Some think that AI will arise spontaneously when computer network complexity and processing capability approach the level of the human brain. This appears to miss the fact that the human brain evolved with many different areas of specialization. It's not the tabula rasa that some suppose.

Others think that the first AI will be achieved via reverse engineering of the human brain -- duplicating its structures without fully understanding how it works.

Another possibility is that the first significantly super-human intelligence will be based on an augmented human intelligence, but would the human portion would remain a significant portion of the whole?