So where all the aliens originally human?

wow you are genius, I never thought of that. They should work this angle in the sequel. Funny how hollywood makes sequels not many people want, but they don't make this sequel that many fans die for.....

No he became like them because of the fluid needed to kickstart the mothership which was very rare. Also, at very beginning of the movie, we see the inside of the ship full of them "prawns"...