Sephiroth was a human whom was unfortunately born son to a mad scientist called Hojo. Hojo decided to experiment on his son (possibly before he was born I'm not sure cus I havent played it in a while) and injected him with jenova cells - the first Jenova experiment which made Sephy special. He then had the mako treatment like every SOLDIER dude. Does that answer the question?
And for future reference could those who want to make INTELLIGENT suggestions concerning Aeris or any other argument please make their writing at least vaguely coherant, I could barely understand this thread-starter at all. Unintelligent arguments can be saved for little schoolkids who believe anything.