.
A clue that leads us to this conclusion is provided by Donald Hoffman in his book The Case Against Reality. Using game theory, Hoffman postulates a quite different evolutionary reason for consciousness: that it is a way of simplifying and hiding reality such that conscious beings out-compete those that spend energy seeing reality in high-fidelity. This is called the fitness-beats-truth hypothesis (previously known as the interface theory of perception), and it is worth taking a moment to explain, because it is bold and profound. From an evolutionary standpoint, Hoffman notes, the coin of the realm is fitness, not truth (veridical perception). Thus, using oxygen as an example, any lifeform that was informed simply when oxygen levels were good or bad, rather than knowing details of exactly how much oxygen was available, etc., would have an immediate advantage. In its efficiency, such an organism would out-compete (to extinction, Hoffman suggests) any life which wasted energy on higher-fidelity modeling of its surroundings.
Hoffman's mistake, I believe, is in assuming that everything in an organism's surroundings offers up an imminent, ready-to-hand fitness potential. That is, everything which surrounds us is fitness-valenced, and all we have to do is detect said valence. What is missing here is this: most of our environment is fitness-neutral. Therefore, any organism which could develop a way to sense this environment in this neutral way, for what it is, veridically, then learn to utilize it to its advantage (for food, tools, etc) - *that* organism is the one which out-competes its predecessors who can only see the immediately fitness-valenced environment. Consciousness is thus our way of perceiving high-fidelity reality, and learning to imbue otherwise fitness-neutral surroundings with fitness-valence, through the real-time evolutionary competition of ideas. Locke himself was aware of this, in his notion that everything in our consciousness comes to us first through clear-glass senses, our ideas eventually colored by judgements built-up slowly from birth via small uneasinesses or desires. Living 200 years before Darwin, however, he merely lacked the framework of evolution to explain as much in modern language.
In his boldness, then, Hoffman almost has it right. His is a profound step along the evolutionary road to consciousness, but it only gets us part way there. The fitness-beats-truth hypothesis explains the evolutionary development of our interoceptive/emotive nervous systems (see this post on Barrett’s work), which are indeed inherently valenced. Our higher order sensations (vision, hearing, etc.), however, being un-valenced and capable of expressing a fitness-neutral landscape, have evolved advantageously to approach reality with as much fidelity as possible.