Consciousness is what separates life from non-life.
Gazzaniga commences with a wonderful summary of the history of thinking on the subject of consciousness from the ancient Greeks to the present day. Worth reading the book for that alone. He then introduces the reader to the notion that the origins of life itself sparked consciousness. By the way, that means we can never build consciousness into machines since machines are not alive. So, he stretches the evolution of consciousness further back in time than just about any other writer on the topic. The idea is certainly thought-provoking even if the question, what is the purpose of consciousness, is left hanging.
Link to book: https://www.amazon.co.uk/Consciousness-Instinct-Unraveling-Mystery-Brain/dp/0374715505/ref=sr_1_1?s=books&ie=UTF8&qid=1533212924&sr=1-1&keywords=the+consciousness+instinct
You may also like to browse other neuroscience books: https://www.thesentientrobot.com/category/neuroscience/neuroscience-books/