Because if Ifalna is born of normal biological descent over two thousand years and is still a Cetra with all the capabilities thereof, and humanity aren't, then that means humanity aren't some… weakened descendants of the surviving Cetra of the Jenova era? So where did humanity come from?
Are we sure? The Cetra are pretty clearly human+ at most, there's no discernable origin for humanity, or... really much of anything. Hojo seems to think they're a seperate species, but Hojo is an idiot.
I think it's more likely they were a society, or caste, or at most a subspecies that developed the ability to commune with the planet through natural or 'unnatural' means. Exactly how this ability is passed down... perhaps it's genetic, perhaps it's something more. More likely it's
both.
However, Jenova rather totally shattered Cetra society, while at the same time leaving the planet in a... stable condition - thus in one swoop destroying those with the ability to communicate with the planet and the apparent urgency to do so. The survivors reintegrated back into larger society, and whether through genetic dilution, failing to pass on the necessary techniques, or more likely both, the Cetra as a collective disappeared.
Ifalna and Aerith are 'the last Cetra' because they're the last to have abilities, but they're not
that genetically/genealogically distinct from normal humans. They're
lucky, those who managed to inherit both the necessary inborn talent and knowledge of the Cetra, as opposed to the majority who didn't - because of genetic happenstance, trauma, circumstances, or just disinterest.
Well. 'Lucky'.
Ifalna: "2000 years ago, our ancestors, the Cetra, heard the cries of the Planet. The first ones to discover the Planet's wound were the Cetra at the Knowlespole."
Likewise: This is before Aerith's birth (I'm pretty sure) and Ifalna, speaking to Gast, refers to the Cetra as
our ancestors.
There's perhaps an interesting note in 'heard the cries of the planet'. At least in Remake, Barrett talks about 'hearing the planet's cry'
all the time. Now I don't think he actually
does have this ability (the whole 'initially approving of the Mako reactor' shows this), but I think it demonstrates that it's
in the cultural context of humanity. This is regarded as a plausible thing that happens, though made into more of a metaphor than 'literally communicating with the planetmind'.
Though this also brings up another note. Do we know that Ifalna has 'all the capabilities thereof'? Or that others don't have some? Humans in this story seem to have some... very interesting superpowers seemingly spontaneously.
Also pretty sure this confirms Ilfana is
not immortal, or it would be 'we', not 'our ancestors'.
Ifalna: "Yes, Professor. The one the Professor mistook for a Cetra… was named Jenova. That is 'the crisis from the sky.'"
This definitely helps explain how Hojo (and Gast!) mistook Jenova for a Cetra. She told them she was! She was able to pretend to be a Cetra well enough to fool the Cetra, why would people in the modern age be able to know any different. Perhaps she didn't
directly tell them, but the whole downfall of the Cetra caused enough confusion that someone
else made the mistake? Or it was thought of as a... civil war?
Also, that Hojo thought Jenova was a Cetra means he believes there were
three Cetra running around: Ifalna, Aerith,
and Sephiroth.
As an aside regarding that... watching the Remake, I noted that Aerith's eyes are almost the exact same color of green as Cloud's. Are SOLDIERs perhaps some kind of... Shinra attempt at replicating Cetra abilities? Or at least something tangental to that?
Is some kind of immersion in the mako stream how the Cetra got their abilities in the first place? More 'meditation in site of power' than 'inject magic juice' thematically, but...
(This theorizing is, perhaps, driven by my deep and irrational hatred of Superior Lineages more than any reason.)
Nothing says that Hojo and co had to break down the door the literal second they tracked down Gast and Ifalna. Just as likely that he found them, put them under surveillance for a minute just to scope out the situation, then learned about Aerith. Considering the man's entire scientific ethos boils down to trying to create the new Reese's Peanut Butter Cups by smashing together whatever bullshit he finds lying around, Gast and Ifalna having a child together would've activated his almonds and then some.
Alternatively, perhaps they just... found her during the raid, and Hojo is like 'Bonus Cetra! How thoughtful!'
