I'll just add that it's also pretty certain that Jenova is capable of close-range manipulation even without the target having her cells. Tifa and other party members were affected by Jenova/Sephiroth's power. They were vulnerable to the illusions, if nothing else.
Which is possibly why Ghast believed Jenova was an ancient. He KNEW that the ancients looked just like humans, yet believed Jenova- who didn't look anything close to human- was an ancient. And, she probably manipulated them into injecting her cells into other beings. I could see Hojo using his child as an experiment, but Lucrecia, too? And for all we know, Hojo was a decent guy before going all psycho. Ghast helped the process, too, so one must question how fishy that whole scenario was.
It's also (conjecture) quite likely she helped the "invention" of MAKO power extraction. Something that came shortly after her return. Weakening the planet enough to make her second play at consuming it, as was her original intention. Sephiroth's "god" idea was something that Jenova (and her race) was likely doing for millenia untold before she was stopped.