Here's a fascinating video in which one of the foremost philosophers of mind, the Australian David Chalmers, discusses the difficulty of explaining consciousness from an evolutionary materialistic perspective. His point is that as far as natural selection is concerned, we could have evolved to be highly effective automata or zombies, operating strictly on instinct, without the phenomenon of being self-aware added on.
In other words, according to Chalmers, there's nothing about consciousness, as compared to the zombie model, that gives a creature an additional benefit in passing along its genetic legacy to its descendants. In that sense consciousness is superfluous and therefore devilishly hard to explain as a product of unguided evolution. Where then did it come from and how did we come to possess it?
Coming up with an explanation for how material processes like chemical reactions in the brain can actually create sensations - such as the experience of red or sweet or pain - and explaining exactly what these sensations are is what philosophers of mind call the "hard problem" of consciousness. The "easy" problem is mapping the parts of the brain that are involved when various sensations (called qualia)are being experienced. The "easy" problem is "easy" only relative to the difficulty of the "hard" problem.