Well, my understanding is that the true form of the Phoenix just wouldn't have been "complete" in 1974.
The duty of the Phoenix is to judge the state of the universe, "burning away what doesn't work," and thus ensure that the universe goes on functioning safely. But the Phoenix
Force is just that, a force; it has no personality of its own and no desire beyond the desire to feel desire (say that five times fast!). In fact, it usually can't even manifest itself without the aid of a mortal host. Even if the Phoenix Force is the original spark at the dawn of creation and the ultimate fire that will eventually consume existence, it's just not a
person any more than magnetism is a person or the Big Bang was a person.
So how can a cosmic
force act as the judge, jury, and executioner of the entire universe? This is where Jean Grey comes in, as the "person" aspect of this equation. Call it destiny or blind fortune or what have you, it just so happens that the omega mutant telepath/telekine Jean Grey of Earth is the ultimate, perfect avatar for the Phoenix Force to embody in order to carry out its work (in this 616 reality, at least). Sublime calls her the ultimate mutation...that is, evolution has placed her at the pinnacle of sentient life, capable of perfectly accessing the most primal and most infinite power in the universe.
Did the Phoenix Force merely wait until its avatar, this perfect "other half" of itself, was born through the random processes of evolution...or was she specifically created by the Force itself to act as its physical self? It's hard to say -- my own opinion is that the course of evolution in all universes would always eventually create this perfect Phoenix avatar and that in our universe it turned out to be Jean -- but the end result is the same: Jean Grey is the Phoenix, the one who embodies the Phoenix Force.
So actually, the question of what exactly Phoenix did before Jean was born is a bit misleading; technically, even though the Phoenix Force has been around forever, the
Phoenix was only ever truly born when Jean was.
When Jean's human personality is dominant, she wears the familiar green costume. When the passion and power of the Phoenix Force takes over and overwhelms Jean's humane aspect, she wears the red costume of the Dark Phoenix. Neither one of these forms is truly capable of performing the duty of the Phoenix; the human Jean Phoenix doesn't understand this duty enough, and the Dark Phoenix doesn't have human understanding to control what she burns and doesn't burn. Thus, we end up with tragedies like the time Dark Phoenix ate that sun and killed an entire alien planet.
When both Jean and the Phoenix Force are in complete utter harmony with each other, however, they become the White Phoenix of the Crown, wearing the rarely-seen white costume. This is the true form of the Phoenix in this universe, the one true vessel to carry out its work of fixing the universe when it doesn't work anymore.
Wow, that was a much longer ramble than I had originally intended
. Hope that cleared things up.
We've seen Jean turn into Phoenix in not just the regular universe (and not just once), but in many many realities. There's a basis for it that has already been shown.
