Humans are essentially a collective of symbiotic organisms which have been working together for so very long that some are no longer distinguishable as separate. We know that we have DNA that came from external sources. We know that we carry around a host of bacteria that are vital to our life and health. Somehow all of this (Scientifically speaking)results in an entity with a sense of "I". If you aren't thinking about it religiously, how else would one think about it?