Jenova was actually only part... heck I bet Sephiroth would still be born... as Malcom from Jurrasic Park said, "Life will find a way." And Sephiroth would probably still go insane...he was still an experiment from Hojo and would probably go insane just the same..but not a powerful.
It was Jenova that wiped out the Ancients. Jenova that Shira thought was and ancient and tried to make a 'clone', got psycho instead, that went on a rampage to destroy the earth. Jenova was the one that weakened the planet. Jenova is the one who started all this CRAP and it's being 'finished' 2000 years after it got to the planet. Can you not see? Don't force me to make an epic post describing Jenova and Sephiroth and what the hell's up with them. Don't. Your eyes will bleed.