To briefly add a comment to this, it has been popular since the age of computers began, to compare the human body to a machine. I'm not sure how well that really fits. It's quite a materialistic attitude and enables many scientists, for example, to equate the brain with a computer, or a processor: the mind IS the brain. Personally I feel that this is too reductionistic an idea about consciousness.

When I think back to what life on earth was probably like a few billion years ago, I think of those single-celled organisms -- blue-green algae and the like. At some point there evolved a kind of symbiotic relationship between some of them: co-operating with each other, to the benefit of each organism, would have saved energy and most probably been naturally selected for. This way of life could have progressed until multicellular organisms appeared. Again, it conserves energy if the cells within the organism are specialised, each carrying out a series of specific tasks, intead of each individual carrying out every single one of the processes necessary for its own life.

Multicellular organisms, including us, can be seen as the ultimate in symbiotic relationships. Even consciousness itself can be seen as a development that somehow emerges from this collection of living things, in order to facilitate the symbiosis. And why can that consciousness not regulate communication between different parts of our bodies, at all levels, from the individual cells to the entire body? I think there's a great deal to be learned about this.

So yes, I think it's entirely possible that this kind of complexity could have emerged from single-celled organisms in the past. It's a matter of finding an analogy that fits the reality we see and experience. And of course there's the physical evidence in the fossil record.