Imagine having consciousness having an evolutionary advantage. Imagine the possibility for a complex neural network to "bind" a part of external consciousness to itself. Then you have external but brain-trapped consciousness as an evolutionary possibility. How to prove: First step is to find some creature with nerves but provably lacking consciousness.
This creature is called Philosophical Zombie. Check its wiki page at http://en.wikipedia.org/wiki/Philosophical_zombie for a list of arguments regarding the Zombie.