This quote from Minsky is how I think consciousness evolved:

Has any of this cleared up what consciousness is or is for?" I asked. "People have such a small number of memory registers," Minsky said, "that we can't think of much. Everything has to be on automatic. Consciousness is not a window. It's more like a debugging trace you use for reprogramming around problems. Humans are really amazing, considering. Just think of what we must be the next step toward. Imagine having a four-megabit consciousness chip in your head."

That is humans were faced with problems that autopilot systems couldn’t handle.

Of course, this shifts the question to why other animals did not have similar evolutionary pressure. I guess there has to be a minimum brain capacity for the runaway evolutionary process to get started.