Time Travel doesn't exist in the Showa Universe of Godzilla and friends.
On the other hand, time travel does exist in the Heisei Godzilla Universe, courtesy 1991's Godzilla vs. King Ghidora, and is a crucial if confusing plot point. But it's not in the Showa, not that we know of.
Fair enough, the Showa universe is full of aliens and lost civilizations, but there are none in the Heisei universe (excepting Space Godzilla).
That's the official story. But I'd like to suggest that there may indeed be a possible case of time travel in the Showa universe. That at least one of the races of alien invaders might actually have been journeying back from the future.
Planet of the Apes, the Charlton Heston movie of 1968, needs no introduction. Flat out, it's one of the most iconic films of the 1960's, crystallizing contemporary tensions ranging from racial issues, to class war, the meaning of humanity, fears of nuclear war on down. The original movie stands as nothing short of brilliant in its creation of a patently ridiculous but absolutely serious alternate world of talking apes.
It was such a success, and so dramatically influential that it was followed up by Beneath the Planet of the Apes, which took us back to the ape world of the future, introduced mutants, and then destroyed the whole thing, back in 1970. And then through a series of 'prequels' -- Escape From Planet of the Apes (1971), Conquest of Planet of the Apes (1972) and Battle for the Planet of the Apes (1973). This was followed by two 'authorized' television series: "Planet of the Apes," in 1974, a television series that lasted 14 episodes and which was later cut into seven movies. And an animated series, "Return to Planet of the Apes" in 1975.
Apart from apes, time travel and the struggle with fate were the recurring themes of the movie series: The first two catapulted far into the future, to Man's fate and the end of the world. The third had survivors of the future fleeing back to the present and the failed quest to stop that future. The fourth had the leader of the apes leading the takeover. And the fifth featured Caesar wrestling with the notion of destiny and whether a better future could be made.
Now obviously, there was an immense amount of psychic capital, a deeply influential vibe, that could lead a single movie to produce four sequels and two separate television series. Whatever it was, Planet of the Apes tapped into something profound in the national consciousness. Maybe it was simply really good masks and makeup, or maybe it was just the idea of apes as a believable intersection of the human and inhuman, or possibly that the series was gifted with truly iconic and talented actors, or perhaps it was the visual look, or the ideas they played with. Whatever it was it hit and hit hard. Yes, the subsequent sequels and series were increasingly inferior from one edition to the next. But that only goes to show the massive impact of the original, the sequels were effectively echoes and aftershocks, growing fainter and fainter. But the original was a blast. Planet of the Apes also had an impact on Japanese sci fi media. There's "Spectreman," another half hour Japanese series from 1971 and 72. Spectreman was an Ultraman clone, most notable for the fact that this character was brown and gold rather than silver and red, and he switched back and forth between kaiju and human sized. Spectreman's human host occasionally had disagreements with the alien controllers who were prepared to sacrifice a few humans for the greater good. But what makes Spectreman interesting to us were the villains, Doctor Gori, an orangutan, and his henchman Karis, a gorilla. (Karis was usually a single henchman, but from time to time, we saw that he was the leader of a squad of gorilla soldiers).
The story goes that Dr. Gori had tried to conquer 'Planet E' but had failed and was forced to leave in a hurry. Coming across Earth, he fell in love with the place, but was disgusted by humans and their pollution. Thus, he attacked Earth's civilization with giant monsters. I'm not sure that follows logically, but it seemed to work for Gori. Gori is opposed by Spectreman and his masters, the Nebula 71 Star. It's not clear who Nebula 71 Star is or whether they're related to Gori or Planet E.
The point is that Dr. Gori and Karis are straight out of Planet of the Apes. Indeed, the fact that they're from 'Planet E' may be suggestive. 'Planet E' is an odd name, perhaps a deliberate hint that rather than being from outer space, they're fleeing from Earth's own future.
Indeed, if we relate Spectreman to Planet of the Apes, and to the subsequent Japanese series "Time of the Apes," then the better inference is that Gori and Karis are time travelers from the future to the present. Of course, there's no reason to do so, apart from the fact that all three series feature races of super-apes confronting humans, and two of them deal with time travel. But then, isn't that enough? Particularly considering the deliberate ambiguity of Planet E?
However, the most obvious example of ape-mania and the influence of the American series in Japan was seen in "Time of the Apes." This was a 26 episode half hour Japanese television series from 1974, following the last film of the official Planet of the Apes series, and at the same time as the American "Planet of the Apes" television program.
The premise was that two children and their babysitter visiting a cryogenics lab are accidentally frozen in an earthquake. They wake up in a future where apes have taken over, and must go on the run for 26 episodes. The apes include Gorillas, Orangutans, Chimps and in a mild twist Baboons. Baboons appeared as one of the ape races in the original PLANET OF THE APES novel, although they're actually monkeys, which suggests that the Japanese had gone even further into the source material.
Apparently, there's one last surviving human in the future, a UFO, a supercomputer and other plot twists, and it's all very frantic and toshy as you'd expect from a mid-seventies Japanese children's sci-fi series.
"Time of the Apes" went on to have a fairly baroque history. American impresario Sandy Frank got his hands on the property in 1987, cut four episodes together into what was by all accounts a spectacularly awful movie and hired the worst dubbing team on Earth. The resulting ejaculate then showed up twice on "Mystery Science Theatre 3000" in 1989 and 1991. But that's neither here nor there.
The bottom line of "Time of the Apes" is that it fits right in with 'ape-fever', and is so relentless in borrowing its plot elements: time travel into the future, a future ruled by apes, human survivors persecuted by apes, that it might as well constitute a part of the Planet of Apes series.
Which finally leads us to the Simeons of Godzilla vs Mechagodzilla in 1974 and Terror of MechaGodzilla in 1975. Who are the Simeons?
Well, they're apes. Gorillas actually. Not literal gorillas, but gorilla-humanoids of the same type as the 'Planet of the Apes' movies and TV series, the "Time of the Apes" series and "Spectreman."
And it turns out that they're from the third planet of their sun, just like us. Except that their whole world is being sucked into a black hole. So, in order to save themselves, they've decided to take over our planet. How? By building a giant mechanical version of Godzilla...
Okay, so that's not the best plan in the universe. That's probably not even in the top ten thousand great plans. But it's their plan, and we have to live with it.
But one thing I find remarkable is that they're from the third planet. This, like Spectreman, almost seems to be a covert reference back to Earth. It's as if the aliens or the writers are almost, but not quite, coming up to a hint that the ape men are actually from our own world. It's reminiscent of the original Planet of the Apes movie where, right up until the final moment, and despite accumulating evidence to the contrary, Charlton Heston believes that he is on an alien world.
Of course, the Simeons say that they're aliens from outer space. Or do they? Are they being coy or evasive when they fail to name their planet, but simply say it's the third planet from their sun? And in any case, can we really trust the presumption or assumption that the Simeons give that they're actually from outer space? Have they ever lied to humans anywhere else through their two movies? How reliable are they really?
Looking at the other alien races of the Showa Universe, it's usually the case that they're quite explicitly being shown as being from outer space. Earthmen visit the rocky homeworld of the Xians in Godzilla vs. Monster X, for instance. The Natals are confronted on the Moon in Battle in Outer Space. Meanwhile, the Mysterians definitely plop down from outer space. The Kilaaks are shown attacking both the Earth and Moon and have their own spaceships. Even the Nebulans are explicitly shown to be from outer space when they summon Gigan and Ghidorah from space, and later when the Seatopians contact them to borrow Gigan again. All of the other aliens are clearly interlopers where we see actual proof that they're from beyond. (Although in truth, two of the 'alien' races, aren't alien at all -- Seatopia and Mu share many features with the alien invaders, but are clearly underground remnants of a past civilization.)
But when it comes to the Simeons, we don't see their spaceships. Where the Mysterians struggle for a foothold, where the Nebulans sneak in covertly and the Kilaaks attack openly, somehow, the Simeons manage to establish large bases and build or import the massive Mechagodzilla. Where's their spaceships? Where are they actually shown in outer space, or shown traveling to Earth, or shown controlling space Kaiju? At best, we have a little bit of ambiguity here.
Of course, all we have is a bit of ambiguity. Ambiguity in the form of a lack of explicit evidence, beyond their own possibly vague statements and presence, that the Simeons really are from space rather than from the future. Ambiguity in the form of a curiously suggestive reference to their own home world as the 'third planet' which might refer to outer space or perhaps to a future Earth.
The game has rules. As influential as it might be, Planet of the Apes is not a Toho production, and not part of Toho's Showa Universe. For that matter, neither is "Spectreman" nor "Time of the Apes." We can tie the Zone Fighter TV series into the Godzilla universe, but that's because Godzilla and other Godzilla franchise kaiju, like Gigan and Ghidorah show up, and because it's a Toho production. But not "Spectreman." We can use them to show the influence of Planet of the Apes, but we can't tie Spectreman into either the 'Planet of the Apes' or the Godzilla franchise.
"Time of the Apes" approaches so closely to Planet of the Apes, borrows so completely, that we can argue that the gap closes completely, and "Time of the Apes" should be considered a part of the 'Future Ape World Timeline.' But "Time of the Apes" contains no laiju and no references that would allow us to link it to Toho continuity or to Spectreman.
What we have at best, with these four series -- the Toho Simeons, the Spectreman nemesis, the 'Planet of' and the 'Time of the Apes' are merely recurring themes of humanoid apes. Two of these themes explicitly include time travel, the other two may be slightly ambiguous. But they are not simultaneous; rather, Planet of the Apes, and its themes, images and ideas are the original inspiration for all the others.
It strikes me that just about everyone admits that the ape-like appearance of the Simeons in Godzilla vs Mechagodzilla is directly inspired by the 'Planet of the Apes' series. That's not a big leap -- but this time, there had been five movies and two television series. It's even speculated that the deformed mutant-like creatures that appear as Simeons, or servants of the Simeons in Terror of Mechagodzilla are inspired by the mutants of Beneath Planet of the Apes, who had recently returned in Battle for Planet of the Apes. And, as in Escape From the Planet of the Apes, the apes are fleeing a doomed world in the process of destruction. That's three different tropes which may be borrowed directly from the 'Planet of the Apes' series.
Well, under those circumstances, it's tempting to suggest that if they're borrowing the apes, the mutants and the flight from a dying/doomed world from the 'Planet of the Apes' series, they might, at some level, be borrowing a fourth, with their origins as being from future Earth. And it's notable that while these movies were airing, at the same time period -- 1974/1975, we had "Time of the Apes" in Japan, featuring humanoid apes in Earth's future, so even if "Time of the Apes" is not part of kaiju continuity, it's still proof of an idea (future apes) that was fresh and live in the air, at exactly the right time.
It seems to me that if we can find ambiguity and suggestiveness as to the Simeons' origins, then perhaps Planet of the Apes and "Time of the Apes," while not being definitive or determinative, should be considered influential or persuasive. The future, like space, is an unknown country. So it may be more than possible that one far flung future timeline for the Showa Universe may be a doomed Earth ruled by apes.
So maybe the true reason that the Simeons want Earth is that as far as they're concerned, it's their damned planet after all, monkeyboys!